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MG Midget and Sprite Technical - Valve pitting?

I have the head off the 998 as the gasket blew for the third time. It has been ground flat now - it was warped 6 thou. Now when cleaning up the parts I found what looks like pitting on the seat area of the valves (see photo). This is an exhaust valve, but the inlets also have marks although less obvious. The seats in the head are fine. I have tried lapping but the marks don't go away. However I can't detect any indentations when probing with a pin. Is this really pitting or just discoloration? Am I worrying too much? This head has only done 13,000 miles and compressions were perfect before the gasket went.

As an aside, I am very fed up with failures of new parts. The first head on this engine after the rebuild was a new Metro casting and it cracked in the roof of the centre exhaust port. I then paid £400 for another new Metro head with bigger valves, and that has warped. It really is too much.

Les



L B Rose

Les

Re. head failure I think you need to establish root cuase. My first thought is that you are running too hot. I wonder if the damage seen on the exhaust vvs is due to excessive heat?

Check mix, coolant flow, timing, valve clearances carefully on re-assembly.
Oggers

I cannot see how you blame the head for warping? How flat was the block before you fitted the head?
The pitting on the valve seat looks like text book carbon indentation to me. This is bits of carbon trapped between seat on valve and in head causing a little 'dink'. Over a period of time all the dinks join together and the seat looks normal. We have seen this early dinking over say 1-2000 miles. The dinking shows up more on the ex valves due to more carbon through put that direction, hotter so softer valve material and the ex valve is 21/4N stainless which dinks more easily than the EN52 inlet material.

Peter
Peter Burgess Tuning

Thanks guys. I had the block decked when I built the engine, and the pistons set flush with it. I have heard of blocks going out of true but the machinist said the head was warped and I think that's more likely. It's not running too hot. I have a thermostatic fan and it never cuts in unless idling in traffic for 5 or more minutes. I calibrated the temp gauge vs a known thermometer and it shows no more than 190 ºF when cruising.

So Peter, dinking is normal? I have never seen it before, over 50 years with these engines. Does modern fuel cause it?

Les
L B Rose

It was first explained to me 30 years ago. I think we were the first to run Bs on unleaded fuel. After 1000 miles testing we removed the head for inspection. We noticed the pitting on the ex valves and sent em back to G&S valves for inspection. They said it was standard process of breaking in. They did not mention fuel as a reason just said standard part of the process. We have seen this on many occasions and had to explain it to worried customers. We tend to strip around 4 or 5 heads a week so we have seen a lot of 'oddities' in 33 years! Remember how often the old 850/100 non stainless low duty ex valves burnt out? Folk used to fit the 12G295 better quality ex valves to improve matters. Small and large bore A series still tend to burn out ex valves.
If everything is set OK why do you think the head warped? What gasket were you running and what material and how many miles from fit before each failure?
The ex port cracking we associate with 1275 heads. I think you may have overheated the ex ports, maybe ignition, fueliing or valve clearances or all the above. What spec is the engine in terms of ex system, in, cam etc etc
Peter
Peter Burgess Tuning

Thanks Peter

You can read the engine spec in my profile, but briefly I closely followed David Vizard's recipe for a road 998:

Kent 276 cam (clearances exactly as in the Kent book)
CAM4810 head casting
10.5:1 CR
SU HIF44 on MG Metro manifold
Inlet valves 31mm
Exhaust valves 27mm
A-Plus distributor with pertronix pack
Maniflow medium bore LCB

I always use the competition type Payen head gasket. I do remember the previous head looked quite discoloured above the centre port, while this one doesn't.

The first gasket failed after about 4000 miles, second one 6000 later, and the third 3000 after that.

I'm inclined to stop worrying about the valves!

Les
L B Rose

I shouldn't worry about the valves. A session on the rollers to ensure all is correct might not go amiss. Fuel is changing year on year and small changes to fuelling and timing can work wonders and ensure safe and reliable running.
Peter
Peter Burgess Tuning

Les. What needle are you running in the carb?
I would have thought an HIF44 would be a bit big for a 998.
My engine is 1133 with HIF44.(can't remember what needle is in it). The car will shortly be going to Peter to be set up after I've fitted a Kent 266 cam, so I'd like to get some idea of which needle(s) to try.

Sorry for thread hijack.
Bernie.
b higginson

Bernie, I was using a BDL needle which is the standard MG Metro one, but WinSU tells me that BCH would be better so I am trying that. Way back when I first built the engine I put it on the roller at Brian Slark's and they found a flat spot so filed the BDL needle. They didn't say it was too rich.

Various people have said the 44 is too big for a 998 but as I say I just followed Vizard. I'm sure a 38 suitably modded would also be fine, but it's what I have now. It is quite thirsty on short runs, but I drive it like I stole it!

Les

PS here is the link for WinSU:

http://www.winsu.co.uk/
L B Rose

How certain are you the pretronix is functioning properly... these things are trash right out of the box... the engine could be pinking/pinging .
1 Paper

Les,

Did the head gasket blow in the same place all 3 times? Where did it blow?

Richard
Richard Wale

Bernie-Les

Just guessing which needle to fit is risky
It needs a session on the rollers as Peter has suggested
If the BDL was lean enough to cause a flat spot, filing it 'can' work but really it's only a means of finding out if a richer mixture in a certain area solves a problem
I would think that ,that process would be followed up by selecting a suitably richer needle to match the results
I would think a 998 is more than capable of getting the business out of a 44

willy
William Revit

"The ex port cracking we associate with 1275 heads. I think you may have overheated the ex ports, maybe ignition, fueliing or valve clearances or all the above"

So yes...running too hot then?

Mix and ex vv clearances would be first port of call for me....

Oggers

Daniels book has a great chapter on modifying a carb needle to make it richer and a better engine match

It's worth the 1/2 hour read
1 Paper

Sorry for late reply folks. I don't know why I never get alerts from this forum. Yes the gasket failed in the same place every time, between cylinders 3 and 4. I took the head to the machine shop and it was warped - skimmed 6 thou to get it right. All back together now. Not sure where to go for a roller session as I wasn't impressed with the last one I had. I really want someone to get the fuelling correct, and advise on the ignition advance curve.

Les
L B Rose

Les,
the only alert on the threads is for first reply.

As for rolling road session, it would be worth the travelling and still good value to visit a certain Peter Burgess that advertises on this site and posts useful info on threads. I've used a few different rolling road tuners over a couple of decades and I thoroughly recommend Peter for excellent value and practical tuning.

(Unless you want a set of exaggerated ego figures then I can think of at least one possibly more.)

http://www.mg-cars.org.uk/peterburgess/

Nigel Atkins

Damned new keyboard, should have read -

(Unless you want a set of exaggerated ego figures then I can think of at least one possibly more other tuners.)
Nigel Atkins

Interested in your comments about rolling road sessions. I have heard good reports of Slark and understood that they have lots of A series tuning expertise. I was thinking of using them myself, so would be interested to hear why you weren't impressed last time you used them?
Peter Blockley

Peter, it was a painful and expensive episode, and if you'd like to email me at lesrose at ntlworld.com I'll be happy to explain.
L B Rose

Interesting I have used Slark 3 times and its all been fine. Car has done some 5000 miles both in Europe and in the uk and all has been fine..........!

Bob
Bob Beaumont

This thread was discussed between 06/01/2018 and 11/01/2018

MG Midget and Sprite Technical index

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