Piston to bore clearances

[yorkspv]

Hi chaps, if I were to be building a road engine, I’d be looking at the piston to bore gap being 2.5thou clearance, is this correct? Now, if I’m building a race engine I’d have to increase this to account for heat soak and increased piston expansion? My question is, presuming the above is correct, if I’m building a sprint/hill climb motor, do I stick with the tighter tolerance as the sustained hard work isn’t going to be prolonged? Thanks

[wantafaster1]

The different piston makers specify different clearances in my experience. Omega ask for over 3 thou in my case.
Gut feeling in your case is to err on the wide side.

[bwaminispeed]

I run 4 thou on my street motors........

[yorkspv]

I read too much clearance for the application and the pistons can rock in the bores?

[lomin]

Hi, the manufacturer puts a specified clearance often doing the measuring 1/2" up from the bottom of the skirt. Modern pistons can be as low as 1-1/2 thou, and forged 3 - 3-1/2 thou. Old designs with a cast in steel strut have more , and often reconditioners add half a thou for a performance /race engine. Go by manufacturers spec, they have built it into their design, but remember every thou extra clearance you make adds just over 3 thou to the end gap of the ring. Ring gap is expected to be within 3 - 5 thou per inch of bore size, but if its too large then the efficiency of the piston to bore sealing drops rapidly. Then you get an engine with more sump pressure and doesn't go as it should. Cheers Lindsay

[Spider]

Refer to the Piston Manufacturer's advice regarding Piston to Bore Clearances and where it should be measured. The Data in the workshop manuals only refers to factory pistons of the day.

For light competition stuff that are using a 'street' piston, depending on what it's doing and what piston it is, I add 0.5 to 0.8 thou on top.

[251 ENG]

If you are using a top quality piston like omegas , just make the bore size what they recommend.
Ideally find someone to hone block that can measure bores accurately and produce one that are tight at the bottom.

Forged pistons run bigger clearance, the pist6are slightly smaller , that's why they sometimes knock when cold .

Modern engines run tight clearances on everything, that's why they make such good power .

A modern F1 engine is seized when cold and has to be pre heated before start up

[yorkspv]

If you are using a top quality piston like omegas , just make the bore size what they recommend.
Ideally find someone to hone block that can measure bores accurately and produce one that are tight at the bottom.

Forged pistons run bigger clearance, the pist6are slightly smaller , that's why they sometimes knock when cold .

Modern engines run tight clearances on everything, that's why they make such good power .

A modern F1 engine is seized when cold and has to be pre heated before start up

So if you purchased some omegas then, for a road engine, do they recommend clearances for different applications?

[251 ENG]

If you have a set of new cast omegas at for example +20 the bore size would be 2.800" giving about 0.0025" -0.003" piston to bore clearance . With the engines we do I would hone bores to as close as possible to 2.800" but not smaller . All bores measured at 3 points down full length . Unless you you have the very latest CNC honer you will get some variation down length of bore . Our sunnen tends to produce a slight taper so I make bores size at top and slightly bigger at bottom, 2.8005".
This is for all cast omegas up to about 11/1 compression ratio maximum.

With forged pistons I would make the bores exactly the same size but they would have approximately 0.0045" bore clearance because the piston is smaller . The exception to this would be pistons that have had skirt coatings added that may require extra clearance.

I have seen people claim they individually measure pistons top 0.0001"and hone each bore to suit , but unless you have full temperature control over everything and super accurate measuring kit you not going to achieve this . Using a micrometer to measure pistons and then to set a bore gauge is laughable when you get to limits of 0.0001"

If you want to visualise 0.0001" , take an average human hair and split it in to 40 slices .

I am not saying the is the right or wrong way but after 35 years of machining and boring and honing block for last 15 it's worked for us.

The other thing to consider is piston quality and even more import ring quality, there is a lot of pistons available that are just not very good.

95+% of our engines use omega pistons , they don't smoke or use oil and make good power

[Hipwell]

As Simon says Omega design the clearance into the piston sizing, to simplify, if you buy a 84mm piston you bore and hone the block to 84mm.

NOW... Every aftermarket piston manufacturer specs this clearance so the pistons don't seize or pick up, Rob Walker does the average Mini sized forged Omega to .003" clearance.

I have done other engines with upto 92mm bore with just .0025" clearance even turbocharged 11:1 comp ratio with Omega's and they don't seize or cause trouble, ring gaps spec'd as low as 6 thou when other manufacturers quote 20. But that was worked out with manufacturer motorsport R&D.