Leak Down Test (Don't Laugh!!)

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What's in the sump? there must be something there, take out the strainer and stick your finger in there, methinks it is not going to be good news. Something has gone pas the pistons and must be lurking in the bottom of the cases.
 
A longshot, were the pistons made correctly, something tells me that they are wider across the damaged area than they are at the skirt and crown - if not detected when fitted it was only a matter of time before they failed.
 
Bugger. But hey, at least you’ve found it!

I would suggest that such damage could quite easily have been started in the first 500 miles and that you’ve been living with damaged pistons for longer than you think.

My guess would be that this was put together with tolerances that were too tight. And although it’s run well for a while, all was not well under the surface. Then maybe a good run on a hot day, or a fill up with poor fuel, or similar, tipped it over the edge.
 
Fast Eddie said:
Bugger. But hey, at least you’ve found it!

I would suggest that such damage could quite easily have been started in the first 500 miles and that you’ve been living with damaged pistons for longer than you think.

My guess would be that this was put together with tolerances that were too tight. And although it’s run well for a while, all was not well under the surface. Then maybe a good run on a hot day, or a fill up with poor fuel, or similar, tipped it over the edge.
Click to expand...

I know the chap who had the bike before me, let the engine run low on oil & seized a piston. He decided to go with the 920 liner/piston kit from Norvil. It went back to the original builder of the bike & he stripped the engine, split the cases, got the crank balanced for the heavier pistons, cleaned up all the internals, got the 920 liners fitted & put the engine back together.

My theory is that the previous owner was nervous due to seizing a piston & ran the bike very rich & did a very gently run-in & might have had some cold seizures (maybe this is not possible on a Norton Commando??)

Whatever caused the problem the solution involves, rebore, new pistons & rings (If I can get oversized pistons!!). I just need to find somebody I trust to do the work ideally over this side of the pond!!

Don't feel up to doing any more work on the Norton tonight, I'll check & inspect the sump for any 'bits' + check the cams for wear.

I'll remove the pistons & measure the ring gap at different points in the bore just to see what results I get.

Really appreciate all your help & advice so far.

 
Those bores look terrible and pistons equally bad. The crosshatch pattern is even worse than terrible. Frankly, it appears to me that the engine builder was totally incompetent. ASSUMING the pistons are the correct size for the bore, then the problems was caused by: the improper cross hatch pattern (and likely an incorrect bore finish) plus poor bore cleaning prior to the assembly. Improper cleaning will produce the vertical scratches (as will poor/no air filtering and the incorrect cross hatch will create an excellent chance that the rings will never seat properly. Of course, improper ring end-gap or gap alignment can cause similar (oil burning) issues.

Fwiw, on reassembly, when you clean the pistons/bores after re-boring/honing, clean the bores with solvent like Naptha or any mineral spirits - gasoline works fine, just don't be smoking at the time - and then clean, Clean, CLEAN with rags/hot, soapy (original Dawn is good) water until a white cloth show no discoloration whatever. Then dry and wipe with a cloth dampened with WD40 to prevent rust and cover the barrels until ready to install the pistons. When you Install the equally clean rings/pistons) do not add any additional lubrication. That's right, install them "dry."

If you are totally uncomfortable with "dry," (some folks are) then wipe some 30 wt engine oil on a rag and apply that to the bores/rings. DO not dip the piston into engine oil, as the old manuals used to specify. That just creates immediate gunk/carbon in the ring grooves when the engine fires up.

Obviously, Be sure to check/verify that the rings are the appropriate size for the piston - IOW if you bore .020 oversize, you must have an .020 oversize piston and a set of .020 oversize rings. Also check/verifty the piston ring end gaps are within spec and that the gaps are appropriately staggered. CHECK all the dimensions, don't assume that the manufacturer got it right when it was labeled/packaged. I have seen (admittedly not very often) piston rings and pistons come marked as a certain size, only to find they were not.
 
As a side note- Strange that the Norvil 920 pistons are heavier than stock.
The RGM 920 pistons are the same weight as stock 850 standard bore pistons +-2 grams.
Roger made sure of that when sourcing the pistons. This way crank rebalancing is not required.

Glen
 
MexicoMike said:
Those bores look terrible and pistons equally bad. The crosshatch pattern is even worse than terrible. Frankly, it appears to me that the engine builder was totally incompetent. ASSUMING the pistons are the correct size for the bore, then the problems was caused by: the improper cross hatch pattern (and likely an incorrect bore finish) plus poor bore cleaning prior to the assembly. Improper cleaning will produce the vertical scratches (as will poor/no air filtering and the incorrect cross hatch will create an excellent chance that the rings will never seat properly. Of course, improper ring end-gap or gap alignment can cause similar (oil burning) issues.

Fwiw, on reassembly, when you clean the pistons/bores after re-boring/honing, clean the bores with solvent like Naptha or any mineral spirits - gasoline works fine, just don't be smoking at the time - and then clean, Clean, CLEAN with rags/hot, soapy (original Dawn is good) water until a white cloth show no discoloration whatever. Then dry and wipe with a cloth dampened with WD40 to prevent rust and cover the barrels until ready to install the pistons. When you Install the equally clean rings/pistons) do not add any additional lubrication. That's right, install them "dry."

If you are totally uncomfortable with "dry," (some folks are) then wipe some 30 wt engine oil on a rag and apply that to the bores/rings. DO not dip the piston into engine oil, as the old manuals used to specify. That just creates immediate gunk/carbon in the ring grooves when the engine fires up.

Obviously, Be sure to check/verify that the rings are the appropriate size for the piston - IOW if you bore .020 oversize, you must have an .020 oversize piston and a set of .020 oversize rings. Also check/verifty the piston ring end gaps are within spec and that the gaps are appropriately staggered. CHECK all the dimensions, don't assume that the manufacturer got it right when it was labeled/packaged. I have seen (admittedly not very often) piston rings and pistons come marked as a certain size, only to find they were not.
Click to expand...

Fantastic advice, I have heard of the 'dry' run-in of piston rings, I think I would use at least a lick of 30wt oil!! I understand the principal & why the 'dry' method is preferred by some engine builders.

I've taken note to check EVERYTHING before final assembly. I do have a tendency to presume that certain things are OK because they are new:rolleyes:

Ideally I'd like to send these barrels to somebody who will check & spec them, fit & ensure new pistons & rings are matched to a rebore etc. I've dealt with SRM in Wales so might start with them & see what they advise!
 
worntorn said:
As a side note- Strange that the Norvil 920 pistons are heavier than stock.
The RGM 920 pistons are the same weight as stock 850 standard bore pistons +-2 grams.
Roger made sure of that when sourcing the pistons. This way crank rebalancing is not required.
Click to expand...

The original Norvil 920 kit had modified Cosworth pistons and they also had the larger diameter gudgeon pins that click mentioned.
 
worntorn said:
As a side note- Strange that the Norvil 920 pistons are heavier than stock.
The RGM 920 pistons are the same weight as stock 850 standard bore pistons +-2 grams.
Roger made sure of that when sourcing the pistons. This way crank rebalancing is not required.

Glen
Click to expand...

My understanding of the history of the 920 coversion from Norvil is that they sourced (Ford?) pistons that did the job but the gudgeon pin was larger. Around the same time RGM used the same piston for their conversion, these pistons were heavier than standard.

RGM then sourced a different 920 piston which was the same weight as a standard piston.

I think? Norvil now offer a different type of piston. I'm actually unsure which one I have, the older heavier one or maybe the same weight as standard. Once I have the pistons off I'll get more info. from Norvil and I'll check the diameter of the gudgeon pin!

L.A.B. just got in before me!!
 
If the crank had to be rebalanced then you would have the earlier heavy pistons, this will be confirmed when you remove them as they also have the bigger gudgeon pins that required the small end bush to be removed from the conrods to be able to fit them.
 
I apologize if you've already posted the info and I've just missed it, but have you measured the bore to see if you can determine what the clearance actually was? Might still have a clean area at the bottom of the bore (or maybe not). Even the cast Ford pistons Fair Spares/Norvil used need at least 4.5 thou clearance if the bike is going to ever be ridden hard. I ran them at .005" in my race bike back in the '80s for quite a while with good results. It's going to be more difficult to get a good measurement on the pistons. They've certainly already collapsed enough to change the orginal clearance by at least .001", probably more. Your results really look just like what I saw back then when I was trying to sort out using forged pistons in the 920. The manufacturers (Arias, Forgedtrue, and Venolia) all said to run them at .002" or .003", and they seized every time. I didn't get any of them to work on the race bikes until I got out to .005" clearance or more.

Ken
 
lcrken said:
I apologize if you've already posted the info and I've just missed it, but have you measured the bore to see if you can determine what the clearance actually was? Might still have a clean area at the bottom of the bore (or maybe not). Even the cast Ford pistons Fair Spares/Norvil used need at least 4.5 thou clearance if the bike is going to ever be ridden hard. I ran them at .005" in my race bike back in the '80s for quite a while with good results. It's going to be more difficult to get a good measurement on the pistons. They've certainly already collapsed enough to change the orginal clearance by at least .001", probably more. Your results really look just like what I saw back then when I was trying to sort out using forged pistons in the 920. The manufacturers (Arias, Forgedtrue, and Venolia) all said to run them at .002" or .003", and they seized every time. I didn't get any of them to work on the race bikes until I got out to .005" clearance or more.

Ken
Click to expand...

Ken, interesting info. regarding the clearance required.

I don't have any bore measuring tools, I don't think a tape measure would do!!!

Because of the knowledge/experience limitations I have, once I talk to a few companies like Norvil & SRM I'll make my mind up on what to do next.

I still need to do a bit of stripping, pistons out to check gudgeon pin size & I'll check the ring gaps etc. BUT not tonight, not in the mood to wrench on the Norton and more importantly I've had too much whiskey:confused:
 
Just thinking - as there’s not much meat on those liners, if the bore is too tight and the scores aren’t bad, might it be possible to bore it to the correct “std” size? ( And use new “std”pistons of course).
 
cliffa said:
Just thinking - as there’s not much meat on those liners, if the bore is too tight and the scores aren’t bad, might it be possible to bore it to the correct “std” size? ( And use new “std”pistons of course).
Click to expand...

Good thinking there cliffa... probably unlikely... but definately worth checking out !
 
:) well, too much for wrenching on a Norton!! As Yoda would say, 'no try, just do'
 
click said:
Fantastic advice, I have heard of the 'dry' run-in of piston rings,

It aint that fantastic, as its a total waste of advice as you have to lube the skirts and gudgeon pins any way .
How did you not see the bores when you replaced the head ?, this problem can't have always been there ,more likely relates back to page 6 were you said you replaced needles and jets
Click to expand...
 
If you are doing a dry install, the point is to NOT have oil on the rings/bores. You do, of course, lube the piston pins on assembly and you can put a smear of oil on the thrust-side piston skirt.
 
MexicoMike said:
If you are doing a dry install, the point is to NOT have oil on the rings/bores. You do, of course, lube the piston pins on assembly and you can put a smear of oil on the thrust-side piston skirt.
Click to expand...

Thanks for the clarification re: gudgeon/wrist pin lube.

I was taking your lead from: "If you are totally uncomfortable with "dry," (some folks are) then wipe some 30 wt engine oil on a rag and apply that to the bores/rings. DO not dip the piston into engine oil, as the old manuals used to specify. That just creates immediate gunk/carbon in the ring grooves when the engine fires up."

So, not dry but definitely not wet!, as I said I understand the principal's involved in a dry install just not 100% sure if I'd use that method! The noise a dry install makes is seemingly quite 'interesting'.
 
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