Leak Down Test (Don't Laugh!!)

FWIW re leakdown tests/bore wear based on many years of doing them: Most wear occurs at the top inch of the bore, which is why LD tests are done at TDC. The wear is easy to observe - even without a micrometer. Take a piston ring and insert it square with the bore (push it down just below the ridge from above with the top of a piston) so it is approx 1/2 below the top of the bore or just below the ridge if there is one. Measure the gap of the ring with a feeler gauge. Then, using the piston, push the ring several inches down and measure again. You will find a narrower gap (less wear). Bores always taper with wear, with the largest internal radius (most wear) at the top.

Further, as noted in an earlier post Leakdown and Comp tests are best performed when the engine is warm - as close to operating temp as possible. This is because the shape of the cylinder bore cold is not the same as it is when hot. IOW, you could have great sealing when cold and poor sealing when hot due to the fact that the bore does not expand evenly with heat. Ideally ou want to know how the engine seals under normal operating conditions, NOT how it seals sitting in the garage ! This is one of the major reasons that two identical engine often produce more/less power. One engine has "rounder" bores when hot than the other, even though both have equally round bores when cold. When building performance/ competition engines we often specifically looked for engine blocks that had more material on the thrust side of the bore than on the opposite side. Those engines always produced more power because when they were hot the bores stayed "rounder." Nowadays, with better casting techniques, there is (probably) not as much random casting offset as there used to be.

Gasket for a 77mm bore being suitable for a 81mm bore? I am assuming Norvil would know.

If the 77mm gasket edge hangs into the bore I would not be too keen about it size little bits hanging into the combustion chamber might form hots spots and detonation.

Net difference in radius is 2mm (0.078")

MexicoMike said:

Most wear occurs at the top inch of the bore, which is why LD tests are done at TDC.
Further, as noted in an earlier post Leakdown and Comp tests are best performed when the engine is warm - as close to operating temp as possible.

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Back in the day, we would perform leak down tests as a routine on airplane engines. They were brought up to temp and checked at 80 psi at TDC. They were plain bearing engines, which meant they could be coerced to "stick" at TDC once pressure was applied. You better make damn sure that's the case, as a six foot propeller with 80 psi in the bore makes for exciting times once it breaks away!

MexicoMike said:

FWIW re leakdown tests/bore wear based on many years of doing them: Most wear occurs at the top inch of the bore, which is why LD tests are done at TDC. The wear is easy to observe - even without a micrometer. Take a piston ring and insert it square with the bore (push it down just below the ridge from above with the top of a piston) so it is approx 1/2 below the top of the bore or just below the ridge if there is one. Measure the gap of the ring with a feeler gauge. Then, using the piston, push the ring several inches down and measure again. You will find a narrower gap (less wear). Bores always taper with wear, with the largest internal radius (most wear) at the top.

Further, as noted in an earlier post Leakdown and Comp tests are best performed when the engine is warm - as close to operating temp as possible. This is because the shape of the cylinder bore cold is not the same as it is when hot. IOW, you could have great sealing when cold and poor sealing when hot due to the fact that the bore does not expand evenly with heat. Ideally ou want to know how the engine seals under normal operating conditions, NOT how it seals sitting in the garage ! This is one of the major reasons that two identical engine often produce more/less power. One engine has "rounder" bores when hot than the other, even though both have equally round bores when cold. When building performance/ competition engines we often specifically looked for engine blocks that had more material on the thrust side of the bore than on the opposite side. Those engines always produced more power because when they were hot the bores stayed "rounder." Nowadays, with better casting techniques, there is (probably) not as much random casting offset as there used to be.

Click to expand...

That's a fantastic amount of real world knowledge, many thanks

I don't think I'll get a chance before tomorrow to do a 'hot' leak down test, if the weather is OK my plan is to fumigate the entire island of Ireland, well a small bit of it. I'm taking the bike on a 2 hour journey to see if there's any change in the smokey situation, highly unlikely!

I'll then remove the head & have a look at the barrels/pistons/rings etc.

Dances with Shrapnel said:

Gasket for a 77mm bore being suitable for a 81mm bore? I am assuming Norvil would know.

If the 77mm gasket edge hangs into the bore I would not be too keen about it size little bits hanging into the combustion chamber might form hots spots and detonation.

Net difference in radius is 2mm (0.078")

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As I said there was a composite gasket on the bike originally when I got it & it seemed to work fine. You make a good point about the original 850 bore being 77mm and their 920 conversion being 81mm, I don't have an answer other than the reply to my email.

When I have the engine stripped I'll measure everything up & get a composite gasket & see how it fits!!

As noted, if you get the engine precisely at TDC, it won't rotate when the air pressure is applied. But, of course, it's best to lock it down somehow. CAUTION, for anyone who thinks you can keep it from moving by hand-holding a breaker bar/socket on the flywheel bolt when pressure is applied, you can get a very painful lesson unless you are using VERY low (and not useful) pressure.

Frankly, click, I wouldn't worry about doing a warm test - clearly there is a problem and the head/barrels are going to have to come off to find/fix it.

MexicoMike said:

FWIW re leakdown tests/bore wear based on many years of doing them: Most wear occurs at the top inch of the bore, which is why LD tests are done at TDC. The wear is easy to observe - even without a micrometer. Take a piston ring and insert it square with the bore (push it down just below the ridge from above with the top of a piston) so it is approx 1/2 below the top of the bore or just below the ridge if there is one. Measure the gap of the ring with a feeler gauge. Then, using the piston, push the ring several inches down and measure again. You will find a narrower gap (less wear). Bores always taper with wear, with the largest internal radius (most wear) at the top.

Further, as noted in an earlier post Leakdown and Comp tests are best performed when the engine is warm - as close to operating temp as possible. This is because the shape of the cylinder bore cold is not the same as it is when hot. IOW, you could have great sealing when cold and poor sealing when hot due to the fact that the bore does not expand evenly with heat. Ideally ou want to know how the engine seals under normal operating conditions, NOT how it seals sitting in the garage ! This is one of the major reasons that two identical engine often produce more/less power. One engine has "rounder" bores when hot than the other, even though both have equally round bores when cold. When building performance/ competition engines we often specifically looked for engine blocks that had more material on the thrust side of the bore than on the opposite side. Those engines always produced more power because when they were hot the bores stayed "rounder." Nowadays, with better casting techniques, there is (probably) not as much random casting offset as there used to be.

Click to expand...

Something you said struck me. If the roundness of the bore can change when it gets to operating temperature, could you bore the cylinders when they were hot? At startup they might not be round, but when they got hot they then might be. The boring could take into account the expansion due to heat so they wouldn't be too tight when cold. By the way, my vote is for just taking off the cylinders and having a good look. But all this speculation is interesting to say the least and I for one enjoy it. Lot's of good ideas and practical knowledge get into print this way.

If you are going to take it out for a long burn up, don`t forget to take plenty of spare oil with you. I`ll shut the windows now!

Al.

Found some shots from 2012 (6 years ago not 10!!) when I took the head off for the first time, some shots of the composite gasket

Agree a real valid leak down test must be moved pass TDC so is dangerous to Commando and mechanic. Already know rings seal great. Must open up regardless so quit delaying but might order new gasket now. Possible to yellow orange over heat copper gasket to seal a while afterwards but once leaking [if detected] that weak poor sealing area will still be weak area too soon again. If annealing as soft as I do must be careful handling gasket as even the holes will distort is only place supported.

I liked composite gaskets best till a decade+ ago seems to change from robust steel lips to mere folded foil lips to have it blow out through oil drain hole thin area jetting directly into factory fuel lines but got stopped before cut through burning everything up.

Far out, not much room between bolts and liner on a 920. Thanks for sharing the photos. I must say, at your expense click, that im enjoying reading this post and fingers crossed you work it out.

Joe Schlaberdowski said:

Something you said struck me. If the roundness of the bore can change when it gets to operating temperature, could you bore the cylinders when they were hot? At startup they might not be round, but when they got hot they then might be. The boring could take into account the expansion due to heat so they wouldn't be too tight when cold. By the way, my vote is for just taking off the cylinders and having a good look. But all this speculation is interesting to say the least and I for one enjoy it. Lot's of good ideas and practical knowledge get into print this way.

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Bolting the cylinder head on after boring also distorts the bore!

For "classic" engines, heating the block for boring/honing does reduce the distortion. The problem, of course is that' it was not practical on a production basis. But companies that specialize in such work routinely heat the block with hot water to bring the block to something like operating temp and attach torque plates (to simulate the stress of the cylinder head bolts) before boring/honing.

I use the term "classic engines" because I'm TOLD ( I have no actual knowledge) that modern engine manufacturing takes this in to account. But I don't know what, exactly that includes.

Connie73 said:

Far out, not much room between bolts and liner on a 920. Thanks for sharing the photos. I must say, at your expense click, that im enjoying reading this post and fingers crossed you work it out.

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I'm actually enjoying the process myself, the knowledge I've gained from this thread alone is amazing

Based on the 6 year old pictures it doesn't look like your head gasket was doing you any favors. It looks like oil was getting under the head gasket in several places and the piston tops seem over coked. Wiser posters may have more informed views, but I'd be inclined to check the deck for flatness, same with the head.

Just looking at the 6 year old picture and the more recent one on a previous page. both pistons have very little carbon around the circumference, they are washed almost clean on the thrust side, the right side does appear to be worse. This type of oil washing is caused by oil control ring failure. Its time to pull the barrels and measure, hopefully all that will be needed is a good glaze breaking and new ring set. Just as a note if you go with the hastings ring set the oil control ring is a tight fit, this is normal so don't trim it to fit whatever you do or things will be worse.

I had similar oil burning on the left side with mine last year, new rings took care of it. I have a composite head gasket which I prefer as they tend to seal better.

Craig

Clear areas around tunnels and the dark areas on gasket are oil path weep evidence for source of coke top pistons. Piston crown bare area near ext valves indicates a good chamber swirl burn pattern focusing most combustion heat near ext valve. The coked area are more stagnate near TDC but normal is pretty clean as that area the intake valve blast strikes.

It's interesting what your educated eyes see, great information.

There is no doubt in my mind that the engine, head & barrels, must come apart, only way of knowing for sure.

If I can, I'm still going to take the bike out for a spin, not a thrash!, before I pull the Norton off the road.

From what you can see, it sounds like I've always had issues with the oil control part of the pistons. I know I got the bike 500 miles after the 920 conversion was done. Maybe it was given a very gentle run in & the oil control rings never bed in and/or a glaze built up on the bore. The only way I'll know is to strip & take plenty of pics and seek opinions from the Norton experts on this forum.

I know what I'm doing over the weekend!!!

I have removed barrels before on my Triumph T100, anything I should watch out for when taking the Norton barrels off??

I am building a 920 and I am a huge fan of composite gaskets as I’ve only ever used them (only from AN) and have never had any head gasket leak or failure.

However...

920 bore is 2mm bigger around the entire circumference. Looking at a gasket in my shed and at your pics, surely that means that a very large potion of the metal sealing part of the gasket would be protruding into the combustion chamber.

This means that a) it ain’t doing it’s job of sealing and absorbing the clamping force of the torqued head bolts and will surely therefore be weaker, and b) that thin metal protruding into the combustion chamber might get really hot, and lead to pre ignition or similar.

There are benefits to copper though:

A benefit to copper, especially on a higher CR engine, is that copper head gaskets may have a greater tendency for minor oil weeps, but the seldom ‘blow’ completely. Composite gaskets are less inclined to leaks, but IF they fail, they are more likely to ‘blow’ completely.

Another bennefit I see with copper is that it’s a more stable material and therefore easier to be more precise and confident when aiming for a tight squish. Of course, this is only of bennefit to those who are actually aiming for a tight squish!

So, if someone made a proper, proven, 920 composite gasket, I’d probably try one, but on balance, IMHO, I would NOT use an 850 composite gasket on a 920 motor.

Ref removing the barrels: one difference to a Triumph motor is the cast in pushrod tunnels, because they’re part of the barrel, any debri that falls into them (with barrels in situ) won’t just fall out of the bottom! My Commando had severely scored cam followers and tunnels that I believe must have been caused by a P.O. scraping off carbon / bits of gasket and allowing them to fall into the tunnel and turn into grinding grit! So a) take care and b) take the opportunity to properly clean out the tunnels whilst your barrels are off.

Finally, for heavens sake, DO keep us all posted as to your findings !

Speaking of oil in the RH combustion chamber: A friend of mine recently posted a CT scan of an 850 cyl head on a German forum showing a distance of less than two mm between the RH intake port and the oil drain bore. I had an oily RH spark plug on my Atlas (ports enlarged to 30mm) for quite a long time before I actually found out that the reason was a tiny little leakage caused by porosity in exactly this area.


Tim

Funny how things work out. I had a two hour window today, got home, prepped the bike, bit more oil than usual, checked fuel, checked tyre pressures, retightened exhause nuts, cold weather gear on, wheel bike out . . . . . . . . won't start

Checked plugs, no spark It's been starting on the button on the bench no problems.

I take this as a sign from the Norton gods!, just strip the engine & get on with it!!!

I'll hopefully get a start today & take the head off but first I want to trace this electrical issue, always something