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Just checking what the experience is with bore clearances for a competition stressed 2000. When i built mine last time round approx 50k ago now i started at 0.0025 no1, 0.0030 no2, 0.0040 no3, 0.0040 no4, 0.0035 no5, 0.0030 no6. Using county pistons. Its been completely reliable although its now showing signs of piston slap when cold. I noted that some guides offer 0.0025 for comp.

The reason for the variation in bore clearances is to avoid partial seizures which i experienced in its original incarnation from a 'specialist' rebuild. Luckily got a full refund and started again. They had applied bore clearances of 0.0016 and 0.0018. Which is certainly quiet but is insufficient to allow for expansion.

What clearances are commonly used with county pistons?

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It depends on which pistons you are using - it is for the piston manufacturer to define really as dependent on the alloy used, these design and whether hard anodised or not there could be a large variation.

IIRC my PI engine (different pistons as 2.5, but were County branded) were between 0.0020" and 0.0025".  No issues with that. 0.0040" seems big for a cast piston in a 75mm bore. Wouldn't normally reckon on a "slack build" for a competition car if it is also road going and can be run in before serious use.  Hill-climber or circuit racer would be a different story.

This interesting
http://www.partinfo.co.uk/files/SB2169%20-%20PISTON%20SKIRT%20CLEARANCE.pdf but no useful numbers.


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Thanks guys. I've decided to go with the same again as after checking out numerous sites the gap recommendations for county pistons range from 0.0025 (supposedly minimum for a road car) through to 0.0060+ for performance applications.

Because my current engine has performed so well I think the variation in bore clearances to take into account the hotter areas of the engine is really worthwhile.



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