Chris(MA)

curious if anyone has preemptivly swapped pistons on a 996 to head off scoring? seems cheaper than waiting for scoring which requires sleeving?

could also pin IMS, add oil pump new hex etc also at the same time.

thoughts?

AWDGuy

if the cylinders have already tapered, adding new pistons will make scoring happen faster.

Montychristo128

I don't believe the issue lies with the pistons, my car is in the midst of having an engine rebuild which includes liners, as part of the process my engine builder measured all the bores and they were all out of shape, I believe this causes the piston skirt to contact the wall of the cylinder, as it rotates past TDC and gives rise to the scoring.

Poor block design is the issue here.

rogazilla

I have thought about this and the best idea is probably put the money away for rebuild when it actually happens.

When scoring happens, my understanding is they mill it off and sleeves it, so it doesnt matter. Just don't wait too long that it destroy other parts in the block. So drive it until it happens.

Or you can do a preventative rebuilt. Send the car to Jake and do a rebuild into 3.8/4.0 and have all the goodies installed that'd take cares 2 birds in one stone.

I plan to enjoy it as it is until I want more HP or until something is wrong before I spend the money.

Woodman71

This seems like one of the most interesting "preventative maintenance" concepts I have heard on the forum.

I mean, if 100%, or even 20% of these engines positively absolutely ended up having this issue, maybe, but...

hitthecones

I have nothing overly meaningful to add other than this is complete lunacy.

Years from now prospective buyers (or writers like Jack B) will read this thread and believe that all 996 pistons are bad and require replacement as a PM.

This is what perpetuates the ill feelings towards our engines!

Chris(MA)

curious to know if they wear oval or somehow plastically deform due to stress and heat cycles?

Byprodriver

Cylinders wear oval

Porschetech3

The cylinders don't wear into the oval shape, they distort at the top of the cylinder where as in one dimension it gets smaller and in the other it gets bigger, in the middle it is still round. I have asked my tool and die maker to make me a tool ( a jackbolt inside of a fitted inner cylinder) to jack the cylinders back round. Then a cutter to true the outer top of the cylinder so that a support ring can be installed to keep the cylinders round. With the right tooling this could be done by hand similar to machining the ring ridge out of the top of the cylinder we had to do on the old school engines that would wear the cylinders so bad that the groove left at the top would damage the pistons on removal, and if not trued and new rings installed it would destroy them in short order. Most all m96 engines above 50k have significant ovaling, some as much as .006-.008 ,

Ahsai

Exactly what I found on a 100k miles 3.6L engine I was rebuilding. It's distortion rather than wear.

Chris(MA)

still seems a strange that piston scoring happens on the skirt rather than the ring land, if the cylinders are most oval at the top you would assume scuffing would start there?

do the early 2.5 boxster m96 motors suffer less? as far as i know they have the thickest walls

do folks feel it is the piston thrust load against the wall that deforms them?

Porschetech3

The pistons are not round,they are cam ground and barrel shaped with the skirt being the largest diameter(actually most piston makers the largest diameter is .5 inches from the bottom of the skirt 90 degrees from the pin bore), the ring land is the smallest as much as .010-.015 undercut so as not to hit the cylinder wall during expansion, the top of the piston is the hottest there for expands most.And yes Boxster 2.5 is least likely. The theories of why the ovaling occurs is that the heat cycling of the lockasil cylinder draws out the internal stress of the material that was internalized when cast.