piston to cylinder clearance (2017)

acotrel said:

I would have thought that if you replaced cast pistons with forged ones without greatly changing the bore size, you could run with less clearance . Cast pistons are more likely to change metallurgically in use and are usually thicker in the skirts than forged ones. Does the thinner skirt on a forged piston cause more distortion when subjected to heat ? Personally, I never worry about bore clearance as long as there is enough. The rings do the work of keeping compression up, not the bore clearance. And who ever heard piston slap in a long stroke motor such as a Commando engine ?

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Fast Eddie said:

Yes, the CR is lower, which would lower the heat being generated, all other things being equal. But they’re not equal, there’s an extra 80 ish cc being burned in the same cylinder head design, which is surely gonna generate more heat?

Maybe the raised crown on the 750 piston is thicker? Leading to great expansion?

I’m shooting in the dark here too Nater !

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Temperatures are higher with higher compression, not because you are burning more fuel because of the difference in cylinder capacity. With an 850 the combustion chamber must be relatively larger than in a 750. If you raise the comp. ratio in an 850 which has minimum bore clearance - ? ? ? - you use more fuel to keep the combustion temperature sane. It stops detonation and the effect of expanding the pistons too much - stops seizures. Increasing the thickness of the piston crown would only delay expansion for a short time. Once it has reached temperature there would be no difference in the dimension it reached .
When you burn a fuel, the temperature you reach is determined by the heat losses and the compression of the process, not usually by the amount of fuel you burn. In a motor, the process is almost adiabatic. Compared with the heat that goes out the exhaust, what is taken away by the oil and the surrounding air is almost negligible. Piston clearance is a balance, if you use methanol fuel you need much less, but if there is too much clearance, it usually does not matter - unless the bike is a Manx where pistons cost an arm and a leg. My A-grade rider friend used to warm his up very carefully - never start the bike and start revving it straight away, if your pistons are loose. My bike has both - loose pistons and methanol fuel. My feeling is that a long stroke motor is always faster if the pistons are not tight in the bores. But not with two strokes, where the reverse is true - because of ring flutter.

The trouble with any of this stuff is that, if you are chasing performance - what you might think is good is not always the case. If I was rebuilding my motor, I would probably fit the pistons with minimum clearance, then find the motor is slower until it started to wear. I think that if the pistons became sloppy, forged pistons might last longer before they cracked.

acotrel said:

When you burn a fuel, the temperature you reach is determined by the heat losses and the compression of the process, not usually by the amount of fuel you burn.

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Are you really suggesting that a larger cc engine, burning correspondingly more fuel than a smaller cc engine, with all else (inc heat loss abilities) being equal, does NOT create more heat?

Bigger fire... more heat. We learnt that in the caves Alan...!

Yes the 750 has a higher CR, and I fully agree with you that that will create more heat.

But the extra 80cc of the 850 will also create more heat.

I don’t know which one will create the most.

If anyone on here’s is clever enough / bothered enough to enlighten us, that would be interesting.

There is a difference between heat and temperature. The temperature of the piston determines it's expansion, not the heat generated in the combustion chamber. While the motor is running, it is continually losing heat. It ends up as a balance. If the balance is skewed, the temperature rises or falls. Changng the fuel can change the heat balance. Changing the amount consumed does not change much as far as temperature, some fuels run at much lower temperatures.

Most of the heat which is not used to generate motion is lost through the exhaust. So changing the cam timing and the exhaust system configuration can help the engine run cooler, and be less likely to seize.

But Alan... we're not talking about changing cam timing, or exhausts, or fuel, or using methanol, etc in this thread.

We're talking about a stock 750 and a stock 850 and we're just trying to discuss, and therefore understand, why the stock 850, even though it burns more cc of charge, and has a bigger diameter piston, runs a tighter piston to cylinder clearance than the stock 750.

Going back to the original posters question, in my own notes (should be reliable) taken from a conversation with Norman White when I was at his premises a few years ago, he told me to use a clearance of 0.0040" to 0.0045", on my pistons / bore. The pistons were cast but I can't remember the make. This was on an 850 if it makes any difference.

I have learned the hard way that .0045" works for cast pistons and that .0055" works for forged pistons; there are exceptions, the one that I have personal experience with are the Arias pistons I had made for an R90S; Arias specified .003".

Another aspect of the CR, that hasn't been, yet, brought to light here is the simple fact that when you compress a gas you generate heat, I can't say how significant the difference would be between the 750 and the 850. Anyone who doubts the effect of gas compression generating heat needs only put a finger on a fully functioning air compressor, no combustion, just compression; they got very hot.

It's not unusual to find out a thread is more interesting than first glance, and this one doesn't disappoint. I'd be curious if the story that Biscuit tells about seizing his pistons going over the pass had anything to do with altitude change, and leaner air. I'm not sure how much altitude change it really takes to start affecting the fuel mixture and piston temp. I know that me messing around with the settings on my carbs can have a big impact on it.

Higher altitude should result in a richer mixture.

The CR difference isn't much, listed most places as 8.9 to one for 750 and 8.5 to one for 850. Can't see that reducing piston skirt gap requirement, especially with the extra cc effect mentioned.
If the piston type changed to a more modern hypereutectic low expansion type for the 850, that change might be the reason for the reduction.

Glen

std 850 3.0315 -3.0325 bore
piston 3.028 -3.0271
and the bore is through 2 grading sizes min clearance from factory . 004''
so you have a page of crap about compression ratio when a error in specs was actually at fault

splatt said:

std 850 3.0315 -3.0325 bore
piston 3.028 -3.0271
and the bore is through 2 grading sizes min clearance from factory . 004''
so you have a page of crap about compression ratio when a error in specs was actually at fault

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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, I would say. I don't think forged vs cast pistons have a appreciable effect on the as built initially clearances as the fact that the aluminum pistons run inside cast iron barrels. We all know the rate of expansion between the two materials is quite different and being air cooled, a Norton engine's piston clearance will vary with ambient temp. I'm thinking that a RUNNING & UP TO TEMP engine is running a clearance of maybe .0015" - .002". The initial installed 'loose' fitting is needed to allow the rapidly expanding alloy pistons to have the 'correct' clearance as they do their thing inside the much slower expanding iron barrels.

Triumph three cylinder Trident engines have aluminum pistons running in steel sleeved aluminum barrels. The initial assembled piston clearance is just .002". Still an air cooled engine but because pistons and barrels are aluminum' expansion rate is very nearly the same. Assembled clearance and hot running clearance remains close to the same.

The 'Kanc' starts out at 680' and the summit is around 2800'.
https://www.google.com/search?rlz=1...0.5.310...0j38j0i67k1j0i22i30k1.0.7RmwUqqt1A0

splatt said:

std 850 3.0315 -3.0325 bore
piston 3.028 -3.0271
and the bore is through 2 grading sizes min clearance from factory . 004''
so you have a page of crap about compression ratio when a error in specs was actually at fault

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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

There's been a lot of argument in some of the car forums about why you need (or don't need) more clearance for forged pistons than cast, with all sorts of info on different alloys, the effect of design features, detailed drawings of piston temperature distribution, and on, and on. Lots of different opinions. But to answer the original question here, you can't go wrong using .0045" clearance with stock cast pistons in an 850 street bike.

Ken

rvich said:

It's not unusual to find out a thread is more interesting than first glance, and this one doesn't disappoint. I'd be curious if the story that Biscuit tells about seizing his pistons going over the pass had anything to do with altitude change, and leaner air. I'm not sure how much altitude change it really takes to start affecting the fuel mixture and piston temp. I know that me messing around with the settings on my carbs can have a big impact on it.

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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. 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.

Omega suggest 3thou for their forged pistons .. seems very tight to me

acotrel said:

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.

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I need to go back to chemistry class! Not sure why I was thinking oxygen molecules being fewer creating leaner mix, but I was. So carry on. I've heard other stories of seized pistons on hill climbs, so it isn't a singular event regardless of the series of events leading to it. I'll go find my pointy hat and stool in my usual corner now. (Actually this is what I get for reading this forum in the middle of the night when I can't sleep and post something during the "tweeting hours"!)

The change to the lower piston clearnace for 850 could be due to a new piston manufacturer at the time using a new LowEx Alloy. Current forged pistons made from the modern LowEx, not 70's spec piston alloys may only need a 0.003'' piston clearance, even with an aircooled cast iron barrel.

The 850 had a lower CR, why we don't really know, but this maybe due to the fact they needed to have oversizes, the change in CR for a corresponding 750 oversize piston will obviously not be the same as an 850. The rate of change is considerable and at the top oversize quite a difference, but with modern fuel not really an issue these days as many engines now run high CR to achieve emission specs, just need a bigger leg to kick it.

Since the 60's Norton pistons have always been metric, and the bores also, the manual was printed with the Imp measurements, as that was what customers knew.

Biscuit said:

Triumph three cylinder Trident engines have aluminum pistons running in steel sleeved aluminum barrels. The initial assembled piston clearance is just .002". Still an air cooled engine but because pistons and barrels are aluminum' expansion rate is very nearly the same. Assembled clearance and hot running clearance remains close to the same.

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Triumph three cylinders were a 68mm bore. One would expect less clearance specified for smaller bores but 0.002" is tighter than can be accounted for in bore differences.