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E: Engine Piston Clearances



erik

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VOC Member
I start to rebuilt my rapide and two cp Pistons should be used.Terry Prince recomends a Piston clearence between o,oo25 and o,oo35 inches. this is about 0,08 to 0,09 Millimeters. other sources saying 0,05 and up to 0,15 mm. what should i do?Erik
 

Oldhaven

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VOC Member
This discussion addresses your question to some extent:

 

Albervin

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VOC Member
I have a pair of CP pistons (7.5:1) with 4 thou clearance. No oil burning and no problems after 20,000 miles. Just like the balance between rich and lean fuel mixture I always go for safety. In Australia, where it can be very hot and the fuel can be suspect I thought I would rather buy more oil than pistons.
 
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MartynG

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VOC Member
I support Alyns comments. With the generally warmer weather in Australia my Comet with a freshly fitted CP pistons (8.5:1) and a clearance of 0.003" as recommended by CP, the motor tended to 'nip' up.

Go for safety and give 1 to 1 1/2 tho more clearance than specified by the piston maker - remember piston makers these days assume their products are going into liquid cooled motors that have less thermal expansion/contraction than our air/oil cooled motors - thus the need for us to have a larger piston clearance.

In my motor I increased the clearance to .005", which required a re-hone of the barrel, and no further problems.
 

greg brillus

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More critical on a single that is working harder, most all twins with either Cp's or Omega's I give them about 3.5 thou no less. just as important are the ignition timing and fuel mixtures. Less full advance and slightly more mixture for our modern faster burning fuels is the general rule now.
 

vibrac

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VOC Member
I would go with marting's suggestion I always ran the racers with 6 thou. Back when these bikes were used daily vastly wider clearance was not uncommon through wear and closer staring clearance gave longer life and people were accustomed to running in an engine. When did you last see "running in please pass" on a tin box?
 

erik

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I live here in Germany in the wine area Rheingau and in summertime rit can be very hot up to 40 degrees Celsius . so may be 5 thou ,0,127 mm would be a good choyce for a twin ?
 

oexing

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VOC Member
Erik, that is way too much clearance. That number would be OK for a big piston of 85 mm in an ALL cast iron cylinder with less heat expansion. Greg´s number of max. 3.5 thou is about right. You can compare a Vincent cylinder with that of a Yamaha SR 500 or XT, same size of piston, shrunk in cast iron liner, reaching way down into the case and not much more oil flow. The SR oil pump is small, all roller crank, no oil jet for the bore, so no factor here. They go for 3 thou clearance. Same story with two valve /5 or / 6 series BMWs , iron liner in alu cylinder.
I am running 5.5 thou clearance in the iron block 3.8 Jag E-type with that sort of pistons so, no, 5 thou is ridiculous. You just have to see the very different conditions in iron or alu engines while looking at the same types of pistons without any extra features like slots or cast-in steel inserts for expansion control.
All statements are to be seen on a basis of correct engine work, meaning perfect geometric honing process and no distortion of the liner in poorly fitting , too tight engine case.

Vic
 

vibrac

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Erik I bet you wished you never asked the question:D
get really confused use the search button and read decades of advice....:eek:
 

davidd

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VOC Member
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
 
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Albervin

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VOC Member
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.
 

oexing

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VOC Member
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
25968

25969

Horex 600 , 4 valve, 1935

25970
 


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