Piston clearance?

[wigglesworth]

Im getting stuff gathered for my 044/046 crossbreed. I am sitting here looking at my my P&C and taking measurements. Both look nice and clean, no scoring, marks or such. My question lie's in this, the best measurement I can get on the bore is just above the base, and it measures 2.046 The "used" oem piston measures 2.040 at the skirt, and 2.037 just below the crown. Is this too sloppy? What is the "accepted" amount of allowable tolerance? I would like to use the OEM piston. Thought's?

 

[FATGUY]

I may be wrong, but that seems really sloppy, perhaps someone with more expertise in this area will chime in....

 

[Tzed250]

I may be wrong, but that seems really sloppy, perhaps someone with more expertise in this area will chime in....

You aren't wrong Nik.

What instruments are you using to measure the bore and piston with?

A dial bore gage and micrometer are the minimum. A set of calipers does not have the accuracy needed for those measurements.


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[wigglesworth]

Im using a set of dial caliper's. Like I said, it's the best measurment I could get with them. I have access to the bore gauge and micrometer's so I guess i'll have to hook up and get a better measurement. I'll post up when after I measure. Thanks guy's.

 

[lone wolf]

you can check piston to cyl clearance by putting Long brass feeler gauges between pist and cyl. and I think 3 thousandths or tiny bit more clearance not a lot of info out there on chainsaw piston to wall clearance what say you guys on the clearance?

 

[timberwolf]

Measure the clearance at the skirt maybe 1/4 inch up from the bottom. The crown is intentionally undersized to allow for thermal expansion so the piston is more or less a perfect cylinder at operating temperature.

.0060 does sound kinda so-so. If I have a cylinder replated I have the bore done 0.0015 to 0.0020 clearance. 0.0100 is no question straight into the scrap metal bin. Engine will likely have a rattle and be on itway out if it is much looser than that.

 

[wigglesworth]

Measure the clearance at the skirt maybe 1/4 inch up from the bottom. The crown is intentionally undersized to allow for thermal expansion so the piston is more or less a perfect cylinder at operating temperature.

.0060 does sound kinda so-so. If I have a cylinder replated I have the bore done 0.0015 to 0.0020 clearance. 0.0100 is no question straight into the scrap metal bin. Engine will likely have a rattle and be on itway out if it is much looser than that.

The guy with the mic's and bore gauges was MIA this morning, so I didnt get to measure, but i did measure the gap with a feeler gauge. I could force a .005 in between the piston and bore, but an .006 would not. TW are you saying that it will have a rattle/be on it's way out with .006?

 

[timberwolf]

No, I have used pistons in that range (some combinations of aftermarket parts can come out nearly that loose), but it is getting near the limit IMHO. You will get back out of it what you put into it if you know what I am saying. The piston is one of the fastest wearing parts in an engine and if you put a worn piston in, it is certain to fail sooner as opposed to later.

 

[wigglesworth]

The piston is one of the fastest wearing parts in an engine and if you put a worn piston in it is certain to fail sooner as opposed to later.

That is a very good point. To me it's just odd that there is very little visible wear on it, which made me think that this clearance might be normal. Any idea what the clearance is on a factory new jug and slug?

 

[blsnelling]

No, I have used pistons in that range (some combinations of aftermarket parts can come out nearly that loose), but it is getting near the limit IMHO.

+1. You'd have trouble getting .001" in some new saws.

 

[wigglesworth]

+1. You'd have trouble getting .001" in some new saws.

That seem's really tight. If it ever got hot, I would think it would seize up with only .001 clearance.

 

[blsnelling]

That seem's really tight. If it ever got hot, I would think it would seize up with only .001 clearance.

I've seen it numerous times, and never an issue. This is only on the skirt though, not the crown.

 

[lone wolf]

brad what cc saw are you speaking of?

 

[lone wolf]

That seem's really tight. If it ever got hot, I would think it would seize up with only .001 clearance. maybe a new saw would be that tight but it Will wear as you break it in so a couple thous. on your saw sounds about where it should start at and we already heard from the other wolf what the maximum limit should be. 5 - 6 thous.

 

[blsnelling]

brad what cc saw are you speaking of?

I've seen several, but honestly can recall what all models. I know I've seen 346s that tight. Maybe 372s as well.

 

[lone wolf]

are you talking about new saws that don't have a lot of time on them im guessing?

 

[lone wolf]

Im using a set of dial caliper's. Like I said, it's the best measurment I could get with them. I have access to the bore gauge and micrometer's so I guess i'll have to hook up and get a better measurement. I'll post up when after I measure. Thanks guy's.

use precision electronic calipers mitutoyo maybe

 

[wigglesworth]

use precision electronic calipers mitutoyo maybe

No. There these..Mitutoyo 505-712 carbide tipped...LINK

 

[lone wolf]

No. There these..Mitutoyo 505-712 carbide tipped...LINK

they look accurate but i like the feeler gage method I think you want the pist to wall to be close to 1.5 - 2 thousands in new cond a few more and your at the wear limit

 

[wigglesworth]

they look accurate but i like the feeler gage method I think you want the pist to wall to be close to 1.5 - 2 thousands in new cond a few more and your at the wear limit

Im thinking Im gonna go ahead and go with a meteor on this one. It seems I could probably get by with this one at .006, but it's at the high end of the spectrum. Grandpa always said "If it's worth doing, its worth doing right"