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MG MGB Technical - U.S. version V engine nominal compression

After a cylinder head rebuild and signs that the head that came off had not been properly torqued, compression is 90 PSI straight across.

The rebuilt head has been on for 5,000 mi almost all highway driving.

1) Does a Payen gasket need to be retorted?

2) 90 PSI seems low when the Midget has always been in the 140 PSI range. On the other hand, the B runs perfectly on regular fuel with no run on while the Midget has a dent in the hood where the engine broke free from its mounts running on in the 1980s when premium fuel was not available.

3) Idle and running are extremely smooth, even with the A/C compressor engaged at idle.

I would prefer to leave torque alone. I torqued the head to 48 PSI when assembled. Valves are quiet. Let sleeping dogs lie?

Thanks.
Glenn Mallory

I don't think Payen need retorquing, I haven't.

The book quotes 130 LC and 160/170 HC.

As fas as I'm aware compression pressure should be done hot, all plugs out, and throttle wedged wide open.

With all cylinders the same I'd be trying another gauge, or your gauge on another car.

But if your Midget is showing 140 now then it has to something with the MGB engine. Valve clearances? Some cars seem to have a weird cam where the greatest gap is not at the usual 'rule of nine' point but is changing randomly all the way round the back of the lobe, varying from lobe to lobe, where in theory it should be a constant base circle. If the gap on inlet valves is biggest shortly before they start closing that will delay their full closure and reduce compression pressure.

But that seems unlikely with all cylinders being equal, and if you adjust both cars the same. Which leaves mechanical components.


paulh4

Just a thought: Was the original cylinder head rebuilt, or swapped for a rebuilt one?
I replaced my original head with a rebuilt one (23 years ago...) and since then have the bad combination of a LC cylinder head combined with LC pistons...Resulting in very low compression of course. Plan to replace the pistons by HC versions this spring/summer.
C.Boerop Kees

The Payen gaskets are supposed to be monotorque, but good practice is to retorque any non-metal gasket if you want it to go the distance
I do them after the first heat cycle and again after a few hundred kilometers, there's always a couple that move, usually out at the ends or along the manifold side
There are a few differing views as to lubed or unlubed studs/nuts, I'm a luber and do them with a smear of a mixture of oil and molly assembly lube
If you do this retorquing is easy, I don't back each nut off before retensioning, no need with the molly lube, just pull them up
works for me
willy
William Revit

I retorqued the head using a different torque wrench (the previous one was borrowed. It took lots of additional torque so I suspect that the wrenches are probably both defective (first one low, second one too much torque).

I had to back valve adjustment off some after the retorque. The engine is running fine but I have not had the nerve to check compression yet. I am going to run it for a couple of thousand miles and then recheck to make sure that any potential over torque didn't lead to uneven compression readings between cylinders.

Time to order a high grade torque wrench.
Glenn Mallory

I agree with Paul on how to perform a cylinder compression check--the engine has to be hot, so that it is checking the engine in the normal running condition, and the throttle must be fully open to allow an accurate reading. Any other method, except one, will give you an inaccurate result. The one other useful test is called a "running cylinder pressure test" and is performed with the engine heated up to normal operating temperature, then a single sparkplug is removed, the compression tester screwed into the spark plug hole and the engine started and run for a minute or so. Whether this method gives you additional information (as compared to the static testing method) remains to be seen. Most of the time it does not, but, on a few occasions, it has shown significantly different readings than the static test, offering some insight as to what may be going on.

As to retorquing of the Payen type head gasket, I agree with Willy. Everyone I have installed over the years showed the need to be retorqued after initial start up for cam break-in. Several people, whose opinion I respect, have told me that they never retorque a Payen and have never had a problem with the engines they have built. Thus, I do not know if there is a definite opinion within the community about this. But, I have never had a head gasket problem when I have retorqued after initial run in, followed by a six month tune up of the rebuilt engine. Les
Les Bengtson

Les, I have done the "running cylinder pressure test" and find results similar to the "all plugs out, coil disconnected" wah, wah, wah, wah test (hot).

After several thousand miles of a rebuild of the Midget engine with a Payen head gasket once, it developed rough running and loss of power. Re-torquing fixed it.

So I think that even a Payen head gasket should be re-torqued. I find interesting the low compression I have observed on the B since I have owned it (90 PSI) compared to the 135 - 140 PSI that has been typical for the 1275 since the car was new. One runs on low octane fuel and the other is picky. Silver lining stuff I guess but driving the Midget is great fun after lugging around in the B.
Glenn Mallory

How much effect does the camshaft have on straightforward comp tests? Does the amount of overlap lower the readings?
Allan Reeling

Valve timing very much has an effect on compression readings where the intake is still open when the piston starts its compression stroke with the standard MGB cam for example. A tight gap will reduce the reading significantly.

The problem comes when you have - like mine and I know of others - a cam where the gap is not consistent round the back of the lobe as it should be. The max gap can be anywhere and vary from valve to valve.
paulh4

The inlet valve closing point that affects the compression readings rather than overlap. One example on an engine with 12:1 nominal CR.
With cam profile closing the inlet valve at 80 deg, cranking compression reads about the same as std compression and std camshaft.
With std camshaft in the 12:1 engine, cranking compression is about 50psi higher.
And very noticeable on the starter.
Paul Walbran

This thread was discussed between 18/03/2020 and 02/04/2020

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