View Full Version : Can someone defone "collapsed skirt"???

05-05-2005, 06:09 PM
Can someone define "collapsed skirt" for me?

I am wanting to verify what my machinist told me...

He says that he wouldn't use my forged pistons in the block they were in (too much piston to bore clearance), but says he would use em in another block machined to fit, especially since I have a spare block...

The piston manufacturer says I should check for collapsed skirts.

When I asked him if he could measure to see if any of the pistons had a collapsed skirts, he said it didn't really matter because he could fit the pistons into my spare stock block....

My prior understanding was that collapsed skirts made the piston improperly shaped, hence no good....

Is he correct?

This is a new relationship with the machineist in question, so I gotta second guess him.....

Thanks guys!

Parker Dean
05-05-2005, 10:09 PM
Essentially a collapsed skirt is when the skirt of the piston has actually given way and is closer to the center of the bore than it should be. This is frequently the result of repeated overheats. I've had a skirt actually break off because of this. Since the piston has changed dimensionally it is technically failed even though it may still be in one piece. I wouldn't even consider running such a piston myself.

Now if the reason for the excessive clearance mentioned was just bore wear and the pistons themselves are fine, that's another matter.

05-05-2005, 10:45 PM
Ross pistons say the skirts should be measured (1/4 inch from the bottom) to ensure proper size. He also said that it should be measured from just above the pin, to the bottom of the skirt, to ensure a uniform taper, and no sudded change in size.

My machinist says that this is a non-issue, as long as there is no severe deformation.

My problem is that my machinist didn't act like it was important to check this. I don't know what they mic out to right now, as he has't checked.
He said that there would likely be a .001 "deformation" in the skirts of any forged piston that has been run, and that as long as this skirt was withing a thou or two, it was a non-issue, because the new block can be machined ot match the piston...

I guess my question now is, should I get a new machinist, becasue he is overlooking important things, or is the Ross piston guy being over anal???


05-06-2005, 12:46 AM
Between my bird, my wife's, and my brother's, we've taken every motor to a different machine shop. And guess what, we got three different experiences. Sometimes, you really can't compare what you're dealing with until you've got something else to compare it to.

Don't be afraid to call up your machine-ist and ask as many questions as makes you happy. It's your money. It can be daunting talking to someone who has a lot of experience in this field, but every good question deserves an answer. If you start to get the run-around, take the advice my first girlfriend gave me, and "Pull Out!", it's never too late. :D

Seriously though, If you ask your guy whether the piston skirts are "collapsed" or not, he should give you a straight answer.

David Neibert
05-06-2005, 08:40 AM
Just tell the guy you want the pistons measured before deciding if your going to use them. It shouldn't take more than 5 minutes to measure all six.

They probably are okay, but since you've already been burned once, I think it's best to check them.


XR7 Dave
05-06-2005, 09:41 AM
There is a big difference between an engine builder who races and one who just builds race engines. An engine builder who races will usually know what you can get away with safely. An engine builder who does not race but builds race engines will always do exactly what is "right". If your engine builder actually owns a race car, then I would tend to trust his opinion. If he does not race, then he is less likely to be in tune with your pocketbook.

My machinist told me exactly the same thing about a set of Ross pistons taken from a motor which has seen a lot of nitrous and a lot of boost. He said that he would custom bore a stock block to match the pistons. He has a race car that runs 9000rpm without breaking so I trust his opinion on things like this. Another engine builder had rejected the pistons because they had "collapsed skirts". We measured the pistons according to Ross' specifications and my engine builder said the pistons were normal/useable.

But ya, that is a tough spot to be in deciding what to do with them.

05-06-2005, 09:46 AM
We use to tap the skirts back out with a brass hammer. :)

But seriously, if it's .002" or less, it's not a big deal at all. We had several engines that came in repeatedly (for various reasons) and occasionally a piston would be a little deformed. I've tweaked them back as much as .0015" without any issues.

05-06-2005, 10:14 AM
Sounds good!

I don't know if this dude drives the car, but the shop sponsors the local bracket races, and they do has a car and always has several other drag cars in the shop.

They also have rice in the shop, which tells me they are not to narrowminded to blow off anything but a SBC, like many others do.

He seemed to be confident about reusing them, and matching the bore to them... I think his words were something to the effect of "there's no reason not to resuse them unless I had a desire to spend 7-800 on new pistons..." He also said he wouldn't feel bad about using them one bit.

He said he'll measure them if I wanted him to, but there was no reason to do that. If he brings out his brass hammer, I'll start to worry ;)

Lets just hope I can get the deal settled, not much luck yet... I'll just have a set of CAT hbeams, and custom Ross pistons sitting in the garage for awhile :eek:

There's no way the wife will approve another $1200+ to get this engine unscrewed!