Leak Down Test (Don't Laugh!!)

Status
Not open for further replies.
click said:
I measured the piston across the bottom in line with the gudgeon pin hole and got 80.46mm.......................
Click to expand...

If I read this correctly, you really need to be taking the measurement on the piston skirt at right angles to the gudgeon pin, not "in line with".

Fast Eddie said:
My money if firmly on inadequate clearances.
Click to expand...

I tend to agree but it's click's money so he really needs to confirm the root of this problem.

A quick assessment is sliding the piston in to the bore starting with say a 0.003" feeler gauge between the skirt and bore and increment up until it feels like it there's really no slack. The measurement should be at 90 degrees to the gudgeon pin.

I like hobot's assessment on the pistons. They really don't look too bad other than the scuffing but I would give them a good cleaning and inspect the ring lands with a jewelers loop to see that they are not damaged or scratched if you should choose to take that path. From your assessment of the bores (lack of feeling a ridge where the vertical scuff marks are) you may be able to clean them up with a light proper hone.

Back to the root cause. This motor has a habit of seizures; see a pattern here? Did you ever have an oil pressure pressure gauge to rule out oil pressure problems. If this is an oil starvation problem, things to look at are the feed side starting from the tank screen and lines. The lip seal on the timing side end of the crankshaft has been known to blow out or maybe the incorrect shim was used behind the doughnut seal on the oil pump. The oil pump could also be a problem though unlikely. You would be amazed at how little indicated oil pressure is needed to keep the rod journals from seizing. So these are all straws to grab at if that time comes. It is rather odd that you have been going along with this conditions without any indication of "an event" like a rear wheel locking up on the motorway at speed.

Again, I would slip the feeler gauges in, even if it's where there's scuffing as that will give an indication of where the clearance ended up. As straight forward as this may seem, it sure is an odd one you are dealing with here.
 
hobot said:
forgot to leave the best piston surface url. Pleased on doing it to mower with chain saw file.
http://www.federalmogul.com/en-US/Media/Documents/HighqualitySAEpaper200909PFL1163manuscript2.pdf

Btw its a freaking Isolastic Commando for gosh sakes so a few points either side of ideal is a non issue, especially if already run with current pistons w/o hands numb or various brackets light filaments or connections fracturing. If not isolating by about 2000 then just bevel the front iso's 45' with slight rim left to get back disappearing act.
Click to expand...

Thanks for linking to the article, really interesting read. I wounder if I could get a sapphire window liner installed in the Norton so I could observe the pistons while the engine is running, obviously while at the same time riding the bike:rolleyes:
 
RoadScholar said:
You posted a question about balance some where's back on this yellow brick road relative to getting new (larger) pistons. Consider this: each gram of imbalance can generate a kilogram of apparent weight at 1000 Gs of acceleration, which is conservative for a Norton. Selecting a balance factor has shown to be a highly personal choice, based on other threads dedicated to that topic, but whatever factor you choose at least the parts will be matched to a uniform scheme.

I recall you writing that you didn't want to split the bottom end, but when you ask questions about balance splitting comes with that territory. Another truism: Open the bottom end, open your wallet.

I wish your Norton a speedy recovery and understand that life gets in the way often...
Click to expand...

I don't think it was me that mentioned balance, but this thread is so long who knows!! I know the engine was completely stripped & the crank was balanced to match the heavier pistons. Unfortunately the chap who rebuilt the engine is no longer with us, he would have got this work done by somebody else and I don't know who this is! so can't verify what crank balance etc. was used.

Interesting info. about the minor weight difference & what it can cause in terms of a difference in weight balance.

I'm positive the Norton will be back on the road for the summer, positive thinking, that's the ticket;)
 
Last edited:
Dances with Shrapnel said:
If I read this correctly, you really need to be taking the measurement on the piston skirt at right angles to the gudgeon pin, not "in line with".



I tend to agree but it's click's money so he really needs to confirm the root of this problem.

A quick assessment is sliding the piston in to the bore starting with say a 0.003" feeler gauge between the skirt and bore and increment up until it feels like it there's really no slack. The measurement should be at 90 degrees to the gudgeon pin.

I like hobot's assessment on the pistons. They really don't look too bad other than the scuffing but I would give them a good cleaning and inspect the ring lands with a jewelers loop to see that they are not damaged or scratched if you should choose to take that path. From your assessment of the bores (lack of feeling a ridge where the vertical scuff marks are) you may be able to clean them up with a light proper hone.

Back to the root cause. This motor has a habit of seizures; see a pattern here? Did you ever have an oil pressure pressure gauge to rule out oil pressure problems. If this is an oil starvation problem, things to look at are the feed side starting from the tank screen and lines. The lip seal on the timing side end of the crankshaft has been known to blow out or maybe the incorrect shim was used behind the doughnut seal on the oil pump. The oil pump could also be a problem though unlikely. You would be amazed at how little indicated oil pressure is needed to keep the rod journals from seizing. So these are all straws to grab at if that time comes. It is rather odd that you have been going along with this conditions without any indication of "an event" like a rear wheel locking up on the motorway at speed.

Again, I would slip the feeler gauges in, even if it's where there's scuffing as that will give an indication of where the clearance ended up. As straight forward as this may seem, it sure is an odd one you are dealing with here.
Click to expand...

Many thanks for the info. on how to measure the piston, it's info. like this that I don't have at my fingertips, I'll re-measure later today & post the results.

I'll check the pistons in the bore, just to be sure, with or without rings installed???

I'll also give the pistons a forensic looking at under magnification.

Never had an oil pressure gauge. Would an oil pressure issue not show signs of heat seizures? I've been told by two different sources that it looks like cold seizures (all from pictures of the damage).

My basic understanding is that what is referred to as a 'cold seizure' is mainly scoring/smudging of the skirt against the bore due to lack of clearance and/or a breakdown of the oil film between the bore & piston.

If this scoring melts/creeps up the piston & locks the rings you are now going to get a heat seizure & the pistons/rings will 'lock-up'

I know this is an oversimplification of what can & does go wrong but I seem to be getting the first part (scoring of the piston skirt) but not the 2nd part (piston rings locking up).

The only indication I have of the robustness of the oil circulation system is observing the oil return to the oil tank which has always been strong.

You said "The lip seal on the timing side end of the crankshaft has been known to blow out or maybe the incorrect shim was used behind the doughnut seal on the oil pump"

If the lip seal had blown or an incorrect shim on the oil pump, what, if any, symptoms would show up?? (apart from damage to the piston!!)

Really appreciate you applying some of your gray matter to this problem!
 
Last edited:
oldmikew said:
Am inclined to agree with Hobot. Nothing seized, so if you clean the piston faces with a smooth file ( do NOT use emery paper ) and get the bores rehoned to get the worst of the score marks out then it should be 0k. Precaution would be to find out from Norvil which piston suppliers they use and what the recommended bore clearence is. And one final point , do ensure that the rings were fitted the right way up by the original builder.
Click to expand...

Thanks for the suggestion from yourself & hotbot.

I'll do all the measuring homework I need to do & the forensic investigation of the pistons first & then see what these results show!!
 
Doubt anyway but pressure gauge and exam of integrity of the pump nipple for sense of leakage there but big ends would suffer first as bores don't need much oil but from crank spray and splash. Over rich fuel can remove oil for similar scuffing harm. Cold seizure mainly implies piston swelling faster/more than bores have time to expand so rubs/binds till bores catch up or piston or rod come apart. Most the seizures I've seen in mowers and Nortons showed piston stopped on up stroke, breaking piston in half or bending/breaking rod.

I'm so concerned about this issue I had Canaga give Mz Peel .0055" at top and .006" the rest the way down. Two reasons, one Ken experimented with clearance till his racers did not bind to land on .005", which I think wise for big bore blocks, two Mz Peel is hi CR supercharged so can go from normal mid even cruise temps to 3x's more heat on piston than a Combat. I also routinely water cross deep enough to lift legs high for the splash and hit summer storms fronts on hwy, so Maney Alu cylinder will contract significantly at times.

If can't find machinist and can wait a bit I've decent bore measure kit to lend. Used to figure out similar decisions/processing on Trixie Combat before finalizing bore with Bore Tech coating. Mainly got it to keep eye on abusing Peel, keeping in mind posts on some oval shaped bores and beat up piston crowns of racers using JMS type pancake pistons that can rock more so clearance matters more. JMS/pancake pistons seem to last fine for the long run but hot rod engines ran at full boggie are all famous for shorter parts endurance to check in and replace hopefully ahead of failure.
 
hobot said:
Doubt anyway but pressure gauge and exam of integrity of the pump nipple for sense of leakage there but big ends would suffer first as bores don't need much oil but from crank spray and splash. Over rich fuel can remove oil for similar scuffing harm. Cold seizure mainly implies piston swelling faster/more than bores have time to expand so rubs/binds till bores catch up or piston or rod come apart. Most the seizures I've seen in mowers and Nortons showed piston stopped on up stroke, breaking piston in half or bending/breaking rod.

I'm so concerned about this issue I had Canaga give Mz Peel .0055" at top and .006" the rest the way down. Two reasons, one Ken experimented with clearance till his racers did not bind to land on .005", which I think wise for big bore blocks, two Mz Peel is hi CR supercharged so can go from normal mid even cruise temps to 3x's more heat on piston than a Combat. I also routinely water cross deep enough to lift legs high for the splash and hit summer storms fronts on hwy, so Maney Alu cylinder will contract significantly at times.

If can't find machinist and can wait a bit I've decent bore measure kit to lend. Used to figure out similar decisions/processing on Trixie Combat before finalizing bore with Bore Tech coating. Mainly got it to keep eye on abusing Peel, keeping in mind posts on some oval shaped bores and beat up piston crowns of racers using JMS type pancake pistons that can rock more so clearance matters more. JMS/pancake pistons seem to last fine for the long run but hot rod engines ran at full boggie are all famous for shorter parts endurance to check in and replace hopefully ahead of failure.
Click to expand...

The over rich bit rings a bell!! Since I've had this bike I've chased good carburetion, it always ran rich on both cylinders but particularly the RH cylinder. I run twin Amal MK1's. This could have easily 'washed' the bores of oil, not something, TBH, that was on my radar! Pete L. also mentioned this as a possible cause.

I would do plug chops & get the carburetion OK for the top end but low to mid was always rich. I eventually changed the needle jets & needles & got better carburetion on the lower to mid range.

I was also chasing an oil leak from the head, valve oil seals slipping off. I think all these other, relatively minor, issues were masking what was really going on!!

Because of the low mileage I tend to do on this bike, this whole process took years to shake through, I'd make a change over the winter, then potter around over the summer, riding the bike & getting it dialled in, then, when the summer was over, the bike is put away & maybe try something else over the winter. Some summers I would not get to use the bike, so you can see that 3-5 years could slip by dealing with one issue. Not ideal, just the way it is. Having more than one bike & a car tends to limit the amount of time you spend on a classic bike!

Many thanks for your kind offer of a lend of a bore measuring kit, the cost of shipping to Ireland & back would probably be the same or more than purchasing one myself :rolleyes: there's also the issue of would I know what to do with the kit :confused:, as they say it's the thought that counts, thanks anyway:)
 
click said:
I'll check the pistons in the bore, just to be sure, with or without rings installed???
Click to expand...
Without rings installed.
click said:
Never had an oil pressure gauge. Would an oil pressure issue not show signs of heat seizures? I've been told by two different sources that it looks like cold seizures (all from pictures of the damage). My basic understanding is that what is referred to as a 'cold seizure' is mainly scoring/smudging of the skirt against the bore due to lack of clearance and/or a breakdown of the oil film between the bore & piston.
Click to expand...
Cannot comment on the hot seizure versus cold seizure. I have a friend and fellow racer who badly scuffed a brand new set of race pistons. The cause was traced to some machine flashing remaining in the feed side spigot to the oil tank. I calculated that he had maybe 1/8 the cross sectional area that he should have. Worked fine for low to moderate speeds but was devastating at high speeds.
click said:
If this scoring melts/creeps up the piston & locks the rings you are now going to get a heat seizure & the pistons/rings will 'lock-up'
Click to expand...
Any time you have scuffing there is a lot of heat generated. I think this is a matter of degree where the piston to bore clearance has a reasonable and safe clearance range. Once you exceed this range due to say overheating resulting from such things as detonation or preignition, one uses up that margin of safety in the bore to piston clearance.
click said:
I know this is an oversimplification of what can & does go wrong but I seem to be getting the first part (scoring of the piston skirt) but not the 2nd part (piston rings locking up).
Click to expand...
If the metal smears enough it can get into the ring lands and hang up the rings. I would think a hung up oil ring would provide more lubrication but a hung up oil ring also provides more oil to the combustion chamber which knocks down (pun intended) the combustion process ability to resist detonation, thus more heat. All elements presented are true, if this is what happens is really on a case by case basis so I can say with certainty - "who knows?"
click said:
The only indication I have of the robustness of the oil circulation system is observing the oil return to the oil tank which has always been strong. You said "The lip seal on the timing side end of the crankshaft has been known to blow out or maybe the incorrect shim was used behind the doughnut seal on the oil pump". If the lip seal had blown or an incorrect shim on the oil pump, what, if any, symptoms would show up?? (apart from damage to the piston!!)
Click to expand...
Well, first, you would still see ample oil returning to your oil tank. With a blown oil seal the problem is the oil would not be getting to where it needed to go with ample pressure. The oil would spurt out into the timing chest, dribble down to the sump and get picked up by the return pump thus you would see ample oil returning to the tank. A little story to share here the last time I ran Daytona with an ultra short stroke Norton (new build) where the bypass valve was jacked up for more pressure. When I came in after my first race a friend pointed to my oil pressure gauge which was reading about zero in the hot pit. The motor sounded great and ran great so I said f*ck it and went out to complete the second race. Upon tear down I found the seal had been blown. The pump will deliver oil to the end of the crank and I believe the centripetal force at high rpm was what assured "enough" oil to the rod journals. Note that I emphasize "enough" as opposed to "adequate" or "ample". Upon tear down of this new motor there was atrocious wear on the cylinder liners (which I attributed to liner metallurgy and fixed through subsequent carbide impregnation process of the liner surfaces) but others mentioned it maybe due to inadequate lubrication. So again, who knows. I fixed the seal and have used the carbide process ever since. Check your oil tank screen and the condition of the oil feed side hoses. Can the hoses collapse? Do you run an after market (they all are) anti drain valve on the feed side of the oil lines? Are the oil lines OEM or after market. These are easy things to check now but I suspect there's maybe a piston to liner clearance issue yet 1.) this engine seized before and 2.) you stated that the fellow who rebuilt and converted to 920 knew what he was doing (or something like that) so why the too tight of a clearance? It does not add up and there's a history there. Suspicious minds want to know.
 
Last edited:
I'm not sure what the current status is regarding piston replacement, but if you find lighter pistons, I would think this might be a good thing regardless of the heavier crank. There are a lot of discussions here about using lightweight pistons without rebalancing the crank. It has been argued often that is a preferred state.

Russ

PS - Forgive me if this has already been discussed. I have been gone for a week and this thread has gone viral!
 
hehehe clickeroo I too thot I needed something fun and simple to take up my spare time, mostly puttering around but dang grimlins like your cascade is such a common theme sharing it helps not to take so personally. It took me about a decade of daily hours on several forums and archives of NOC and Capt. Norton to be able to comprehend what I'd gotten into or discuss much. My Pre-peel Combat was abused for last reputation a couple decades before it found me, as no concept any Norton dinosaurs survived to 1999. It ran great able to out accelerate 1200cc over cammed wheelie bar Sportster drag bike up to 70's before passed. Knew it should be gone through as had 13 leak sites and mildly smoking to discover nothing left to just refit w/o fixing or replacing crank to head gearbox to swingarm and forks/brakes. That set me back so bad bought a modern thinking to be rid of Commando clunker beyond my capability. I put a dash of booze in coffee this 20's F am to calm mind before communing here and flashing back.

Didn't know so far away but certainly can find mechanic/machine shop to measure bores and pistons to decide next steps. Conquering Commando crap has made me braver facing other unknowns in life but about killed me doing it.
 
I calculate 906 cc s actual displacement with an 80.5 mm bore.
The new type RGM kit measures the same.
The decals still say 920 tho.
I might just put the decals on the side panels and forego the rest of the 920/906 exercise.
In 1965 I added an STP decal to my go kart and got an immediate 10 mph increase in top speed.

Glen
 
Last edited:
Don't forget an oil leak into chamber, like this case, creates ceramic hard coke particles which can also cause such scuffing and aggravate clearance issues.
 
Dances with Shrapnel said:
Without rings installed.

Cannot comment on the hot seizure versus cold seizure. I have a friend and fellow racer who badly scuffed a brand new set of race pistons. The cause was traced to some machine flashing remaining in the feed side spigot to the oil tank. I calculated that he had maybe 1/8 the cross sectional area that he should have. Worked fine for low to moderate speeds but was devastating at high speeds.

Any time you have scuffing there is a lot of heat generated. I think this is a matter of degree where the piston to bore clearance has a reasonable and safe clearance range. Once you exceed this range due to say overheating resulting from such things as detonation or preignition, one uses up that margin of safety in the bore to piston clearance.

If the metal smears enough it can get into the ring lands and hang up the rings. I would think a hung up oil ring would provide more lubrication but a hung up oil ring also provides more oil to the combustion chamber which knocks down (pun intended) the combustion process ability to resist detonation, thus more heat. All elements presented are true, if this is what happens is really on a case by case basis so I can say with certainty - "who knows?"

Well, first, you would still see ample oil returning to your oil tank. With a blown oil seal the problem is the oil would not be getting to where it needed to go with ample pressure. The oil would spurt out into the timing chest, dribble down to the sump and get picked up by the return pump thus you would see ample oil returning to the tank. A little story to share here the last time I ran Daytona with an ultra short stroke Norton (new build) where the bypass valve was jacked up for more pressure. When I came in after my first race a friend pointed to my oil pressure gauge which was reading about zero in the hot pit. The motor sounded great and ran great so I said f*ck it and went out to complete the second race. Upon tear down I found the seal had been blown. The pump will deliver oil to the end of the crank and I believe the centripetal force at high rpm was what assured "enough" oil to the rod journals. Note that I emphasize "enough" as opposed to "adequate" or "ample". Upon tear down of this new motor there was atrocious wear on the cylinder liners (which I attributed to liner metallurgy and fixed through subsequent carbide impregnation process of the liner surfaces) but others mentioned it maybe due to inadequate lubrication. So again, who knows. I fixed the seal and have used the carbide process ever since. Check your oil tank screen and the condition of the oil feed side hoses. Can the hoses collapse? Do you run an after market (they all are) anti drain valve on the feed side of the oil lines? Are the oil lines OEM or after market. These are easy things to check now but I suspect there's maybe a piston to liner clearance issue yet 1.) this engine seized before and 2.) you stated that the fellow who rebuilt and converted to 920 knew what he was doing (or something like that) so why the too tight of a clearance? It does not add up and there's a history there. Suspicious minds want to know.
Click to expand...

Now that's an education right there, really appreciate you sharing your knowledge/experience.

I'll add the checking of the oil system to my list, oil tank screen, oil lines etc. I'll also check the crank oil seal. I'll probably be on again asking questions about this but one bit at a time.
 
hobot said:
hehehe clickeroo I too thot I needed something fun and simple to take up my spare time, mostly puttering around but dang grimlins like your cascade is such a common theme sharing it helps not to take so personally. It took me about a decade of daily hours on several forums and archives of NOC and Capt. Norton to be able to comprehend what I'd gotten into or discuss much. My Pre-peel Combat was abused for last reputation a couple decades before it found me, as no concept any Norton dinosaurs survived to 1999. It ran great able to out accelerate 1200cc over cammed wheelie bar Sportster drag bike up to 70's before passed. Knew it should be gone through as had 13 leak sites and mildly smoking to discover nothing left to just refit w/o fixing or replacing crank to head gearbox to swingarm and forks/brakes. That set me back so bad bought a modern thinking to be rid of Commando clunker beyond my capability. I put a dash of booze in coffee this 20's F am to calm mind before communing here and flashing back.

Didn't know so far away but certainly can find mechanic/machine shop to measure bores and pistons to decide next steps. Conquering Commando crap has made me braver facing other unknowns in life but about killed me doing it.
Click to expand...

I think a lot of classic bike owners experience these 'feelings', I'm also aware of some guys who purchased their bikes from new 40+ years ago & have used & maintained them as they were intended & have had very little problems (damn them :D )
 
worntorn said:
I calculate 906 cc s actual displacement with an 80.5 mm bore.
The new type RGM kit measures the same.
The decals still say 920 tho.
I might just put the decals on the side panels and forego the rest of the 920/906 exercise.
In 1965 I added an STP decal to my go kart and got an immediate 10 mph increase in top speed.

Glen
Click to expand...


Why didn't I think of that, just buy the stickers, genius
 
click said:
I'll also check the crank oil seal. I'll probably be on again asking questions about this but one bit at a time.
Click to expand...

I would not jump to pulling the timing chest cover to check the seal quite yet. Look at it as a decision tree process. If you find the piston to skirt clearance tighter than a clams ass, then it's a reasonable assumption that you found the problem though I find it tough to reconcile with your assertion that it was put together by a knowledgeable person with some good standard of care though we all have our off days. Checking the oil lines and oil tank sump screen is easy peasy as they are right there out in the open. You can just look at the lines to determine whether they are OEM or after market. Do you have an anti drain valve in your oil feed line? Digging deeper should only be done if indicated. Installing (temporary or permanent) an oil pressure gauge would be less intrusive and more telling when you get back under way though you may not want to wait if the current cylinder to skirt clearances suggest they were set up correctly.

hobot has a good point about cylinder washing if over a prolonged period of time. Jetting from the top down is the normal course of action yet I venture to say most of your road time is usually in the low to mid throttle settings. I hear about "fuel washing" with diesels but not so much on spark ignition engines - probably because diesels continue to perform whereas a spark ignition would not behave very well with an over rich condition, over rich to the point of washing the lubrication away. On the other hand, what you don't have in extreme over richness you can perhaps make up with time - driving it too long with a too rich condition. What did the condition of the oil tell you after each oil change? Did the viscosity appear to be down when cold and did it reek with the smell of gasoline (petrol)?
 
Status
Not open for further replies.
Back
Top