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Piston ring gap turbo build

General Engine Building Discussion

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I am currently assembling a bmw s50b30us which has been bored over to 86.4mm for a turbo build. (Using s52 crank and cams so it’s basically an obd1 s52 and/or s52b32) As far as ring gap is concerned a buddy of mine suggested to gap the top ring .024 and the second ring at .026 (forget the percentage these figures equal to over stock gap)

Does this sound right or would you recommend another size or percentage over stock spec?

You haven't mentioned what fuel you're intending to run or how much power and boost you're wanting to run so it's hard to be too specific. For moderate boost use on pump gas 24 thou is quite a lot a little looser than I'd suggest running. Usually I work on the basis of around 5 thou per inch of bore for the top ring so on an 86.4 mm bore I'd be around 16-18 thou. You can add 2-3 thou to the second ring.

Sorry it is a moderate turbo build on pump gas (91octane) Another aspect I am still learning and trying to understand is how to “state” what it is I am running rather than saying 15psi. My turbo is a Turbonectics 62-1 with a T4 flange .81 ar that I plan to change to a T3 flange 62ar. It is a track car that will be in the upper rpm range most of the time. The compression I have calculated is roughly 9:1 to 9:5. I say roughly bc I can seem to get any solid info on my block deck height as well as head. I have the specs but nothing to compare it with. The head and block have been decked at least twice that I know of which is almost forcing me to run a .140 hg to avoid piston to valve contact.

On another note I have a question about new lifters and re using cam trays. Is this something that can be done? Should I post a new topic for this question? Thanks again for all your help.

One more question on ring gap. Where in the cylinder do you recommend the ring be placed for the gapping process? Hastings recommendation is at the very bottom where the ring does not travel since it is not worn. This does not seem logical as this is not where the ring will be expanding. What is your process and thanks again for all the help?

Given your use my initial recommendation would still stand. For circuit use with sustained high load use you could choose to increase the gap by 2 thou for a little peace of mind.

The particular engine you're dealing with isn't one I've had experience with so I'm not 1005 sure what the valve actuation is. Can you fill me in here so I can answer accurately. I'd assume it's either a hydraulic bucket arrangement or a finger follower design but I can't find any easy info with a quick search.

When gapping the rings I place them around 20mm down the bore. You'd struggle to accurately locate them at the bottom of the bore and most ring squaring tools are only going to move the ring down about this much.

It’s a hydraulic bucket style. I know when re using the same lifters you want to put them in the same position rather than mix them up. I plan to buy new lifters due to pitting on the current ones. The last thing I want to have to do is buy new trays as well. Thanks for all your help.