Atlas piston issues (oil fouling)

Inspired by Click's recent (and epic 31 page) thread. https://www.accessnorton.com/NortonCommando/leak-down-test-dont-laugh.24506/

I've got this '63 Atlas I've been working on for a while now. It runs well enough until it oil fouls the right side cylinder in a couple of miles and I can visually see white/grey smoke from the right side exhaust.

Single Monoblock
TriSpark in a points body. Issue does not follow plug leads.
Head just back from the menders. About 120# compression on both sides and that's with original dished pistons.
Rocker flats facing inwards.
Verified oil drain passage clear to open timing chest with some carb cleaner and straw.
Cylinder marked as +.010 and measurements confirm this. Decent crosshatch pattern still visible. No obvious scoring of the bores. I thought it looked "just fine".

I've spent a lot of time chasing this issue. Lots of good practice in removing and installing the head and fitting pushrods. And much like Click, if it's not the head then off with the barrels.

The barrel measures up at 2.886 with a couple of 5's measured top and bottom, front to back and side to side, and the cross hatching is still visible. Ring gap is .019 on the right and .020 on the left. Should be between 14 and 9.

The pistons are another story. These are original dished pistons marked 24282 (left) and 24283 (right). Stamped as +010. However, they are knurled front and back and I don't think they left the factory this way? The pistons measure up at 2.882 front to back and 2.872 side to side.

Honestly, I'm starting to think that someone bored the thing +.010 and knurled the pistons and probably didn't even bother to fit oversized rings. Oh yeah, at some point the pistons had been reversed as there were witness marks in the exhaust pockets.

So really what I'm looking for is advise. Try a re-ring or save the money and spend it on the rebore it really wants.

If rebore and flat top pistons, what have my fellow Atlas owners done regards balance and fitting a compression plate?

This bike's definitely got some interesting stories to tell.

I noticed that you mentioned that the rocker spindle flats were turned inward. Have the original scrolled spindles been replaced by later ones?
If the bore measurements are good, maybe try to find a set of new pistons. Chances are the old ones, with the knurling and worn ring lands, should be put on the shelf. If you have to fit Commando pistons, I would install a compression plate to get the ratio closer to original.

IMHO decompression plates are a joke. you just made a second junction for an oil leak and more difficult to keep the cylinder stable on the crankcase's. i prefer to machine .020 to .030 off the tops of the pistons to lower it a 1/2 point. and NO you will not get it as low as the dished pistons but it should be in the 8.5 to 1 range.

My Atlas recently started pissing raw oil out the left exhaust port. Sent the head to Comnoz who found left exhaust valve guide was leaking. Perhaps this is source of your problem, although I see you state head just rebuilt. Perhaps the rebuilder overlooked the guide. My guide was leaking under the guide seat, not the valve bore.

Pistons did not come from the factory knurled.

New dished pistons are unobtainium. I would replace with new JCC ..... toss the rings supplied with the pistons, and use Hastings. Also use flat circlips rather than wire type supplied by JCC.

I replaced my pistons with JCC 9:1 CR. I considered a de-compression plate but decided against it. I did not do a rebalance, not wanting to split the cases. I do not find anymore vibration than with original pistons, although that is very subjective. Actually, since i got my magneto firing exactly equal on both cylinders, i subjectively feel the vibration is less. Should you decide to use a plate, I can calculate the plate thickness required if you tell me your target CR.

Slick

Thanks Slick, that's pretty much the kind of info I was looking for. Just do the rebore. Think I'll ring up Phil Radford later today and spend some more money.

Still undecided about CR. Mostly just a desire to be able to run on 87 octane and not try and "hop up" the engine. Phil's got a plate listed in his spares list.

On second reflection on your problem, I am withdrawing my suggestion that a leaky seat under the valve guide may be the cause.

In my case, there was no smoke or plug fouling, simply because the oil never got into the combustion chamber ... it simply leaked from the rocker box into the exhaust port.

I have heard firsthand of a case where the right cylinder was smoking. The engine was torn down three times!! Finally an old Norton guru in our area found the rocker box oil drain hole in the cylinder barrels was partially obstructed where it entered the sump. Check that out.

I am running well on 93 octane ethanol free, CR = 9:1 (actually more like 9.3, as Comnoz had to mill my head some), and full advance = 32 degrees. You may very well be ping free on 87 octane, if you set the full advance to 28 degrees.

I would avoid the de-compression plate, and run 93 octane pump fuel if pinging is a problem.

Slick.

BTW ... a shim thickness of 0.029" will drop a CR = 9.0 to 8.5. This agrees with bill (reply #3).

Ooops! re-reading your original post, I see you verified the oil drain hole is clear.

More second thoughts ... rocker spindle flats turned inwards is correct for low pressure feed to the rockers. I assume your top end oil system is still low pressure, or has PO converted to high pressure tap?

20 over pistons on the way from Fair Spares. I asked Phil about a compression plate and he told me not to buy it so I didn't.

This Atlas head is still a low pressure setup.

Not my first Norton motor, but definitely one of the more troublesome ones. If I'd have just popped for the rebore at the same time I had the head done, I'd probably be riding the Ratlas down to the OVM show in Corvallis. Guess I'll just have to ride the Commando.

Keep us informed of your progress.

Slick

With bores still showing cross hatch and a 4 thou clearance ,I can't see how the decision to rebore was arrived at. Rings often wear much more than the grooves ,A set of rings would most likely have done the job. Mind you I have made a few daft decisions myself!!.

norton bob said:

With bores still showing cross hatch and a 4 thou clearance ,I can't see how the decision to rebore was arrived at. Rings often wear much more than the grooves ,A set of rings would most likely have done the job.

Click to expand...

and a light hone!

Peavey Jeltz said:

20 over pistons on the way from Fair Spares. I asked Phil about a compression plate and he told me not to buy it so I didn't.
This Atlas head is still a low pressure setup.

Click to expand...

Pistons are obviously rather worn. The piston-to-bore clearance measures up to 0.014" - initial clearance should be 0.0045" as per workshop manual.

I wonder how much the ring side clearance is? The workshop manual says 0.0015--0.0035 in. Too much ring side clearance and the rings will act as oil pumps to the combustion chamber rather oil scrapers.

Your ring end-gap of 0.020" is far too much. The manual specifies 0.012 -- 0.013 in and a wear limit gap of 0.014".

Your barrel appears to be in good nick. The oiling problem could still be caused by the inlet valve guide leaking, but I would do the pistons and rings first.

-Knut

According to the owners measurements the thrust faces of the pistons have only 4thou clearance in the bore.

norton bob said:

According to the owners measurements the thrust faces of the pistons have only 4thou clearance in the bore.

Click to expand...

I saw that but it doesn't help much when the transverse faces leak like a reversed watershed.

I think I know what you mean,I have seen staining below the rings and pin bores. Look closely at the top ring ,they can wear up to 5 thou without damaging the grooves. Skirt staining can also come from the dirty oil fling from rod drillings. Still ,you have gorn and dunnit now! new pistons. I just hate to shorten the life of scarce barrels if there is another option. And I have short arms and long pockets.

Its the rings that provide the seal, not all pistons even have sides on them below the rings. Just have to wear ear defenders as they do increase piston slap.



From a BSA B44.

mdt-son said:

Pistons are obviously rather worn. The piston-to-bore clearance measures up to 0.014" - initial clearance should be 0.0045" as per workshop manual.

I wonder how much the ring side clearance is? The workshop manual says 0.0015--0.0035 in. Too much ring side clearance and the rings will act as oil pumps to the combustion chamber rather oil scrapers.

Your ring end-gap of 0.020" is far too much. The manual specifies 0.012 -- 0.013 in and a wear limit gap of 0.014".

Your barrel appears to be in good nick. The oiling problem could still be caused by the inlet valve guide leaking, but I would do the pistons and rings first.

-Knut

Click to expand...

As others said, the clearance at the side of the piston is not what you measure and quote.

Also- you cannot depend on new ring gaps matching a 50+ year old manual spec . New ones are quite likely to have a bigger gap and it’s not a problem.

Triton Thrasher said:

As others said, the clearance at the side of the piston is not what you measure and quote.
Also- you cannot depend on new ring gaps matching a 50+ year old manual spec . New ones are quite likely to have a bigger gap and it’s not a problem.

Click to expand...

It's true that pistons wear in the direction transverse to the crankshaft and one would normally expect to find the most wear on the front and rear surfaces. However, pistons are circular when new, aren't they? And combustion gas pressure and heat is equaully dispersed on the piston crown, right? Then, side clearance does matter! Even though the piston rings do the sealing work, it seems likely that gas blowby, oil transportation, and heat conduction will be affected by having too much of a gap. Mechanically, the larger than normal gap may lead to pistons rocking, affecting wear and sealing at other parts of the circumference.

Your second statement puts the reality upside down. AMC wrote this spec to minimize blow-by and oil being pumped to the combustion chamber. Of course, this spec was written for the type of piston rings used at the time. If the same type of rings are used, the factory spec should be maintained. I can't see a good reason not to do so. Allowing a bigger gap likely means the engine will not work at its best. It's not difficult to adapt rings to meet the specification. Remember, the factory made their recommendations based on extensive testing and examinations. For use of different types of rings, the maker's recommendations should be adhered to. Type of engine (air cooled), cylinder bore material and maximum rev will affect their recommendation. As a fitter of non-standard rings, you are on your own basically.

-Knut

mdt-son said:

However, pistons are circular when new, aren't they?

-Knut

Click to expand...

No, they are cam ground or turned to a shape, the shape is not round and not even the same from top to bottom hence why they quote a clearance in one place only. How can side clearance matter when most modern pistons have no sides.

kommando said:

Its the rings that provide the seal, not all pistons even have sides on them below the rings.

Click to expand...

My gap statement was for the ring land. The steady-state temperature at the piston crown is typically 500 'F (260 'C) while bottom of skirt sees 200 'F only (93 'C). Consequentally the piston deforms the most at the top. However, it also deforms elliptically along the circumference due to varying stiffness. Extension /widening) fore/aft is larger than transverse extension.
https://www.shrirampistons.com/structural-analysis-through-FEA.html

Kommando, you are right. I missed one component. The trust force causes the piston to deform elliptically in the direction of the wrist pin. Thus there has to be a bigger clearance here.
So, mechanical forces and thermal load combat each other in terms of piston displacements.
http://blog.wiseco.com/pistons-arent-round-profile-and-ovality-explained

Did you know there is a major and a minor trust force side? The major trust force side is at the aft of our pistons, so there is where the the piston and cylinder bore are expected to wear the most. Apparently the trust force is of substantial magnitude, prompting said mechanical piston deformation.

Learning something new every day

-Knut

mdt-son said:

It's true that pistons wear in the direction transverse to the crankshaft and one would normally expect to find the most wear on the front and rear surfaces. However, pistons are circular when new, aren't they? And combustion gas pressure and heat is equaully dispersed on the piston crown, right? Then, side clearance does matter! Even though the piston rings do the sealing work, it seems likely that gas blowby, oil transportation, and heat conduction will be affected by having too much of a gap. Mechanically, the larger than normal gap may lead to pistons rocking, affecting wear and sealing at other parts of the circumference.

Your second statement puts the reality upside down. AMC wrote this spec to minimize blow-by and oil being pumped to the combustion chamber. Of course, this spec was written for the type of piston rings used at the time. If the same type of rings are used, the factory spec should be maintained. I can't see a good reason not to do so. Allowing a bigger gap likely means the engine will not work at its best. It's not difficult to adapt rings to meet the specification. Remember, the factory made their recommendations based on extensive testing and examinations. For use of different types of rings, the maker's recommendations should be adhered to. Type of engine (air cooled), cylinder bore material and maximum rev will affect their recommendation. As a fitter of non-standard rings, you are on your own basically.

-Knut

Click to expand...

I can’t much be bothered arguing, but if you think cold pistons are round, you know nothing about pistons.

My second statement is right. You wrote a long post full of garbage that you’ve dreamed up.