bore and sleeve

this friday im going to bore and sleeve a small gas engine and i was wondering how close i should fit the piston to the bore. the bore is 3 3/4".
on my model gas engines with about a 1" bore i have about 2 tenths of a thou. clearence but i think this would be too tight for this bigger bore.

what is anyones thoughts or experiences in this matter?

thanks
chuck

[Ihorse]

.75 of a thou for that size per in of dia--
d,j,

[Bill Brock]

Hi Chuck, When boring automotive engines of that size we never went less than .002 and that was considered snug. I would say any place between .002 and .004 would be good. Air cooled is slightly looser than water cooled.
Hope this helps Bill

[CharlieBiler]

Fellows,
Rule of thumb is 0.002" to 0.004" of clearance per inch of bore. Big old long stroke engines like tight bores. Small high speed engines need the extra clearance to make power. Racers cut even more clearance. They do not care about life, oil consumption, or efficency. They just want power. Ever notice how an old engine has lots of zip after you give it a valve job? Why doesn't a new engine give that? The piston is too tight and power is lost.

If you bore to the tight side, do a good job of honing. Make sure the cross hatch is close to 90 degrees. I also recommend a thirty minute break in at fast idle, with no shut down. That is just my hillbilly opinion. Best of luck, and make sure you have fun.

[CharlieBiler]

Correction; air cooled needs more room. Go 40 percent more to make up for the little hot runners.

[J.B. Castagnos]

[QUOTE=CharlieBiler]Fellows,
Rule of thumb is 0.002" to 0.004" of clearance per inch of bore. Big old long stroke engines like tight bores.

I think .001" per 1" would be plenty when setting up a new bore, if it's worn run what ya got, nothing to lose.

[Keven Withers]

This is what I grew up with in the machine shop .001 per inch plus .001 to spare so on a 4" piston you would want .005 clearance I am a third generation machinist and have been around old engines for 40 years. Keven

[Craig A]

I'm going with Keven on this one and as the bore increases so does the clearance. As I stated in an earlier post, we allowed .020" on a 9 1/2" piston. On those big pistons it doesn't take much heat to cause .015" of expansion. On my 12-25 Avery (6 1/2" bore) I allowed .010". Craig
By the way Keven-----have you played with the 35hp Ingeco????

[Ken Majeski]

Well... I pretty much agree with Kevin and Craig as far as Cast iron Pistons are concerened.... But no one has mentioned what the Piston in question will be made out of If you are going to run an Aluminum piston in a cast iron cylinder and are going to work it you better allow a couple thou More, I have heard of several Model T Fords (3 3/4) Bore where the .001 rule was followed and they scored pistons and cylinders when going to aluminum. Also if the Piston is Cam ground it will help some....

thanks for all the replies to my question, the piston is cast iron and it is not cam ground (it is perfectly round).
this engine will not be worked hard it just has to run and not blow the oil out the backend of the cylinder. right now it runs but it has so much compression is going past the piston due to pit holes in the bore from the piston being siezed there for 50 years that the oil just gets blown right of the piston and all over the crank and flywheels.

i will let you guys know haw it turns out, i should be boring it next saturday.

thanks
charles foster

[J.B. Castagnos]

One of my Model T's has alluminum pistons and it did score one, but it wasn't from lack of clearance. A friend used it for a parade and let it run at a very slow idle for over twenty minutes. These engines are splash lubed, at the slow speed the oil wasn't reaching the piston. Aluminum isn't as forgiving as cast iron to lack of lube.

[Keven Withers]

Yes Craig we ran the engine for 4 long days at Rollag this year I will get you a little more info. Keven

[CharlieBiler]

Originally Posted by Ken Majeski

Well... I pretty much agree with Kevin and Craig as far as Cast iron Pistons are concerened.... But no one has mentioned what the Piston in question will be made out of If you are going to run an Aluminum piston in a cast iron cylinder and are going to work it you better allow a couple thou More, I have heard of several Model T Fords (3 3/4) Bore where the .001 rule was followed and they scored pistons and cylinders when going to aluminum. Also if the Piston is Cam ground it will help some....

Sorry.....Stupid mode was engaged. I keep forgetting about iron pistons. Keven Withers is right. My mind keeps thinking about forged aluminum pistons. My bad. I use iron pistons as paperweights.