piston to cylinder clearance (2017)

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There are two things at play. As you go higher, the air pressure becomes less so the mixture becomes richer, but the air temperature drops, so the mixture becomes leaner as the air density increases. The leaning-off effect is very pronounced. If the motor is already carburetted very lean, the combustion can become detonation. The simple fact is that the change in the cooling of the barrels is slow when the ambient temperature drops or rises, but the combustion temperature rise is almost instantaneous when the ambient temperature drops. So you get the seizure when the pistons expand too much.

QUOTE]

I can buy this, although I can't really comment on ambient temp drop and with it an increase in combustion temp causing rapid piston expansion alone as being the main cause of the pistons becoming too big for their bore. To prove this to myself I guess I would have had to ride the same bike on a 35°F from the start and see if it seized. My point is, as a road bike an engine should be built to specs that will suffice under all likely earthly riding conditions.
 
I assume you are saying that the actual factory spec for 850 bores is 3.0315 - 3.0325", not the 3.030" is the service manuals. That would make a lot of sense. Could you explain where you found those numbers? I couldn't find anything except 3.030" in any of the factory documents I have available, but it's a pretty limited collection.

Ken

Out of a copied 1973 Norton Villiers commando workshop manual in the technical data, you must pay attention to the fact in the manual that that cylinders are sized in 2 grades A 3.O315 -3.O32, B 3.032- 3.0325 So the smaller sized pistons would have gone in A and larger in B. Pistons had a tolerance of 0.0009'' nearly one thou.
0.002 was pretty much std for iron sleeved air cooled alloy cylinders although I believe now it has closed right down
 
This looks like where I got my initial brainstorm to build my engine with only .0035" clearance. Quick math using simple subtraction combinations looks as though the acceptable range to be .0035" - .0055". Misleading at best,



Yes at a quick glance it looks like you say but as they had 2 barrel size grades they would have matched small piston in small bores big in big bores, probably keeping it at 0.004- 0.0045 and then any thing heading to a hot country probably got small in big
 
Would it have been too hard to just specify a clear spec for something as important as piston clearance ? :rolleyes: After all, seems like every British Workshop Manual devotes two pages on how to put a tire on a rim!
 
Gents, I think we’re giving Norton far too much credit here. I do not believe they would have matched low pistons to high barrels for hot markets!

They’d have matched low pistons to low barrels and high pistons to high barrels and that’s it!

And that leaves lots of scope for those pistons and barrels at the edge of each tolerance to be within spec, but far from ideal.

And that’s probably why some bikes seized, some smoked, and some were fine !

But they were all ‘in spec’...!
 
Don't know that they went so far to have different piston clearances for warm climates but some British makes did vary compression ratio for fuel quality at the sales destination.
My 47 Rapide was sent out new to Malta at a lowly 5.9 to one! The fuel available in Malta at that time was very low quality.
Home market Shadows were 7.3 , US got 8 to one as the fuel quality was better than in the UK.
BSA Super Rockets were higher compression for the WestCoast US than East Coast US because the West Coast dealer, Hap Alzina, said " We have premium fuel, give us high Compression"

So there was some thought put into where the bikes were going.
Whether or not Norton in the 1970s was that thoughtful about warm climates is a good question.
Looking at the way the company was lurching along back then ....it's doubtful.

Glen
 
As you say all within spec, probably had 1/4 thou tolerance and that would have been fine, until you run out of one size and used up what ever was left over.
We know to well that Norton refused to use different comp pistons, we will machine the cylinder head instead,
They must have had a fair idea where machines were going, good way to get rid of the extra oversize bores and very small pistons AUSTRALIA
 
As you say all within spec, probably had 1/4 thou tolerance and that would have been fine, until you run out of one size and used up what ever was left over.
We know to well that Norton refused to use different comp pistons, we will machine the cylinder head instead,
They must have had a fair idea where machines were going, good way to get rid of the extra oversize bores and very small pistons AUSTRALIA

Norton used split skirt pistons which would presumably require a different clearence from the solid skirt ones they changed to... the late John Hudson
advised owners to change , apparently some of the split skirts did just that..
 
Norton used split skirt pistons which would presumably require a different clearence from the solid skirt ones they changed to... the late John Hudson
advised owners to change , apparently some of the split skirts did just that..

But they never used the split skirt design on the 850s, only earlier 750s. And the factory gave the same clearance specs for split and solid skirt 750 pistons.

Ken
 
Agreed about the 850s, with the 750s I think there may have been differences in the clearences they actually used . Took the head of mine after about 2500 miles and was amazed to note the bores did not even look remotely run in.. The notorious split skirts , still happily intact
 
There are two things at play. As you go higher, the air pressure becomes less so the mixture becomes richer, but the air temperature drops, so the mixture becomes leaner as the air density increases. The leaning-off effect is very pronounced. If the motor is already carburetted very lean, the combustion can become detonation. The simple fact is that the change in the cooling of the barrels is slow when the ambient temperature drops or rises, but the combustion temperature rise is almost instantaneous when the ambient temperature drops. So you get the seizure when the pistons expand too much. It is very noticeable at race meetings. Many guys jet their bikes too rich and on very cold mornings, their bikes become faster. Anyone whose bike is jetted lean has the problem that the bike becomes a pig to ride. With an air-cooled two-stroke race bike, really cold weather can spell disaster.
Sorry for being late to this thread. I just can't let the misinformation in Al's 2nd sentence pass. Al, I suspect that you actually understand how all this works. However you mis stated how. Air temperature drops approximately 2 degrees C [or 3.5 degrees F] for every 1,000 feet higher you go up [above sea level] in the atmosphere. This rate varies, due to variations in humidity & temperature. See

Standard Lapse Rate

The "mixture" does NOT get leaner with altitude, the air gets less dense. That means less oxygen, so the mixture gets richer at higher altitudes, on an engine with a fixed jet carburetor. Cooler temperatures decrease, not increase the density altitude. These factors are why piston powered gas aircraft all have a mixture control, as well as a throttle control. Pilots have to lean the mixture manually, as their aircraft gain altitude. If Al's statement above was correct, then every aircraft engine operating manual printed in the past 100 years is wrong! General rule of thumb is that you will lose about 3% power for every 1,000 feet you climb above sea level, due to thinner [less dense] air.

The Fuel / Air Mixture

You are correct that temperature change also affects fuel mixture. Colder air is more dense, hence more oxygen in the same volume of air. Carburetors with fixed jets are only optimized for one set of atmospheric conditions. As Al states above, an engine jetted on the ragged edge of being to lean, will be critically lean if the temperature drops significantly. Modern electronically fuel injected engines compensate for changes of air temperature, via the Inlet air temperature sensor aka IAT sensor. Changes in altitude & humidity are compensated for via the mass air pressure aka MAP] sensor , which reads barometric pressure. This is referred to as a speed/density system. Another fuel injection method is The Mass Air Flow [aka MAF] system. MAF systems directly measure the density of the air entering the engine's throttle body via the mass air flow aka MAF sensor. MAF systems have the advantage of being able to compensate for engine modifications, such as changes in exhaust system or camshaft.
Because of these issues, Amal and other manufacturers jet their products a tad rich, to compensate for these variations. Comnoz [Jim C] lives at 4,700 feet elevation. He lives about 90 miles south east of my sister. So I know that his motorcycle travels can take him from 4,000 to 14, 000 feet just in the local area. I suspect that is why his Norton has been converted to electronic fuel injection. Those 920cc also help compensate for the loss of power at those altitudes!
Last time I visited my sister, she complained to me how the local John Deere dealer could not fix her Gator [4 wheel ATV]. The machine would often stall at idle, once warmed up. I fixed it in 3 minutes by adjusting the carburetor's fuel mixture screw. The problem is that my sister's ranch is at 9,500 feet. The dealer is 2,500 feet lower. He adjusted it to run properly at the altitude of his shop. To run properly, the mixture had to be adjusted leaner at it's altitude of operation.
Back to the actual subject of this thread, driving in hilly or mountainous terrain increases the load on the engine. This creates more heat and expansion of the piston skirts. So, .003" piston clearance may work well on an engine that is carefully broken in and only operated in "the flat lands" like me in Florida. It might not work out for someone operating in hills or mountains. Personally, I'll err on the side of caution and use more clearance. Never know where I might travel with it in the future. ;)
 
As mentioned earlier in this thread. .0045" for cast pistons and .0055" for forged pistons (2618 alloy).

But there is one more variable - .0045" for 4032 alloy pistons.

piston to cylinder clearance (2017)
 
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I assume you are saying that the actual factory spec for 850 bores is 3.0315 - 3.0325", not the 3.030" is the service manuals. That would make a lot of sense. Could you explain where you found those numbers? I couldn't find anything except 3.030" in any of the factory documents I have available, but it's a pretty limited collection.

Ken
Ken,
I don't know where he got those dimensions. However, if Norton actually did size the bore by metric standards, 77mm = 3.031487. Close enough that I'd call it 3.0315
 
Just looked at this thread again, and realized I never explained what was wrong with the 850 numbers in my posts. Sorry if I mislead anyone. It was a really stupid mistake on my part. I recently went back to the service manuals, and discovered that I had been looking at the specs given for the engine bore, 3.030", not the specs given a page later for the "engine block" bore, which were the correct figures. I cut and pasted the relevant sections from the MK3 service manual below to show what I mean. If read correctly, the nominal clearance is .0045", not the .0020" - .0029" in my posts. I'm feeling pretty embarrassed about it at this point. I certainly never used less than .0045" clearance for an 850, but it's been decades since I last looked at the factory specs to decide what clearances to use. I'm sure I looked at the correct specs back then. When I looked at them recently to post an answer for this thread, I screwed up. I still find it hard to believe I did that. I certainly knew that Norton wouldn't actually use such a tight clearance, and I should have not posted it until I sorted out what had happened. It seems unbelievable that I could just think that it must be a mistake that had carried through in all the 850 service manuals. But that's what I did. Just more proof that its not just our bodies that degenerate with age.:(

850 Piston Clearances.JPG


Ken
 
Great thread.
Has anyone got specs on the .040 over JCC/Emgo pistons, the actual size, or what size bore was made to fit them?
I pulled apart my 73 850 it has .040 over pistons the bore is probably OK but the pistons are junk. Would be nice if I can hone and reuse the bore BUT I need an idea of how the clearance is going to work out, if what I have is too big I'll just bite the bullet and go to .060 over pistons and rebore.
Also kind of wondering if there are any reliability issues bored out .060 over?
Thanks for any help.
P.S. looking for a stock or mild cam.
 
Post #10
I don'y think the bore size matters ?

 
Pre-Evolution Harley Davidson cast pistons had a steel strut to reduce skirt expansion. The spec was .001"-.002". I always set them up at .0015" clearance and avoided prolonged high rev operation for the first 1,500 miles or so. Engine ring life was excellent.

I wonder why Norton didn't pursue something like that?
 
Have cylinder down to local engine shop, he's going to do a light clean up hone, have a fresh set of .040 over on order.
When he looked at my "loose in the bore" .040 pistons he said seizing, over heating like that will "shrink the pistons"
Head is headed to Jim's shop for a full monty.
 
Pre-Evolution Harley Davidson cast pistons had a steel strut to reduce skirt expansion. The spec was .001"-.002". I always set them up at .0015" clearance and avoided prolonged high rev operation for the first 1,500 miles or so. Engine ring life was excellent.

I wonder why Norton didn't pursue something like that?
Because of the danger of these fancy pistons falling to pieces?
 
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