Ariel Owners Motorcycle Club

Has anyone run a compression test on a square 4. Recently I had the carb off, so out of interest I decided to do a compression test. My gauge is held by hand against the plug hole and seals against bevelled rubber, rather than screw in, so not ideal. 3 cylinders at 115 psi and one at 113 psi which is even enough, but not as high as I would have expected. Has anyone got any other figures. Compression tests should be done at wide open throttle, so that means carb off or prop up the slide with the butterfly wide open on the SU carb.
David

Check your gauge against another. Valve timing can affect pressure. Being even is good enough really. Nev

A few years ago I compression tested my mark 1 square four after running it in following a rebore with new pistons and rings. It has mark 2 pistons with the ‘top hat’, and the head is skimmed (too much by previous owner). My compression tester is a screw in type with a non-return valve so it keeps the peak reading on the dial. My readings came in at 125psi on each cylinder with a +/- 1psi reading on some. Your readings sound fine.

Hi David. A compression test on my Mk1 last year, with original standard 6:1 pistons and freshly honed bores got me 100 psi on two cylinders and 105 psi on the other two. After 400 miles this rose to 108 on all cylinders. It runs really well. I would expect the Mk2 to be a bit higher. Yours seems to be right in the ball park. Cheers, Chris.

I did a compression test on my ‘48 4G last year, when I was having some running issues and needed to rule compression out. Using a good quality tester I borrowed from work and with WOT I was a pretty even 95psi on all 4, so I’d say your figure on a MK2 with higher compression pistons is in the right sort of area.

Rob

Thanks for the replies, and my reading may be a little low, although I was happy with how even they were. What I did not mention is that my 4 has 7.2 pistons and the head and cylinder were shaved .040” and .030” for a total of .070” due to distortion and some corrosion. My compression ratio would be about 8,3 to 1. So I will look to borrow another compression test gauge.
David

hi my 4 is about 110 psi with a good gauge

Hi, I can't remember the actual readings but I know they were all just over 100 and my Mk 1 runs very well.