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MG Midget and Sprite Technical - Compression figures

+60 thou 1275 (is that 1330?) with a kent 276 and a fairly standard Metro 1300 head lightly tickled by me.

A few years back I melted a piston (long story) and replaced pistons and rings with just an in-situ tripod hone. At this point the ring gaps were larger than spec. (I don't remember how much but at least 20-30% over I'd guess).

The car doesn't seem to have been running right for a couple of years and I just can't get it in to a happy state of tune. The peakiness of the 276 seems to be more exagerated and an even, consistent idle elusive (it's always been tricky as I also have a lightened flywheel).

This evening I finally got around to checking the compression:

Dry: 200, 250, 240, 200
Wet: 230, 290, 290, 260

Would you agree I don't have a compression problem?

However when I pulled the plugs they did look quite oily so maybe I have an oil control problem. Could this be the slack rings or is it more likely to be the valves stems. The head had done quite a high mileage before being transferred to my car and all I did was the seals and that was a fair few years ago.

Please muse, speculate, question further etc.

Thanks.
Jeremy Cogman

Well, Jeremy, that amount of jump between the dry and wet readings would lead one to believe that there is an issue with rings. However, how much oil did you use to do the wet test?

The oil takes up combustion chamber room, so if too much is put in, it will inflate the difference. This is easy to do with dished pistons. You only want to get oil onto the wall/piston interface to temporally seal the rings.

Charley
C R Huff

I agree with Charley. Plus your figures for #2 and #3 are very high even if you skimmed the metro head. I suspect you added too much oil as Charley says. You also need to be very consistent across each measurement, spinning the engine the same number of times for each one. ( Do 10, with the other 3 plugs out and a good battery)

My engine spec is as yours;+60 thou, 276 cam, metro big valve head skimmed to a 10.5:1 ratio, lightened flywheel, balanced crank. I measure compression every 3000 miles so l get a good long term history. It generally gets all figures between 200 and 215 for wet and 190 and 200 dry. A couple of years ago dry figures dropped to 140 - 150 ish so I fitted a set of CORD rings. I am on about 90k since the +60 rebore.
GuyW

Thanks chaps.

How much oil do you add and how do you get it round the whole circumference? I added three squirts from a can, trying to space 120 degrees, I guess that was too much.

Jeremy Cogman

I add maybe 3 to 5ml. Work on the assumption that if the rings are leaking the oil will quickly be drawn to the ring. At the same time l don't want excess oil which would effectively reduce the chamber volume and give high compression readings. But I am no expert and only think that consistency is probably the most important.

Another thing though, if you put too much oil in it will get messy as excess oil is blown out of the other plug holes as you progress along the cylinders.
GuyW

You could take your oil can and put 3 squirts into a measuring device. I suspect that is quite a lot. Probably enough to completely fill a piston dish, or more. Just filling the dish would be like installing flat top pistons.

You will have to run the car to dry it out before trying again. If you can get the tip of your oil can touching the wall, and just slowly dribble some down the wall, within a minute or so it should migrate around the bore.

And, like Guy said, be consistent with your process.

But, if your dry figures are anywhere near correct, I don't think low compression is your problem. They are a bit wide spread, but that may be down to not being consistent.

Charley
C R Huff

A trip to Peter Burgess' rolling road to get it running right?
Jeremy MkIII

This thread was discussed between 25/08/2018 and 28/08/2018

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