Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Cylinder Compression

What should normal compression be for a 1275cc engine?


KTM Moore

175 +/- 25
David Smith


Thanks very much for your response. Can you tell me where the information can be found as I have not read this it anywhere?

KTM Moore

Hi Keith,

The Compression Ratio is quoted in the Handbook (I had to get that in before Nigel) and various manuals.
For a 1275 it is either 8.8:1 or 8.0:1 for low compression engines. If yours is a California car it may well be the LC version. Its based on a calculation, so if the engine is unmodified it will be exact.

The (dynamic) compression is a measurement that can vary depending on a variety of factors - which is why David quotes an approximate figure with a proportionately wide error margin. The state of engine tune, the means and equipment used to measure it, the method of actually taking the measurement, rotation speed of engine,air temp, air pressure, humidity, engine temp ....etc etc. All can have an impact on the compression measurement.

What you really look for in the compression measurement is big differences between cylinders and changes when you add a bit of oil to the cylinder before measuring.

just what I've seen on my own engines and read experiences of others on forums like this over the years...
David Smith

Thanks guys,

I've seen the compression ratio numerous times, it was the dynamic compression measurement I had not been able to find. Even though I didn't state it in my question I was really looking for a compression number for an engine in optimum condition so I can compare the numbers I get when I do my own test. I am also going to perform a leakdown test, which I have not done before, so that will be interesting. Is there a compression measurement which would allow for replacing old piston rings, to increase compression,
rather than having the cylinders overbored.


KTM Moore

Mine generally records around 195 to 200


most important is how it runs!

If it runs fine and shows somewhat equal compression values I would leave the head on and not do a leak down test.

An interesting extra test is to preform an compression test dry and wet (a measured amount of oil in each bore) test.

If this shows an increase in number than the bore/ring combo is worn.

Do not get to hung up on the number as it really is dependant on to many factors to give an exact value
Onno K

These are the actual numbers from the Workshop Manual (Sprite and midget issue 1) Tuning data 1275cc engine to engine 12CC/Da/H16300. For 8.8:1 Compression Ratio.Compression pressure 120lb./ 350 r.p.m.
F Pollock

This thread was discussed between 07/04/2012 and 10/04/2012

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.