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MG Midget and Sprite Technical - Compression Test 155 155 90 155 'SOLVED'

I took the head off and removed #3 exaust valve and this is what I found. The midget only has just under 13,000 on it after the rebuild, is this normal wear for that many miles? Anyway, I guess I'll be getting a valve job done.

Jon Saylor

Another shot ...

Jon Saylor

And one more ...

Jon Saylor


Doesn't look normal to me for that mileage.

C R Huff

Do you have high lift rockers?
Maybe the valve lash was too tight to let the valve cool down.
I'd check the others to.
Alex G Matla

The reason I described a possible "V" notch on the earlier thread was that is how a burnt valve usually starts, beginning at a point on the circumference where the valve contact is poor. In this case it is burnt all around, rather than having hot exhaust gasses forced through a narrow point. It looks as if that valve has been stuck open for a while. Maybe the valve stem bent enough to prevent it closing?

Guy Weller

I agree with Guy. Just my opinion here, but the the way that valve looks rough and black all around the rim makes it appear that it's been seating very poorly - like it hasn't been making full contact with the seat, that is. You can't see any conspicuous places where it's been leaking (i.e., no V-notch), so it's been allowing blow-by around most of its circumference.

Keep us posted!

Gryf Ketcherside

How are the others?

Was it running lean?
Which makes it hotter

How was the fit in the guide?
Can have bad effects on the cooling if it is to loose.
Onno Könemann

I'm going to have to agree with Gut also. I'm going to use one of the exhaust valves off a spare head I have (although the valve stem seems straight). I was thinking the springs on #3 might be a little weak (?).
I can have those checked.

It might have been running a little lean. I'm going to richen it up some.

Here's a couple photos of a two other valves. All the rest looked pretty much the same as the on here.

Exhaust valve. The white ash looking color it good ...... yes??

Jon Saylor

intake valve ..

Jon Saylor

What do the head, top of block, and valve seats look like? Was there clearance on that valve before you took it apart? That is a lot of deposits for the mileage, and the bad one looks like it is maybe rusty = possible head gasket leakage. Does it use oil? Chunks of crap or rust breaking off can lodge on the seat causing burning. In any event, the seats can be expected to have suffered as much as the valve, so take it all to a machine shop for a real valve job.

Fletcher R Millmore

For only 13,000 miles the "good" valves are a bit of a mess too - at least the exhaust one is. They should be much cleaner than that and the deposit on them should be more of a pale straw colour rather than that white. It does suggest that it has been running weak.

All the valves certainly need lapping in again - even that inlet has signs of seat pitting. A slow job by hand - you would be better to get the valve seats re-cut first.

Guy Weller

Hi Jon

it look like you have some oil burning? Number three valves tend to run hotter in A and B series. The picture of the second ex valve looks like there is black oily carbon on the margin. The inlet valve looks oily too. Is the carbon on the the chambers oily to the touch, ie leaves a little wetness on your finger tip rather than feels dry and hard? The ex failure looks similar to what I would expect from oil contamination issues with A series which usually presents iteself with burning out number three ex valve.

P Burgess

Well i have to agree with the above posts
re do the valves and seats
and have it setup properly perhaps on a roling road to be sure.

Onno Könemann

This thread was discussed between 19/08/2010 and 20/08/2010

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