Bigger 885 valves on 731 head - will sleeving be required?

[agisthos]

I have seen some 731 heads with the 885 valves, which are 2mm bigger on both the intake and exhaust.

Any problems doing this without sleeving (EDIT valve seats) the head? You would presume the heads are very similar so no, but the 731 has a smaller combustion chamber so perhaps the valves are seated differently?


And this is not for an ETA block build, but for the standard M20B23 bottom end. I am not sure if bigger valves will introduce any clearance issues on the B23 bottom end.

[mbonder]

Not sure what you mean by Sleeving the head? Sleeving would be done on the bottom end where you repair a damaged bore by inserting a sleeve, say if you get a score in the cylinder wall.

You would have to get your head machined to accept a larger valve (regardless of how much larger the valve is), but the size is measuring it's diameter, not it's length, so there wouldn't be any additional protrusion into the cylinder by having a larger valve. Only thing to be concerned with is depending upon your pistons, you may need to machine the piston crowns to fit the larger valves.

[digger]

iirc you will need new seats (sleeves?). if the OD of seat insert is larger than valve GTG. i just dont have a head closeby to measure

[agisthos]

Sorry, yeah I mean to say valve seats, not sleeves.

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Only thing to be concerned with is depending upon your pistons, you may need to machine the piston crowns to fit the larger valves.

That's exactly what I don't want to do, take out and disassemble the engine. I just had the bottom end fully balanced and rebuilt.

But the 323i bottom end uses flat top pistons, not domed like the 325i, so maybe no clearance issues using 885 valves? That would make everything so much easier.

The flat top piston has like the faintest notch only on the intake side, and its probably only there for aggressive cam timings.