yikes. that is pretty extreme scoring. worst i've ever seen.

this is what the frm sleeves in my h22 looked like when i tore it down... my pistons were *all* broken, chunks of ring land fell out in my hands, but the cylinders are still smooth, after driving with busted lands for several thousand miles :





"The guy who built the engine apparently didn't know that forged pistons can't use the stock piston to wall clearance...."

do you know what clearances he used? i still suspect some incompatibility between the forged pistons and frm sleeves, but i have never really understood why it would be an issue since the piston itself should not contact the cylinder. when i rebuilt my motor, almost everyone told me that forged pistons + frm sleeves would cause the exact problem you have. what is even more confusing is that the s2k uses forged pistons out of the box, but apparently they have a special coating which may be for detonation protection (heat distribution) or it may really have to do with the sleeves.

 

Chris,

The wall clearances really depend on the forgings. With Arias pistons I've found that you often have to run large wall clearances, but a high quality forging from someone like JE usually doesn't need much - but its all in the alloy choice.

I wonder, were your aftermarket pistons coated on the skirts? I believe the stock pistons are and if your aftermarket slugs were not, that might have created a friction issue (oil retention between wall and skirt may have been bad, don't know).

UL

 

This is probably a stupid question. I thought that the rings prevented the piston skirts from contacting the sides of the cylinder wall making the piston go through a smooth and linear motion up and down? If the piston moved (jiggled?) through its motion up and down in one plane which im guessing would be the same plane in which the connecting rod articulates on the wrist pin wouldn't there be some type of compression loss due to the inablility of the rings to mate perpendicularly with the cylinder wall through its entire range of motion as well as some stiction due to the changing angle of the rings as it moved up and down in relation to the cylinder wall?
I guess i'm not understanding correctly. I guess i just dont see how the material of the piston would affect how the rings mate to the cylinder wall unless the rings were to small or some huge load in the same plane as the above mentioned joint would cause the piston skirt to contact the cylinder wall?

 

Oh and just looked at Chris' block on his webpage and the scoring is consistent with the articulation of the wrist pin with the connecting rod. Did the spacing of the rings change with the new pistons? For instance, do the new pistons have a smaller distance between the top and bottom ring? If something like that were true wouldn't that cause an increased load for the rings to bear in that plane making it more likely that the rings would be overwhelmed and cause the piston skirt to contact the sleeve? I think that perhaps even changing the position of the rings in relation to the wrist pin (moving them up or down) from the location of the stock rings might cause more stress to be put on them!
I just don't understand how the material of the piston would change how the rings prevent the piston from making lateral movements.
Perhaps you could post a picture of the two pistons side by side so that the center of the wrist pin holes line up, then we could see if there was a difference in location or spacing of the rings
Also was there any difference in material or design of the new rings?

 

You've got to remember that the piston to bore clearance is about a thousandth of an inch. That's small enough that a thin film of oil is necessary to protect things. The rings take up a lot of the thrust load, but not all of it.

The modern coatings used on pistons are supposed to reduce friction, and may do that thru improved oil retention (wetting) among other things. I'm no expert on coatings, but if you're putting them on the piston skirt (be they low friction, anti wear, whatever) there must be a reason.

UL

 

Pistons had the same measurments as stock, including piston pin location (height from top of piston to top of piston pin) and same ring sizes and same ringland sizes. You could hold a stock piston (with rings) up to the JE and the piston rings would slip right in to the JE piston.

I hear what you guys are saying about the coating which could retain oil better, however I can't believe it would do THAT much damage.... The theory that makes most logical sense to me is that the JE pistons simply expand more than the stock pistons, and therefore it dug in to the cylinder wall when it heated. Mike says the scoring is very bad, and could only be repaired with a large boring. I think he estimated .050"?

As I said earlier the stock piston to wall clearance was used (as described in the service manual.) Apparently these clearances are very tight in comparison to other cars....

Chris

 

Yes, with only a 1 mil clearance, I'd certainly think that piston expansion would have to be carefully controlled! UL, can you make sense (or nonsense ) of this caveat about not using forged pistons with FRM sleeves? I'd have thought forged metal would expand less than cast?? What expands more, steel or Al? Are the different alloys of Al extremely different in this respect? And what about expansion of the cylinders?

So many questions...

 

Now we need to have the pistons machined, and the rods need to have new bushings put in to match the different pin size.....

argh..

 

Well twohoos, everything I've seen says the stock F20C pistons are forged to begin with, so I'm not sure about the FRM thing. Plus, I've seen plenty of FRM prelude blocks with forged pistons in them.

The piston manufacturer should have had a recommended wall clearance included with the pistons. Even if the clearance was a little tight, I'm surprised the thing just didn't seize when it heated up.

Oh, and forgings tend to expand more than cast, and Al tends to expand more than Steel (extreme alloys not withstanding).

UL

 

have you seen any of them actually run for any length of time? the only people i know who put forged pistons into a stock prelude block ended up with no compression and scored walls after less than a thousand miles. i know they *could* all have been bad build-ups, but it seems unlikely.

since then, everyone i know sends their H blocks to GE or a local machine shop for sleeving with iron sleeves, and have been running strong since.

sleeved h22a4 with endyn (mfr by wiseco) 9.5:1 pistons, crower rods :



fwik, the expansion of forged pistons is larger than most cast pistons, but it depend greatly on the type of alloy used. the clearances recommended on an endyn piston, for example, are quite a bit less than arias used to specify.

[QUOTE]Originally posted by ultimate lurker
[B]Well twohoos, everything I've seen says the stock F20C pistons are forged to begin with, so I'm not sure about the FRM thing.