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Ring Gap Clearance Recommendation for BMW B48/B58

Practical Engine Building

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Good day,

I am currently looking at doing an engine rebuild on a BMW B48 engine (basically 4-cylinder to the B58 supra) with CP Pistons and CP Rods.

I have viewed the engine rebuild course.

For reference, BMW recommends the following:

1st ring 0.015mm-0.025mm which is about 0.006"-0.010"

2nd ring 0.003mm-0.045mm which is about 0.011"-0.018"

However, the pistons are upgraded from stock cast iron to forged CP pistons.

The cylinder bore is stock 82mm (3.23") and the compression ratio will be left stock 11:1.

CP recommends the following:

1st ring 0.018"

2nd ring 0.021"

Typically, piston manufactures specify a ring gap clearance which are a bit conservative.

I have a friend who used the same pistons and the following ring gap he used (found online on CP website for less than 750hp):

1st ring 0.016"

2nd ring 0.017"

My friend has no issues and running 27psi boost on bigger turbo.

I have seen many b58s and b48s rebuilt, getting blow-by issues and plenty oil in intake.

See attached, I included a ring gap calculator from CP, the recommended ring gap which came with the pistons and also the recommended ring gap I found online for less than 750hp applications which my friend used.

Based on your experience what piston ring gap clearance would you recommend?

The main application would be daily street use.

Thank you.

Regards,

Shane

Attached Files

Hi Shane,

Typically for a forced induction, street driven, performance engine with forged pistons I’d be looking somewhere in the order of 0.005” per 1.0” of bore for the top ring which would put you right around where your friends build is.

From experience if I’m unfamiliar with the engine I would always err on the side of caution and go a touch bigger as it’s much cheaper and easier to replace a set of piston rings if the blow by is too high over potential lunching the motor by setting them too tight.

Ultimately though CP will know their products and that application and their recommendations are based on experience and engineering so that would be a starting point.

We have to factor in that the latest GDI engines are designed to specific emissions regulations so will run hotter due to higher boost levels, lean burn conditions, increase combustion efficiency so ring gaps will have to be increased to suit.

Hi Scott,

Thank you for your inputs, what ring gap do you recommend for the 2nd ring and the oil ring?

Regards,

Shane

Hello Shane,

As suggested previously I’d go with the CP recommendations, they’re a touch bigger than what we’d see conventionally but it is much easier to deal with blow/by than rings butting up.

We usually reply within 12hrs (often sooner)

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