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MG Midget and Sprite Technical - Valve seat angles and widths

Hi folks, happy new year. I hope you are all well. Maybe one for Peter B here...

My head gasket went pop over the festive period, clearly a sign that I needed to tear everything apart and get on with my engine build. I am currently looking at a few aspects of the head.

I have replaced the valve guides and need to re-cut the valve seats. On the 1500 the standard cut is a single angle 45 1/2 degree seat with a 45 degree valve. This creates an "interference" that creates the correct valve seal and seat width.

So... hypothetically speaking, If I were to use a 45 degree cutter to re-cut the seat, how would I determine and ensure the correct valve seat width? Would there be the downside of having too large a seat width? Or would you just cut to the standard 45 1/2 degree seat (the problem being it is more difficult finding a cutter)?

Also, does a three angle seat give much benefit on the 1500 head? Or does the port shape/valve shrouding negate this?

Yes, I could just blindly pay some shop to do just do this work (and may eventually do so), but I like to learn and understand and experience for myself what is going on.

Does anyone have a set of valve seat cutters? :-)

Cheers all,
Malcolm.
Malcolm Le Chevalier

Hi Malcolm
No probs with 45 degrees seat and if you were to reface valves at 45 degrees. Having a slightly different seat/valve angle allows quicker seat sealing when the engine is first run, the 1/2 degree difference cannot maintain long. You can only control the width of seat by cutting back the top and bottom of the seat either 90 degrees and zero degrees or better with intermediate top and bottom cut angles say 60 and 30 degrees which will give a flow benefit.
We usually cut 45 degree seat and valve face with our multi angle cutters and with our Peg seat grinder. Our SP Neway individual cutters tend to be 46 degree to give the interference you mention.
Peter
Peter Burgess Tuning

If you did a straight 45, you would have a much larger seat area than doing say a 30-45-60, right? Does this have a significant negative side, or just cut it, lap it with some fine paste and be done with it?

Cheers Peter,
Malc.
Malcolm Le Chevalier

The 45 will work fine Malcolm.
Peter
Peter Burgess Tuning

Right-o :-)
Malcolm Le Chevalier

The problem malc is cutters are brutally expensive, if you Do 100 heads it's not to bad, but for a one to 3 time job, it's not very economical

Prop
1 Paper

Here is a new set... yikes

But there is a used similar almost new set for a 1/3 The price set

You can buy junk alot cheaper... but considering the precision needed for valves I wouldn't... just use a sheet of sand paper and do it by hand you would get the almost same results

But neway is a decent name

http://m.ebay.com/itm/Neway-Cutter-Valve-seat-cutting-system/253314505226?hash=item3afab89a0a:g:urMAAOSwNAJZk3sv

Prop
1 Paper

This thread was discussed on 02/01/2018

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