E: Engine Piston Clearances
- Thread starter erik
- Start date
Erik,
I think .0025-.0035" is to small for a standard Vincent with a low expansion piston. That number would work well for a Vincent with an aluminum Nicasil liner and a low expansion piston. This type of liner sheds heat readily and there is little chance of oil migration between the cylinder and muff.
An older high expansion piston would need more than .004" clearance. Vincent tried the .004" figure and had a rash of seizures reported by dealers, particularly in the US.
Due to the design of the stock Vincent, overheating of the barrels is not uncommon. For the barrels, the liner and muff allow oil migration in between the two and this creates a layer of carbon that inhibits the free flow of heat to the muff. Additionally, the cylinder sits very low in the crankcase compared to other motorcycle designs. This means that a good portion of the cylinder is not exposed to air flow. These factors tend to make for uneven heating of the cylinders. They can get hot spots.
This uneven heating is a much more serious problem on the Comet motors because they cannot share the load with another cylinder. Thus, they need a little more clearance than a twin to deal with rapid spot heating that cannot be air cooled quick enough. So, for a Nicasil aluminum cylinder, .0015 is recommended and I use .0035 on my single racer, as an example. If I were running it on the street I could go lower, but I do not see any advantage to that because I get years of wear and good compression with the higher clearance. If the compression goes down, I put new rings in, like everyone should.
If you are not running air cleaners, you will need to renew the top end in 25,000 miles. I always took that bet and I don't use air cleaners. The stock liners will take a beating, but I found the Nicasil liners will last decades under the same conditions.
In this thread, as in the other threads, no one has listed any problems with running larger clearances, while there are a lot of excellent reasons to avoid running small clearances. I would say that erring on the large clearance side has no penalty.
David
I think .0025-.0035" is to small for a standard Vincent with a low expansion piston. That number would work well for a Vincent with an aluminum Nicasil liner and a low expansion piston. This type of liner sheds heat readily and there is little chance of oil migration between the cylinder and muff.
An older high expansion piston would need more than .004" clearance. Vincent tried the .004" figure and had a rash of seizures reported by dealers, particularly in the US.
Due to the design of the stock Vincent, overheating of the barrels is not uncommon. For the barrels, the liner and muff allow oil migration in between the two and this creates a layer of carbon that inhibits the free flow of heat to the muff. Additionally, the cylinder sits very low in the crankcase compared to other motorcycle designs. This means that a good portion of the cylinder is not exposed to air flow. These factors tend to make for uneven heating of the cylinders. They can get hot spots.
This uneven heating is a much more serious problem on the Comet motors because they cannot share the load with another cylinder. Thus, they need a little more clearance than a twin to deal with rapid spot heating that cannot be air cooled quick enough. So, for a Nicasil aluminum cylinder, .0015 is recommended and I use .0035 on my single racer, as an example. If I were running it on the street I could go lower, but I do not see any advantage to that because I get years of wear and good compression with the higher clearance. If the compression goes down, I put new rings in, like everyone should.
If you are not running air cleaners, you will need to renew the top end in 25,000 miles. I always took that bet and I don't use air cleaners. The stock liners will take a beating, but I found the Nicasil liners will last decades under the same conditions.
In this thread, as in the other threads, no one has listed any problems with running larger clearances, while there are a lot of excellent reasons to avoid running small clearances. I would say that erring on the large clearance side has no penalty.
David
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Well said David. The issue of air space between liner and barrel/muff can be dealt with by a heat conductive "filler" but I would not totally rely on that. I have used nikasil bores on two strokes with 0.0015 clearances but for a four stroke I would not dare. Likewise I would not entertain a clearance of 0.0035 in a Vincent; twin or single.. The fuel is really problematic and so also are carburettors. Unless you can check the air and idle jets you are playing with fire and $$$$$$$$$$$. What Vic is missing is the oil flow. An E-Type oil pump is like a turbo jet compared to a Vincent oil pump, and water cooled.. Heat is dissipated in an air cooled engine by "air" and oil flow. The example of the SR/XR Yamaha is probably more applicable. The Yamaha weighs a good deal less than a Vincent, has a five speed gearbox and has clean air in front of the cylinder. It is OHC design so the oiling is completely different. BMWs have cylinders in open air. Go less than 0.004" at your peril.
Sorry, I cannot see the logic in some arguments: The fact that the liner reaches down a lot does not matter much as that part of the liner is "oil cooled" by the crank case due to the minimum gap that is filled with oil no doubt. And there you´d see less than 100 degrees C my guess, less than at the air cooled part. The Yamaha have same shrunk in liner, see picture, so should have same troubles - but don´t, even though the cylinder is a stressed member unlike the Vincent part. . These have shorter stroke, the chain tunnel one side so no even heat all around but still about 3 thou clearance. The oil flow is only down that chain tunnel from the ohc, not very much with that oil pump to be a factor. Free air flow for this single is not a statement I´d put too loudly as there is a lot of shading from the front mudguard. This may lead to lower cylinder temps for the REAR cylinder on V-twins .
Anyway, I am running 5 thou in my 600 cc ALL iron head and cylinder Horex, same hp like a Comet. You can see the mighty oil pump at the crank position and nobody will deny that cooling for iron engines and heat growth is comparably miserable. So how can one ask for same 5 thou clearance in an all alu engine ?? Where´s the logic ??
As to oil creeping up the liner, stand the cylinder in an oven at min. 150 degrees C for a while . When you find the muff at the bottom, scrap that junk or get an oversized liner. No reason to argue about this, nor about distorted liners or out of round honed items. You cannot give advice on junk parts. So far I´d go with Greg for 3.5 thou safety, he got most of experience here I guess.
Vic
Yamaha vs. Vincent
Horex 600 , 4 valve, 1935
Anyway, I am running 5 thou in my 600 cc ALL iron head and cylinder Horex, same hp like a Comet. You can see the mighty oil pump at the crank position and nobody will deny that cooling for iron engines and heat growth is comparably miserable. So how can one ask for same 5 thou clearance in an all alu engine ?? Where´s the logic ??
As to oil creeping up the liner, stand the cylinder in an oven at min. 150 degrees C for a while . When you find the muff at the bottom, scrap that junk or get an oversized liner. No reason to argue about this, nor about distorted liners or out of round honed items. You cannot give advice on junk parts. So far I´d go with Greg for 3.5 thou safety, he got most of experience here I guess.
Vic
Yamaha vs. Vincent
Horex 600 , 4 valve, 1935
When faced with an academic disagreement (and aside from my stated preference based on my or others experience) it's best to ere on the side that can do the least damage and bearing in mind the reduced mileages nowadays that is self evident go wide clearance