Wondering if .003 Piston to Cylinder Clearance is good

[woodsracer250]

Hi everyone,

I had a guy bore a cylinder for me and even tho it's too late to change anything I was just wanting to make sure that .003 is ok.

The cylinder is a liquid cooled 85 Honda ATC 250 Cylinder with the stock sleeve still in it and the cylinder has been heavily ported on (I'm putting together a basket case project BTW) and the cylinder is going on a TRX250R (yea... the bad ass 4 wheeler of the late 80's) but I was just wanting to make sure that .003 cylinder to piston clearance is ok?? It's not too much is it??

Any input on this will be greatly appreciated and I thank you all a bunch in advance!!

 

[woodsracer250]

If anyone could comment on this I'd sure feel more at ease.

(I was thinking .003 was a lil loose but what do Y'all think??)

Thanks again (in advance)

 

[Rich Rohrich]

Where is the clearance being measured in the bore?

 

[High Lord Gomer]

...and how?

(Meaning, can you fit one .003 feeler on one side or one .003 feeler on one side and another .003 feeler directly opposite it?)

 

[woodsracer250]

Hi Y'all...

No, I just asked what kind of clearance he gave it and he said "I give em about .003" and I said "Oh... ok" and walked.

I havent measured it with a feeler gauge or anything. I was just thinking .003 sounded a lil (a lot) loose. Is'nt it spossed to be about 2.5 (max)??

 

[Rich Rohrich]

Hi Y'all...

No, I just asked what kind of clearance he gave it and he said "I give em about .003"

Anyone who would say something that stupid is best not trusted. I would strongly urge you to have it measured by someone who actually knows what they are doing before you put it together.

Once you get a proper piston to wall clearance and a reading on the cylinder taper, check with the piston maker's spec for setup.

Good cast pistons like the Japanese OEM stuff can run fairly tight, while forged pistons like Wiseco need a little more running clearance.

 

[High Lord Gomer]

Rich, my question was an honestly ignorant one...how do you measure piston clearance?

 

[helio lucas]

gomer, i know you ask directly to rich but i will try my best to answer...

piston clearence is usually measured, per manufacturers service manual, at 90 degrees of the piston pin, some 10mm above the bottom of the piston skirt, at the tighest spot, with a micometer. the bore has to be measured with a internal micrometer (bore gauge) some 10-20mm below the top of cylinder. the bore must to be perfectly round or the proper clearance will be wrong.

really, most service manuals provides some good ballpark limits for clearance, piston size and cylinder bore wear (they usually wear more around the intake port)

some just fit a feeler gauge between the cylinder and piston and check for the tightest spot.


i love the response the owner of a engine repair shop (the best around) gave me once. i´ve asked what clearance he usually put when rebore a cylinder for a 50cc bike(small LC cast iron cylinder with a cheap piston).

he said "i don´t give any clearance, the manufacturer of piston do. if it´s a 40mm piston i just bore it to 40mm. if you look closely the piston is somewhat smaller than the 40mm, say 39,95mm, that is the clearance it needs..."
after this we talked to very long and i have not used or recomended other shop besides this. :cool:

 

[High Lord Gomer]

Thank you!

 

[Rich Rohrich]

Helio did a great job of explaining it.

I'd only add that generally I will measure the piston with a micrometer, and use that reading as the reference to zero out the bore gauge. With the gauge zeroed at the exact size of the piston you and run the measuring anvil of the gauge up and down the bore to check for taper, and turn it 360 degrees at various positions in the cylinder to verify out of round. Even an inexpensive mic and gauge can be used in this fashion, and with enough care very consistent results will be had.

That's why anybody who claims to be a machinist and says ""I give em about .003" is best avoided.

You can't accurately measure piston to wall clearance with a feeler gauge, but I realize that won't stop the Luddites of the world from trying. :whoa: