T H E O L D G R E E N B E E M E R
RIDING A 1974 BMW R90/6 FLAT TWIN
NEW GUIDES ARE IN ... GRINDING THE SEATS
Using the old valves as a grinding tool
Why not ... reshape the old valves and use them to regrind the seats back
into shape. I must say it can be done this way ... provided the new guides
and the old guides that were in the head first, were bored exactly in the
middle. If this is not the case, you are out of luck and need a Seat
Cutting tool $$$ too bad. So check your new guides first before
you install them in the heads. This can save a lot of trouble.
So back to the lathe to restore the 45 degree angle to new
Enough material left on these old valves to give this idea a shot
Grinding paste mixed with WD40 works better than without, work clean, make
absolutely sure not to allow grinding paste to come in contact with the
valve stems or guides as this can ruin the whole thing!
These are hardened seats, the heads of this bike were adapted for
low lead fuel long ago, so this job is going to take some time.
Patience and a lot of elbow steam was needed. If you are
still running the old soft seats this must be easy to do.
Don't forget to oil the valve stem when doing this!
My goodness this does the trick very well, I saved some pocket money for sure !
Next, a final lapping round with new valves, but ... I took the liberty to change
the contact area into somewhat larger over what the specs recommend.
2.5mm both inlet and exhaust, 45 degrees and no correction angles.
Specs are just specs, if you have ever seen a well used BMW
airhead valve you probably know why I'm doing this.
Update: The engine runs like a charm, has a nice constant idle
and produces a real strong locomotive power.
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