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MG MGB Technical - Compression Test results

Well , what do these numbers mean . My car is bored to 1950cc , big valve head and 271 cam . I have not checked the compression since this work was done 8 years ago and just took a look today .I measured twice , the second time with a couple of squirts of engine oil in the cylinders .

1 162/170

2 162/167

3 167/172

4 162/162

There may be clue here to which cylinder the rattly tappet is on , possibly no 3 and a recon rocker assy should fix that , but what does no 4 mean . I did the test twice , and it came out the same , does this indicate that the valves are the limiting factor here , although clearly not to the point where I need to do anything immediately .
S Best

What your test indicates is an engine in very good condition. Yes, with cylinder number 4, it indicates that the seal in the cylinder was not better after you had added oil to the cylinder and re-checked. This, most commonly, is related to valves. It is less commonly related to a broken piston ring or a holed piston.

Considering all the numbers you list, I cannot find any "fault". The fact that number 4 did not come up any, while the other three came up slightly, might indicate that number 4 will need the valves lapped in before the other three will. It might also indicate a slight trace of carbon on the valves preventing a perfect seal on number 4. Such things can be rather transient, appearing on one check and not appearing after the car has been driven at highway/motorway speeds to "blow the carbon out".

Your compression readings are sufficiently high and sufficiently consistant, both dry and wet, that all they can tell you is you have a good engine. Problems are diagnosed by having signficantly differing readings and analyzing why they are different. Les
Les Bengtson

Amen, wish my 8 year old engine had specs like that! And a 1950 to boot!

Rattly tappet? Sometimes the end of the rocker can get a groove that will "fool" the feeler gauge and you get an abnormally loose setting. Otherwise, the only place left to "rattle" would be the tappet box itself and that isn't a nice place for bad noises.

Mike!
mike!

I would be inclined to dribble some Redex into both carbs while idling and then immediately switch off and leave overnight.

Next day start and run the engine up to temperature ( the neighbours will enjoy the clouds of white smoke!) and retest the compressions.

This should clear out any carbon deposits on the valves and the rings.

I would expect higher readings from a 1950cc engine unless the pistons have been left too far down the bore.
Chris at Octarine Services

Chris,
My .60 over motor has 234lbs +/- 2 PSI dry in all cylinders. The test was done with the throttle plates completely open. Are those numbers in the park? Too high maybe?
David
David

I did not jam the throttle open , just cranked until the gauge did not move up any further , it's an old gunson with a peak reading valve on it so I dont think it matters where the piston ends up .
I might try one cylinder with the thrttle open and see what reading I get . Thanks for the feedback guys .
S Best

David,

It all depends on how high the pistons are in the block and wheter the head has been skimmed or modified.

I would expect anything between 180 and 240psi from a 1950 in excellent condition.
Chris at Octarine Services

After the inputs above I did some tests with a block of wood wedged under the throttle arm , this got it about 3/4 open. This made a big difference to where the gauge ended up. I tried it cold first , and got some odd answers so ran the engine up to temp and repeated the test with and without oil.

cold dry/"oiled"

1 175 176/172

2 175 165/170

3 170 170/175

4 150 170/172

earlier nos cut and pasted to help compare ( done hot)

1 162/170 2 162/167 3 167/172 4 162/162

Also re Chris comments on pistons, yes , my block was machined , the top is about 1mm above the engine no plate so comporession ratio is OK, the head is a Peter Burgess big valve.I have no idea how close the pistons come to the top of the block though . Any based on this I may change the head gasket this winter , and measure the distance from piston crown to block deck this winter . (Sorry Chris did not get waht u meant berfore )
S Best

Thanks, Chris.
David
David

Those figures look pretty good to me. I suspect you have a cam with a somewhat large overlap, hmmmm?
If you want the numbers to get bigger try increasing the valve lash to take up some of the overlap.

Compression tests should always be done with the engine warm and the throttle open as wide as possible.

Mike!
mike!

Thanks Mike , yes the car is cammed , an Oselli 271, about as much overlap as you want on a road engine . It also has a Burgess big valve head . Tappets are set cold to factory numbers .
S Best

This thread was discussed between 12/07/2004 and 20/07/2004

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