390xp

[NORMZILLA44]

Good stuff my friend!

 

[Outlaw5.0]

Well I will try another caliper since my numbers are off in left field.

 

[CJ1]

JMO, I think its worth the time just for the fact that later on down the road anybody can put a new Piston in if needed. 2 of my saws have popups now, but I think in the future Ill be sending them off to be cut on. Probably not that big of a deal if ya have the stuff to do it, But when Im old and grey, I dunno that I want to trying to find someone to cut me a piston for a 372 I ported 25 years ago .....just saying....:msp_smile:

I agree 100%. That is why I want a stock piston so it can be replaced without any trouble. And there are advantages to a flat top also. CJ

 

[Outlaw5.0]

If you have a pop-up piston, and use the saw for a living, I highly suggest keeping a machined spare piston on hand.

 

[parrisw]

Well I will try another caliper since my numbers are off in left field.

Ya, I just thought it was odd is all. I guess its possible though.

 

[parrisw]

If you have a pop-up piston, and use the saw for a living, I highly suggest keep a machined spare piston on hand.

Good idea!

 

[tlandrum]

99% of the time when i lose a top end the cylinder is toast from shrapnal. i have never burned up a saw from an air leak or straight gas. its usually mechanical failure and the p/c are toast. so id need a cylinder for that spare piston.

 

[parrisw]

99% of the time when i lose a top end the cylinder is toast from shrapnal. i have never burned up a saw from an air leak or straight gas. its usually mechanical failure and the p/c are toast. so id need a cylinder for that spare piston.

Yup, that can happen too. But I've also done many that only need a piston.

 

[mdavlee]

The 390s I checked were .024-.028" on squish without machine work. You might have got one that has real high squish and could really benefit from tightening. I had one with the base machined that was around 175-180 on compression. The squish was at .020" on that one.

 

[parrisw]

The 390s I checked were .024-.028" on squish without machine work. You might have got one that has real high squish and could really benefit from tightening. I had one with the base machined that was around 175-180 on compression. The squish was at .020" on that one.

Wow, that's pretty good, just for a jug drop. Wonder what mine will be at with a cut combustion chamber and .020" squish. Don't want it too high.

 

[mdavlee]

Yeah I think it will put it up pretty good with that amount of drop. I don't know what they will hit with popups. I don't remember what the one was at that had the machine work done to it. I know I dropped a 7900 down about .013" and gained 15 lbs.

 

[parrisw]

Yeah I think it will put it up pretty good with that amount of drop. I don't know what they will hit with popups. I don't remember what the one was at that had the machine work done to it. I know I dropped a 7900 down about .013" and gained 15 lbs.

I did a popup on one once for a guy and he said it hit 195psi.

 

[mdavlee]

I would imagine close to 200 lbs would be right on with a popup.

 

[parrisw]

I would imagine close to 200 lbs would be right on with a popup.

Yikes. gona sound like a dirtbike!

 

[mdavlee]

The last one I had with a wrap handle had the most compression. It did sound a little different and it's one of the saws I used the decomp on.

 

[wyk]

If you have a pop-up piston, and use the saw for a living, I highly suggest keeping a machined spare piston on hand.

Most of us whom use saws for a living would have a couple of other saws on hand... I don't know many that swap out pistons in the field.

 

[Metals406]

Subbing. . . opcorn:

 

[parrisw]

Machine work is all done. Just did the cyl base this eve. Squish now is at .023, it ended up there, so I figured I'd leave it there, don't want compression to get out of hand.

Post machine work numbers.

EX 97.5atdc 165dur

Intake closes 76.5ftdc 153dur

Transfers 121atdc

Blowdown 23.5

My thoughts, ex will stay were it is, just widen.

Intake maybe bring up to 155-156dur close to 80ftdc

Transfer maybe raise up to 119?? little unsure about the transfers still.

that would leave 21.5blowdown

 

[RiverRat2]

Machine work is all done. Just did the cyl base this eve. Squish now is at .023, it ended up there, so I figured I'd leave it there, don't want compression to get out of hand.

Scccccchweeeeeeet,,,, Again very nice work William cant wait to see the vids of this one slinging chips!!!!!

 

[mdavlee]

Looks good Will. I would leave the intake where it is for now. You might want to drop the cylinder some more. :cool2: