Piston-cylinder clearance: So uh..yea, anyone see anything wrong here ***Pics***

[Kennay]

or am I having a moment? :scared:


Cylinder 1 yields ~150psi, with a ton of piston slap by feel
Cylinder 2 yields ~138 psi, with a ton of piston slap by feel

I don't have a dial caliper handy, that's why I didn't use one. I took the head off of a 650 kawi I have sitting here, and there is much less play in the piston than on this engine, and who knows how many hours it has on it. It has a 1mm overbore, and about 160psi.

To make sure I wasn't going nutty, I put a .012" one one side, and straight across the piston, I slid in a .013"

 

[Scraggly]

is this the new 760?

 

[Kennay]

yes

 

[Scraggly]

damn that sux

 

[750SX]

usually it is measured lower on the skirt. Either at the bottom, or a couple mm up. Depends on the piston manufacturer.

 

[SUPERTUNE]

Take the feeler gauge set apart, flip the cylinder upside down then use about .004 up to .007 blade and lay it on the intake skirt (90 degrees from the pin boss) and select the proper gauge that will let the piston push through and not try and get stuck. The way you are showing is wrong, as pistons are designed to be much smaller at the top of piston (tapered to the bottom of skirt) and also oval, being much smaller at the wristpin sides.

 

[Kennay]

Well that's good to know. I will check that later. I just don't have anymore money to spend after buying this thing and I'm a bit jumpy.

 

[BROSS]

Well that's good to know. I will check that later. I just don't have anymore money to spend after buying this thing and I'm a bit jumpy.

That engine was built at my shop 1-2 years ago, my father did the machining to the cylinder and they should have .0045 clearance and the engine should be fresh, never started. A stock 760 had different compression due to crank twisting or something like that. Anyone correct me if I am wrong.

PS. the guy is a good dude, don't think he would sell questionable products.

 

[Kennay]

I really didn't want to mention names Bross, but thanks. Selling something questionable and selling something you don't know is wrong are two different things.

Thanks for throwing it out there though. I feel better since you cleared it up. If you don't mind, would you edit the name out please? I'm not trying to drag anyone through dirt or bring out names.

Can you elaborate on this crank twisting?

 

[BROSS]

The 760 came in big sitdowns and with the load on the prop I think it caused the cranks to twist slightly there for the different compression, every stock 760 I ever checked comp on was about 10-15lbs different. IMO

 

[Kennay]

This is a new crank

 

[BROSS]

Thats good, the comp diff is made up in the cylinder head, but a new welded crank is deff good.

 

[Mouthfulloflake]

that makes no sense to me.

The 760 came in big sitdowns and with the load on the prop I think it caused the cranks to twist slightly there for the different compression, every stock 760 I ever checked comp on was about 10-15lbs different. IMO

 

[mybad]

I have a alot of piston slap at the top of my big bore. 849 sxr. Its been running strong all season. The builder guaranteed his work and said just ride it. At Daytona I accidentally ingested sand into the cooling system. The motor overheated big time. Everything was sizzling. If it wasnt for the extra clearance I think it would of been toast. IMO

 

[BROSS]

Like I said the comp is off on all stock 760 I have checked. This is all my opinion and what I have heard, been working on ski for a living for 10 years. I may be wrong...

 

[Kennay]

I have a alot of piston slap at the top of my big bore. 849 sxr. Its been running strong all season. The builder guaranteed his work and said just ride it. At Daytona I accidentally ingested sand into the cooling system. The motor overheated big time. Everything was sizzling. If it wasnt for the extra clearance I think it would of been toast. IMO

Running forged pitons vs. cast makes a big difference when you start talking about piston slap. You probably have forged

 

[Mouthfulloflake]

a twisted crank shouldn't change compression, it would only alter the timing.

a bent ROD would change the stroke, and therefore change compression though.

Like I said the comp is off on all stock 760 I have checked. This is all my opinion and what I have heard, been working on ski for a living for 10 years. I may be wrong...

 

[WaveDemon]

is this motor brand new and not broken in yet?

 

[DrewNJ]

Bross is right about the compression on the 760. I've done compression tests on about 4-5 760's with stock heads and all of them were 10-15lbs lighter on the back jug. These were all on bone stock 760's in sitdowns....

 

[Mouthfulloflake]

why though?

are the cranks indexed with the PTo cylinder having slightly retarted timing to prevent DETO under load on them?

I still dont see how that would effect compression?

Bross is right about the compression on the 760. I've done compression tests on about 4-5 760's with stock heads and all of them were 10-15lbs lighter on the back jug. These were all on bone stock 760's in sitdowns....