[Benno]

Hoping to get some guidance from those of you in the know. I am preparing to rebuild my early 1992 GTS motor to help address oil ingestion into the combustion chamber. All the research I have done has pointed to the lack of oil drain holes in the GTS pistons as being a major contributor (not the only one). Several experienced engine builders recommend drilling the GTS pistons.

I wanted to get clarity and specificity on how many holes and where to drill them so I could properly advise the machine shop when the time comes.

Here is what a GTS piston looks like. You will note at least 2 blind holes half in/out of the oil groove (third line). I presume there at least 2 more




Here is what an S4 piston looks like. You will note the two holes on the lower skirt. The holes in the oil groove appear to also be blind holes.


Early S4 pistons did not have these lower holes. However, there is a TSB that instructs techs to add them when pistons are removed.


So my question is where to drill holes?
- Do I add the 4-holes in the lower skirt like the TSB suggests?
- Do I drill the blind holes in the oil groove through the piston wall? There seem to be 8 blind holes. This seems to be what 928 engine builders reference doing and it's what is pictured in the JE image below.


FYI: For reference here is a nice piston terminology reference diagram from JE pistons. I had to educate myself on what all the terms mean.

[maddog2020]

when you remove the oil rings you will see dimples where the holes should be in the ring grooves.

[Strosek Ultra]

It is the craziest thing I have ever seen that there are no oil drain holes at the oil ring grooves. Never seen it on any other engine.
Åke

[Benno]

@maddog2020 Thank you for the input. I will be focused on drilling the 8-blind holes in the oil groove. What do you think about adding the 4 drain holes in the skirt like the Porsche TSB for the S4 piston recommends?

[maddog2020]

not sure about the holes in yellow per the TSB. We started drilling out the oil ring groove back in 96.

[worf]

We started drilling out the oil ring groove back in 96.

GTS pistons? Or did “we” find pre-‘92 pistons without ring groove holes?

[maddog2020]

GTS pistons. Local dealership had been replacing engines for excessive oil use. My business partner was able to let him partially tear down one of the long blocks before it went back to Porsche as a core, and he was the one that found no oil drain back holes. They sent us a customer that had already had 1 engine replacement under warranty and was out of warranty, who wanted the issue resolved no matter the cost, so we tore the engine down, and bought new pistons and had them drilled out, and installed them with new rings. Bores were like new with less than 5k miles on them. It solved the oil burning issues and the GTS had quite a bit more pep.