Supertech valves

[zaq123]

M20B25; 885 head

Shop will be cutting valve seats and SuperTech valves will be installed (they were cheaper than OEM M20 set). I have a few questions for you guys:

ST valve spec specifies 6.95mm stem, OEM calls for 7mm (Bentley). I measured my OEM valves and none of them 7mm.
1.Is Beantley just not being precise with this measurement or my poem valves just worn out that much (hard to believe)?
2.Will there be a problem with OEM Spec valve guides and OEM valve seals with SuperTech valves?
3. Shop said they do not have 15-45-75 to cut the seat. They do have 45 but the other two will have to be special ordered just for me. Big $$$. How important is it to keep OEM spec on the seat? Anyone has any experience with this?

Thank you in advance

[digger]

7mm is the nominal bore size of the guide not the size of the stem. normally the valve stems are a bit smaller something like 6.96mm+/- depending on manufacturer so there is clearance. i dont know what the BMW clearance specs are other than the bentley method which is often different to how others specify it

the shop should be able install new guides and ream/hone as required if the current ones are worn or not suitable. i wouldn't assume that they can pop straight in. the clearances need to be good to be able to cut good seats to so dont skimp. you do want a multi angle valve job. the stock one might not be absolutely the best youd need to flow test various configurations but perhaps find a new shop if they can only cut 1 angle and dont have much equipment as its not a good sign. it may not matter much if the seats are good and they just very lighty clean up the 45 assuming you are using standard sized valves

[zaq123]

Thanks for the reply. Shop can do 5 angle cut if I ask but they just don't have OEM 15-..-75 setup.

I kind of figured that valve stem can't be exactly 7mm but Bentley threw me off since they specify that under parameter "d- valve stem thickness" as 7mm.

I'm trying to understand possible geometry changes when valve seats are recut:
1.I'd take valve springs will need shims to maintain correct spring pressure.
2. Does OEM installed height for the valve guide change?
3. anything else will be affected by the recut of valve seats?

[jeffnhiscars]

The guides should all have the same inner bore but it's the outer bore that varies to properly installed in the head. My guy always pops out one of the old ones before ordering replacements. Stock is 13.00 then they go to 13.05; 13.10 etc etc. get the wrong size and you can easily crack the head or have that not stay in place. They freeze then get drifted in so clearance is crucial.

New valves call for new guides and if you are doing a cam you should either face or replace the rockers

Cutting seats should not impact your spring pressure any more than age will. They can be gauged or bought for about $6/spring.