Piston to bore clearance

[oily mitts]

Looking at my engine and have found I have a nearly 5 thou clearance between bore and piston. This gives me a 17 thou ring gap on the top ring. The bore itself is pretty much as from the shop at 3.190 .

What is standard practice as the engine man thinks about 2 thou clearance on the piston/bore clearance is better, with a 12 thou ring gap.

[tristancliffe]

Is that the smallest gap or the largest? Pistons are very much NOT round at room temperature because of the additional materal around the gudgeon pin expanding a different amount to the 'thin' parts. This varies from piston to piston of course.

But I would say a gap of between 3 and 5 thou on the smallest gap at room temperature (assuming your bores are round at room temp, which they probably will be) would be about right. Set the ring gap as per the manufacturer/designer/piston manufacturer.

As a very general rule of thumb, set the top ring to a minimum end gap of .004 inches times the bore diameter, and a thou or two more for each lower ring. But don't use that figure without checking it with someone who knows your engine.

A race engine is happiest with slightly larger tolerances as you want least friction not most longevity.

[Al Weyman]

Also depends on the piston material always check with your manufacturer. I had this recently I bought a set of pistons from SRP for the small blockchevy, the same company as JE Pistons. Now SRP make a more budget line of forged pistons and in two different materials one expands more than the other so requires more bore to piston clearance (cold). Well I got them back and they had put the wrong clearance slip in with the goods and I nearly had kittens thinking their website was wrong and I had order the wrong kit so in the end I actually had to phone the states where they confirmed the website was correct. In another engine I was building att he same time I had pistons from the other material that had to be about 3 or 4 thou tighter. Forged pistons also expand more than cast or Hypereutectic manufacturers originals which mean the engine can be built tighter with less piston slap noise wen cold.

Finally it also depends on the usage and how long you want between rebuilds. If was me I would contact the piston manufacturer and put the question to them as there are many scenarios.

[Notso Swift]

Bore diameter, piston type, use, Induction (NA/Forced)
You need to answer all of these questions before we have enough info for a reasonable opinion, other wise it is just ****ing in the wind!

The box the pistons came in will generally give a range. Chances are the manufacture knows what they are talking about. stay with in these figures.

[Al Weyman]

Unless they put the wrong slip in the box!!! Also Nitros requires more clearance I believe.

[oily mitts]

The engine is a 1600 kent formula ford, n/a, bore 80.9

[R59]

But who made the pistons, what's their expansion characteristic?

[Gungebucket]

Originally Posted by oily mitts

The engine is a 1600 kent formula ford, n/a, bore 80.9

5 thou is a bit excessive for a Kent FF engine. The pistons are low expansion and standard production piston to bore clearance is 0.0019" to 0.0025. I used to build all the FF's with 3.5 thou clearance. One problem with using larger than production clearances is that you end up with excessive ring gaps. At one time we used to use rings that were I think plus 3 thou which overcame the problem; I doubt that these are still available though. When measuring piston to bore clearance it is best to use a feeler strip between the piston and bore down the thrust or anti-thrust side of the piston. You should ideally get the feeler to move with a force of between 7 and 11 pounds pull. It is quite tricky to get consistent results with a pull gauge. With a bit of experience it is best to just "feel" the piston move in the bore with the feeler strip in place.

Hey Ho... distant memories of building many many engines! I am starting to remember all the little things that make winning engines..... better not rattle on about it here but if you want more info I could always try stirring the grey matter, or dig out some of my build notes.. Good luck.