Piston to Bore clearance for racing motor

[Gertvr]

Hi,

I am seeking some expert opinions on the proposal below:

I am currently in the process of getting my Nicasil cylinders bored and re-plated for the 3.0 race motor I am building. I have new Wossner pistons which requires the bore at 95.00mm, this will provide .05mm ( 0.00196") clearance as the pistons are 94.95mm.

I have spoken to an engine builder that proposed a bore spec of 95.00mm + .001" for additional clearance due to it being used in a race motor so total piston to bore clearance (0.00296"). Is this standard practice for a race motor or are there problems with specifying an additional clearance over the manufacturers spec that will cause problems down the line

Thanks for the help

Cheers

Gert

[Flieger]

I am no expert and do not have any race engine experience but...

Increasing the clearance to the "loose side" of spec (very slightly and still within spec) should decrease friction. This frees up more horsepower, like accelerated breaking in of an engine. For race engines, the decreased running life should not be as much of an issue due to the more frequent rebuilds necessary due other consequences of their hard lives.

[Steve@Rennsport]

Gert,

Everyone has an opinion so I can only offer mine,....

Wossner pistons, if I remember, are Alcoa 2618 and need more clearance than .00196". Precise clearances are determined by the design and shape of the piston as well as top ring land dimensions.

I've not used Wossner's so I cannot offer specific suggestions, however I would not run them tighter than .0024", especially in a race engine.

We do runs some 2618 CP's, but these are made to our specification to permit running tolerances around .0019" or less.

[Gertvr]

Steve et al,

Thanks for your opinion..... , you are right, they use 2618-T6. I got hold of the manufacturer's local tech rep in the mean time to get his opinion on the matter as well. He did not have a problem with the additional 0.001" but indicated that i will have an additional .003" of ring end gap, increased engine noise and wear due to the increased piston clearance until things have warmed up to operating temperature, basically babying the motor until that time.

His opinion was that 0.0005" additional would be ideal which puts the total clearance to 0.00246" close to what you have as a minimum.

I am not sure how noticeable the increase in ring end gap will be on the final outcome.

Cheers

Gert