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MG Midget and Sprite Technical - Pocketed Exhaust Valves!!!

I'm in the process of rebuilding my engine (1330 A Plus engine with 266 cam) and discovered that the exhaust valves are very badly pocketed. It is a 12G940 head, heavily reworked with 1.41" inlet valves and 1.15" exhaust but no unleaded inserts! It's done a lot of miles after rebuild in 1999 and I believe the head has never been removed and decoked.Also the PO never used any fuel additive, which probably didn't help one little bit!
I had the seats recut but the exhausts are too far gone and are about 100 thou further in than the inlets. As I see it I have four choices;
1. Use it as is, with uneven spring pressures between exhaust and inlets, and differing geometry on the rockers.
2. To try and have inserts fitted, but my local engineering shop is very doubtful about how succesful this would be, as they feel the pressure used to fit the inserts could easily crack the inlet ports. Anybody had a ported head retro fitted with valve seat inserts?
3. I think I could fit 33mm valves instead of the 29 mm ones that are in there which should have the efffect of lifting the valve out of the pocket. Does this sound feasible?As I only do limited mileage and do use additive maybe this could be the cheapest option, just needing me to do some enlarging of four exhaust valve throats.
4. Scrap the head and use a spare,unleaded big valve Metro head that I have purchased, in good unmolested condition. Any idea how much would need to be skimmed to get a decent compression ratio?
Any comments or thoughts?
Graham.
Graham P

266 cam is pretty mild so I'd go for the Metro head, not worth buggering around spending out on the current one with all the risks.
David Smith

Nick Swift fits inserts to rescue some race heads so might be worth approaching if you want to rescue your head...

I'd be inclined to stick in a 276 or 286 cam if you're wanting a worthwhile improvement over a standard cam. You need to run about 10:1 CR too... I'd go for a new cam rather than high lift rockers on a road engine

James B

I had a head (exact origins unknown) that had a cracked exhaust seat and heavy pitting on the others beyond that which would be removed with a simple lap of the valves. Also had a reasonable amount of porting done etc with the 35mm inlet and 29mm exhaust valves.

I was sceptical if it could be saved but Scholar Engines in suffolk fitted new exhaust seats quite happily.

If they try the new seat and it does crack the ports what have you lost - an hour or so labour from the Machine shop ?
Dean Smith ('73 RWA)

If your old head is that bad then I would try the Metro head and see if you are happy with the performance. Your old head might be that bad that a good new head would give better performance.

I don't think it needs skimming that much to be honest, you would have to measure the chamber cc and do your calcs to find the compression ratio.
Tarquin

I had a chat this morning with Ian hargreave of AVR classics and his conclusion was that inserts probably wouldn't be useable due to the amount of pocketing! However he suggested trying 31mm Coopers S valves which will fit in the existing throat and guides which 32mm would not, so he is sending me one to try out, in the meantime I shall use the Metro head after a regrind and clean up.
Graham.
Graham P

My MGB big-valve head has the valves only 30 thou apart and has inserts fitted. What we have done to get around the cracking is to insert both inlet and exhaust, with the exhaust insert cut into the outside of the inlet insert a bit. This means that the inserts push on each other rather than just one exerting pressure on the thin bit between the ports. It has been a very successful arrangement, allowing the throat configuration and diameter under both valves to be at their optimum.

We adopted this arrangement after inserting just the exhausts caused one of the inlet seats to crack within a couple of years and I was reluctant to scrap a head that was the best flowing B head we'd tested. Fitting the inserts to the inlet did not affect the flow once they were blended in.

That was in the 1980's, so I guess you could say it's been successful.

We didn't go unleaded here until the '90's, the reason for the inserts were pocketed exhaust valve seats. At the time, most inserts being fitted were cast iron but the machine shop suggested new fangled stainless ones (which were on the market here at the time for CNG/LPG conversions which abounded at the time) as they were stronger and so could be machined much thinner, allowing the throat size I wanted while still just avoiding the inlet seat.
Paul Walbran

This thread was discussed on 03/05/2012

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