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MG MGB Technical - Compression test results - follow-up

You may remember this thread from a little over a month ago:


TheMiz, Missouri, USA

OK, if anyone read over on B General about having PopTop towed home Thursday...

We just did a compression test. Results follow in this order: Cylinder #, dry, wet

1 - 136 - 153

2 - 105 - 119

3 - 122 - 135

4 - 140 - 153

She's still running, but a little rough.

Other issues:

Puffs of blue smoke on acceleration (rings)

Burns 1 quart of oil per 100 miles

No mixing of coolant and oil

No loss of coolant

#2 plug is fouled and threads have oil on them


So, she did her last run of the year yesterday and today I pulled the head.

This is what I have found:

Piston 2 is clean and shiny.
Pistons 1 3 & 4 are seriously coked up.
Bores still have honing marks on them, there is no wear ridge at the top of the bores.
Bores all measure 81mm (stock is 80.26) using an internal micrometer.
Pistons look standard, there are no markings on them, there are large gaps down the side. Seems DPO had the block bored but didn't fit oversized pistons!

Onto the head:

It's bent, about 3 thou out of true.
Chambers 1 & 4 are pretty clean and valves were sealing.
Chamber 2 is very badly coked up, exhaust valve is badly pitted and obviously not sealing at all.
Chamber 3 is also coked up but not as badly as 2, exhaust valve is pitted and burnt.
Valve springs on 2 & 3 are weak, I can open the valves by hand pretty easily.
DPO forgot the valve stem seals - there are none there!
Valve guides seem good, sideways play was almost imperceptible when I pulled the valve out and wobbled it.

Next stage is to pull the engine and strip it to see what else DPO did.
The Wiz

Wiz: thanx for posting your continuing saga. The heavy oil consumption goes right to lack of valve stem seals. I would definitely magnaflux the head and see if it is salvagable. Check the cam for wear too.


gary :>{D

"heavy oil consumption goes right to lack of valve stem seals"

No sh!t. OMG, I about came unglued when he told me that!!!

I cannot BELIEVE the stupid cr@p that guy did to MY car.

He ported and polished the head, but did stuuuuupid stuff like the small pistons and the lack of guides.

WTF was he THINKING????


Miz ---

Sympathize with DPO's spouse. She has to put up with him, small piston and all.

Dan Robinson

Nobody's been dumb enough to marry him yet.


I just put some new valve stem seals on my car, from the hand written invoice they are part number 12B2104. Although I bought them at Summit I believe they are Moss parts. They seem to work well and the tappets are quieter (not silent of course, it is a B series engine). There was some discussion whether you could fit them to inlet only or all valves. In the end they went on everywhere and so far so good.
Stan Best

Wiz & Miz are you sure the valve seals are missing ?
I/m not totally up with the cars as sold in the states but the ones here in OZ are usually fitted with the little O rings that sit under the valve collets and can be easly missed. A lot of people fit them wrong and just slide them onto the valve stem which does nothing. When fitted correctly are superior to the mushroom seal that some fit and last much longer. It sounds to me that you have ring problems as your compression is all over the place and seals wont do that. Crook seals will cause smoking at idle but if the guides are not very worn will not cause very heavy oil consumption. Denis

I find it hard to believe that your block was bored and didn't have new pistons fitted. You cannot really tell from the top unless they are marked and the part above the top ring is usually considerably smaller in diameter that the nominal bore. The only way to measure the piston is by taking it out and measuring on the skirt just below the gudgeon pin and at right angles to it. The bores are going to need checking again and at the very least new pistons fitted. No2 is very suspicious to me, was one piston fitted or is this the result of an antifreeze leak cleaning the crown. I,ve seen this before and the piston gleams like new. Whilst the lack of valve stem seals has done nothing for oil consumption I cannot believe that so much oil can ever be sucked down there even without the seals. The pistons/cyls are the culprit as witnessed by the poor compressions. I normally only fit the O ring seals on the inlet valves and have no trouble but you could try the hat type on them all if you wish. New springs might be a good idea as well.
Iain MacKintosh


"A lot of people fit them wrong and just slide them onto the valve stem which does nothing."

Can you elaborate? I went with the top hat seals last time but have reverted to O-rings as the former disintegrated pretty rapidly.

Steve Postins

I agree with Iain - its highly unlikely that the block is bored with stock pistons. Your cylinder walls would be beat up from the piston skirts slapping them. The engine would have knocked something terrible and due to the improper rings, you would have had very little compression - maybe in the 80 range.

If I had to guess, you have a bad valve guide at cylinder #2 which is coking up the head and causing detonation problems. That's why the piston is clean. Either that or the coke built up so thick on the piston that it was hitting the head and it came loose. Recheck the #2 exhaust guide at around 1/4" valve lift. That is where it will have play.

I hate to say it, but maybe you should take a deep breath until the machine shop really evaluates the situation. It may not be as bad as you think!

Jeff Schlemmer

Steve's point about the life of the hat type seals is well made. They have to put up with the movement of the valve stem so will wear quickly. The O ring type are fitted AFTER the spring is compressed and lies immediately below the cotters. It therefore forms an effective stationary seal and will be both effective and have a long life. I think you must wait until the block and head are inspected by the machine shop.
Iain MacKintosh

Steve What Iain said. The O ring is captive in the top cap and compressed by the collets if fitted correctly and will last the life of the valves. Denis

I was told not to put any sleeves on the exhaust by several well versed engine builders as the exhaust valve stems need all the lube they can get. Good ol lead used to lubricate the valve stems but not anymore.
Mike MaGee

I don't use seals on the exhaust valves and have no oil consumption problems. In theory at least its only the inlets that need the seals as the induction will help draw the oil down the guides.
Iain MacKintosh

Well those naughty top hat seals, they both wear too fast and let oil through, but it's not enough to lubricate the exhaust valves. Anyway interesting to see where the 4 or 8 per question came from. The top hat seals have really cut the blow by in my engine which matters to me as I cant get it to behave itself with the PCV valve on. (bored to 1950 and probably just squirms around too much to work with what was always a marginal system). Also very interesting that you can run without exhaust valve seals although the scavenge effect in tuned engine might suck some oil out.
Stan Best

Oh there's another issue Stan. You say that the hat seals have cut the blow by and this can only be after the exhaust valve opens and the gas is pushed out of the cylinder. You say that they have a short life and you may have to suffer this as you can easily change them without having to lift the head butit seems to me that O rings should be fitted on inlets and hats on exhausts.

Any comment?
Iain MacKintosh

I've just rebuilt my cylinder head using O-rings but it sounds as though I took the wrong option by fitting them on the valve stems. In my defence, I've always placed them immediately below the collets in the past, but the workshop that did the seat work insisted they should sit on the stem and would find their level when the valves open. It made sense but they were not mg specialists. Oh well. What they did say was to never use top hat seals on the exhaust valves for the reasons above, as they are a bit too good when they work.

On that subject, the last time I did this head I used top hat seals on the inlets only and they managed about 12,000 miles. By then, all 4 were split and hanging off the bosses. I was getting a tell-tale blast of oily smoke on starting but more worrying to me was that two of the exhaust valves have not been sealing thanks to lumps of hard brown ash on the valves.

Looking at the mess on the valve stem above the lumps, I suspect they formed from oil coming down the stem, so have used the O-rings on inlets and exhausts this time.
Steve Postins

Not sure it's that simple. There will be pressure pulses in the inlet*. As the valve shuts off the inlet flow there will be a rise in pressure, and with the overlap of the 270 cam it will have exhaust gases there at tickover speeds.There is also that annoying puff of blue smoke as you start up caused by oil draining down the valve guides, nothing to worry about but to the unitiated it looks like you have a worn engine. My car only did this sometimes and I would expect it to be less while the new seals work. As you say you can swap them out without removing the head using the rope via the spark plug hole trick although even removing the head that is not that big a job on the B.
* if anyone has a link to some traces it would be great.
Stan Best

Steve,I wrote the above before your post and link. 12000 miles is about 4 years for my car so I can file this in too difficult for now. I will lift the rocker cover and check them at the close of the season next year, everything else being equal of course!
Stan Best

Interesting point about the hat seals and o-ring style seals. I will check my valve train for these style seals.

Many years ago, I had a Ford 302 which used a quart of oil about evry 300 miles. It only had 71k. Replaced the valve stem oil seals. We discovered about half of them had broken of with time. what was remaining was brittle and pretty much shattered when removed. After the job was done, the oil comsumption dropped to 1500 miles / quart. We did replace the seals with the hat style.


Gary :>{D

Tried to change my valve seals last weekend, removed the #1 rocker and with an overhead spring compressor could not quite compress the spring enough to completely expose the collets (only about half came up after a light tap). I stuffed rope into the cylinder, but also have the air compressor set up to screw into the plug hole (did not try that). I hate to pull the head if I don't have to because it has never been off in 103K miles, compression is good, even across the cylinders with no apparent leak in the head gasket. Any ideas before I give up on trying to replace the seals with the head on?

Joe The compressed air system works terrific, I/v been using it for 50 years on all types of OHV engines and never had a problem. Two things to look out for are to have the cylinder exactly on TDC compression and in top gear, hand brake on, to stop the engine suddenly turning over, and give the spring cap a solid tap with a socket or something similar before using the spring compressor. This frees up the collets. Most mechanics I know use this system as it has the advantage of holding the valve hard up where the string method is still a bit soggy.
I agree with Iain about the hat type seals being a problem on the exhaust valves but it is ok to use the O rings on all valves as they control the amount of oil that gets on the valve stem but do not stop it completely. I have been repairing these and all other types of BMC engines for a long time and will only use the O ring seal. My SC B has them and at close to 80000miles still uses little or no oil. They work well even with bronz valve guides. Denis

We used the shop compressed air on the Ford 302. After removing the valve rocker arm assy's we just let the compressed air push the piston to BDC and it held the valves in place fine.

gary :>{D

"After removing the valve rocker arm assy's we just let the compressed air push the piston to BDC and it held the valves in place fine."

You were lucky it did go well. If you had lost air pressure the valve would have dropped in the cylinder and the head would have to be removed. That's why Denis said to turn the cylinder you are working on to TDC compression stroke, you can reach the valve stem if you lose air pressure.

FWIW, Clifton
Clifton Gordon

I am considering removing my stock cast iron exhaust and having it ceramic coated as one of my winter projects. I also plan on replacing the tappet cover gaskets with the manifolds off, to take care of some slight leaks. The local coater charges about $50 for coating, plus I would have the cost of gaskets. I would be doing this for reduced heat under the hood, better appearance, and perhaps a slight improvement in power by keeping the intake cooler. What are the opinions on this improvement, is it worth the effort (I have plenty of time in the snowy winter for such a project)

This thread was discussed between 30/10/2005 and 05/11/2005

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