mahle ptw clearance?

[talonDSMerr]

I'm wondering what kind of piston to wall clearance I should aim for using the Mahle pistons in 85.5mm bore size. These are the 2.3L stroker pistons with 9:1 cr. I called the sport compact tech at Mahle and he recommended .0016"-.0024" but that seems a little tight to me, even for a low expansion piston like the Mahle's. I'm looking to run around 30psi boost on e85 on this build, and it will mostly be street driven. For now, I told the machinist to go for .0024".

Any recommendations or advice? Opinions?

 

[turbo23]

.0024 does seem a touch tight, i'd probably run .003, but with the e85 0024 will work. mahle's strut desgin, along with the small bore of the 4g, wont expand much.

 

[TSIfreek]

Ha I have mine set at .0015 (on the coating) and its at about .0023~ with out the coating as you dont messure the coating for PTW clearance as it will wear off. Mahle pistons are desighned to run tight.
My machineist recomended me run the bore size on the piston box as that will give the best ring seal and piston support. 500 HP is what im after street driven.
If you run larger than is what is recomended your piston ring gaps will be excessive big right out of the box. My top rings were the only ones that needed to be filed down. My second rings came out at .018-.020 right out of the box.

 

[talonDSMerr]

Ha I have mine set at .0015 (on the coating) and its at about .0023~ with out the coating as you dont messure the coating for PTW clearance as it will wear off. Mahle pistons are desighned to run tight.
My machineist recomended me run the bore size on the piston box as that will give the best ring seal and piston support. 500 HP is what im after street driven.
If you run larger than is what is recomended your piston ring gaps will be excessive big right out of the box. My top rings were the only ones that needed to be filed down. My second rings came out at .018-.020 right out of the box.

Thanks, that was the info I was looking for and I believe we have similar power goals. I'll remind my machinist not to measure from the coating and aim for .0024" clearance. How much boost pressure are you running and how has compression turned out after engine break-in? Mahle recommended me to run .017" ring end gap on both top and second rings, while I've heard running a tighter top ring and looser bottom helps with running high boost. What did you decide to run your end gaps at?

 

[TSIfreek]

Thanks, that was the info I was looking for and I believe we have similar power goals. I'll remind my machinist not to measure from the coating and aim for .0024" clearance. How much boost pressure are you running and how has compression turned out after engine break-in? Mahle recommended me to run .017" ring end gap on both top and second rings, while I've heard running a tighter top ring and looser bottom helps with running high boost. What did you decide to run your end gaps at?

.016 tops and .018-.020 second ring gaps. 30 psi is the max boost if ever this motor will see maybe next year but not any time soon. I will be on a 16g for a while until I can afford a big upgrade. Not sure on the compression #s as the motor is waiting to be installed this next week.
Reason you want to run a slightly larger second gap on boosted engines is that it allows the some cylinder pressure to get past the second ring to keep the top ring sealed tight and not flutter.
Your machinest has to guess how thick the coating is. He should be told to measure 8mm from the bottom of the skirts as spec per Mahle.

 

[TSIfreek]

Started my motor for the first time today and those mahles make NO noise.

 

[blazebolt]

When I did my build, I emailed Mahle directly about the clearance specs for their 8.5CR Stroker Pistons and below is their response. I told them that I'm only running 20psi, so you will want to go with slightly bigger ring gaps.

Ø3.366" is the finished boresize.
Ø3.366 should give you 0.0015 - 0.002" of clearance, over the grafal coating.
It will actually be 0.003" to the aluminum under the coating, but you cannot measure that.
If you're planning on 30+psi, add 0.0005" to that. Or if you're going to run nitrous.

gaps:
top = 0.017"
second = 0.016"
oils are okay out of the box.