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MG MGB Technical - MGB L series oversize valv/ head query

Just acquired a used MGB BIG VALVE head, a series L

Apparently these heads require EXHAUST VALVE CUT OUTS to the block

(see )

As a newbie, I am curious as to what that actually means?

I want use this head on my 1800cc Mk1 engine and would appreciate any info

I dont intend to carry out any specialised machining myself, that's best left to the guys that do it on a daily basis.

But would appreciate some background info as to process and requirements as well as any insights and/or pitfalls
Mark Thompson

Peter this is one for you to answer. But Mark just Google Peter Burgess, you will get all the story about the block cut out. You can do it yourself with care. ( is that not right Peter? ) Just make sure there is no pushrod holes in sight! I have not lived this down yet. Mike
J.M. Doust

Need to have two "eyebrows" cut into each cylinder of the block. If you are not on standard bore and up around 30 or 40 thou over then I beleive the eyebrows are unnecessary. They are there as a precaution in case the valve were to come too close to the block. Check with your local Mg guy or machinist about cutting them in.
A J Ogilvie


is this covered in Peter Burgess book?

And if so, is it the

"How to powertune MGB engines for Road and track"?


Mark Thompson

Yes, but check his web site. Good fotos and actual dimesions. Mike
J.M. Doust

You will only need cutouts for the ex valves as the inlets are nowhere near the block face even with 45mm race inlets. 18v (open chamber style) heads were shallower chambered than the early heads and the ex cutouts are necessary. To be honest the block needs doing even for early heads as not many are still standard depth.

P Burgess

As Peter says, it is the head depth / valve lift that governs whether you need eyebrows, not valve size. I believe the valves overlap the bore a bit on all heads, at least for the MGA.
Art Pearse

Have a look here - this should answer most of your questions:

David Overington

This thread was discussed between 03/10/2011 and 04/10/2011

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