Uh oh..Broken Piston Skirt please help

[antwanray]

1982 L28et Piston #5 rebuilding a used engine found this

 

Any help on how to solve this?  Is there any chance in just running it as is?  What risks would there be?  Or do you suggest I just purchase a new piston kit?  Any thoughts on the Ross kit 1mm+ bore?

 

[AydinZ71]

Try cleaning the pistons and look closely for cracks. Very, very good chance there is more damage than just the missing skirt section. Often happens when a boosted engine detonates (knock). Were you able to remove all the rings OK?if you see skirt damage, the ring lands are often cracked or some rings are seized in the groove. Check your cylinder walls. It’s possible you just need a new piston kit, and everything else may be salvageable. My guess is this was the #5 or #6 piston. 
 

Machine shop can tell you if your block  needs to bore or only re-hone. If you do re-bore, remove as little material as possible (pistons are available in 0.5mm increments, at least from kameari). Folks have shown cylinder wall rigidity to be more important with boost than a slight bump in displacement. Thin walls lead to plastic deformation = blow-by. 
 

Ross forged pistons are fine. I’d take an opportunity to opt for a CR closer to 8.0-8.2:1. Good balance between off-throttle response, lag, and room for boost. You won’t do well at 9:1 + boost on an L-series, although I see it often on an RB. Chamber just isn’t optimized well enough to avoid detonation, and you will be limited on boost. 
 

let them know it’s for a forced induction application so they design the ring lands appropriately. OEM rods are good to around 400ft-lbs in my experience, but you will likely struggle with heat management and detonation up there. 

[antwanray]

3 hours ago, AydinZ71 said:

Try cleaning the pistons and look closely for cracks. Very, very good chance there is more damage than just the missing skirt section. Often happens when a boosted engine detonates (knock). Were you able to remove all the rings OK?if you see skirt damage, the ring lands are often cracked or some rings are seized in the groove. Check your cylinder walls. It’s possible you just need a new piston kit, and everything else may be salvageable. My guess is this was the #5 or #6 piston. 
 

Machine shop can tell you if your block  needs to bore or only re-hone. If you do re-bore, remove as little material as possible (pistons are available in 0.5mm increments, at least from kameari). Folks have shown cylinder wall rigidity to be more important with boost than a slight bump in displacement. Thin walls lead to plastic deformation = blow-by. 
 

Ross forged pistons are fine. I’d take an opportunity to opt for a CR closer to 8.0-8.2:1. Good balance between off-throttle response, lag, and room for boost. You won’t do well at 9:1 + boost on an L-series, although I see it often on an RB. Chamber just isn’t optimized well enough to avoid detonation, and you will be limited on boost. 
 

let them know it’s for a forced induction application so they design the ring lands appropriately. OEM rods are good to around 400ft-lbs in my experience, but you will likely struggle with heat management and detonation up there. 

The Kameari pistons look like they drastically change pin height if this is the case, you also need to buy their rods?  Sound right?

If you suggest running a higher compression ratio, you are suggesting going flat top forged pistons instead of dished?  Is this what you mean?

 

thanks!

[AydinZ71]

I have not looked at the whole Kameari line of pistons in a while. I just know they are rather affordable at the moment with the exchange rate, and they offer both cast and forged racing pistons. I thought they had an OEM pin height, but I could be wrong. Check out RHD Japan, and search through their offerings. They had what I was looking for in stock, which was much better than CP who flat out would not return my e-mails or calls. 
 

Yes, the Kameari forged race pistons have a 4mm. Shorter pin height (going off memory) which would pair with the 137.5mm rods they also sell, for a V07 crank. I have this setup in a 3.1L at the moment, which will be NA. I wouldn’t recommend it for a turbo. It’s meant to be a lightweight rotating assembly for reliable high revs. And lower parasitic loss. 
 

if you go with Ross, JB, CP, eagle etc. you will be happy. They are all decent quality IMO. Go with whomever can make or stock what specific size you need within your budget. Nominal differences in mass will not matter for a turbo setup anyways. Your focus should reliability and survivability. 
 

just doing some math, a P90 (you said it’s an L28et) with flat top pistons will yield you ~8.7:1 w/ oem style 1mm crushed gasket. A little high for my taste. You have the option to go with a 2mm (from cometic for example), or go with dished custom pistons and shave the head to your liking. Lots of ways to go about it. 
 

I have no idea what your goals are, but if you want to keep it budget: buy a set of new (assuming that’s an option) or used OEM pistons matching your next bore dia. with new rings and run with that. You don’t need forged pistons for 300+ Ft-lbs of torque, but you will need a reliable conservative tune. OEM was something like 7.4:1 CR which was super conservative. 
 

Bottom line: do NOT attempt to drive with a crude, ballpark tune and OEM cast pistons. Take it to a reputable shop that can reliably identify your engines knock limits. Super common that you will not hear light knocking under boost, and will be in the same place again in several months. 
 

 

[jeffer949]

Save your money. Unless your looking for over 400 hp you dont need forged pistons. I believe people have even gone over that mark.  You do need to look at the rest of the pistons to make sure they are ok and if your bore is good. Just buy a new single piston or find a used one. Ive broken a few cylinder number 5 skirts in my day from being young and not wise on tunning.  Hone the cylinders and put new rings in and let her eat.  No need to go crazy on an engine that will likely never see the benefits from the expensive pistons. I may be wrong but with the questions you asked initially on if you could run the piston like this im guessing your new to engine building and are on a budget and wont be adding enough supporting mods to make really big power.  

[AydinZ71]

@jeffer949 I agree ☝️ 

 

you are driving light-to-light, not racing. Save your money for the tune, reliable fuel system, injectors, engine management, etc.

 

you CAN go cheap on those internal engine components as OEM goes a long way. Just don’t cut corners on the machining (ring gaps, bore, hone, tolerances, etc.), and especially not in the tune. 

[antwanray]

11 hours ago, jeffer949 said:

Save your money. Unless your looking for over 400 hp you dont need forged pistons. I believe people have even gone over that mark.  You do need to look at the rest of the pistons to make sure they are ok and if your bore is good. Just buy a new single piston or find a used one. Ive broken a few cylinder number 5 skirts in my day from being young and not wise on tunning.  Hone the cylinders and put new rings in and let her eat.  No need to go crazy on an engine that will likely never see the benefits from the expensive pistons. I may be wrong but with the questions you asked initially on if you could run the piston like this im guessing your new to engine building and are on a budget and wont be adding enough supporting mods to make really big power.  

Thanks for all the input!!
 

Suggestions on where to get a single stock piston replacement?

[antwanray]

These look pretty suspect, but the only thing I can find…

 

https://www.ebay.com/itm/196145602545?hash=item2dab305ff1:g:Fz8AAOSwPJhlhIBZ&amdata=enc%3AAQAIAAAA4MhCdnVELY1q75Tfvon%2BMAiL12uNyFchATM62bdscNE6Wms8TFF84hBvERtcgvcxT5KXjvZpf65rR9%2BJ9E%2BeovNq5mJSy8fBhudppCpiGG1Blja3%2Bmv91jSKjjQDqS8YtRp8e5Uao7ipfVSWbEbowpXnAJT2Gv05yIFO%2BBOV0d1PIDnmz2TKd%2BN9ECYmfZPxsxLw22YCuhbKJSLIVrm10Mqi6uTYU2sAz7wYRKgCY1nBO4zGcxG54uRqJciiyEhXKJ1p%2BbNr1utAtfPJNX%2BIcth%2FNhnadtGTTQV4emkLN4IV|tkp%3ABFBM0pmB5ZVj

[zboi]

Seems fine, its the right size you need right? ENGINE PISTON KIT -- .030 OF AN INCH; COMPRESSION RATIO: 7.4:1 86.1MM
Cast pistons should be cheap.