Blow-by Boy has more data

[Jim Lee]

Finally did a compression test and got this:
Cylinder
1.............140
2.............120
3.............145
4.............150

So Cylinder 2 (counting from left to right standing on the drivers side) is definitely out of whack. I tryed doing a 'wet' test by squirting some wd30 oil in that cylinder and it did not make any difference. Since the head was rebuilt not very many miles ago does this cylinder 2 low compression
correlate with the noticeable blow by coming from the oil cap? Is it pretty clear that I need new rings on the cylinder 2 or is the 120 'livable'...which I know is quite a
subjective thing. Is there anything I can do for one cylinder with low compression to help it along until I get positioned to take the head off and replace the rings?

Thanks very much,
Jim Lee

1959 TR3a
TS50500L

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[Matthew E. Herd]

If you know of someone that has a tool called a leakdown tester, you can pinpoint the problem via the sound of leaking air as it escapes from either the valves or the piston rings (assuming you've rotated the engine so that the valves are closed on that cylinder). It is a valuable tool (which isn't really very expensive, but I have yet to purchase one). The leakdown tester can also tell you what percent of leak you have in a given time period.

However, I'd say that your compression values look ok to me. Is there a specific problem that you have? The cylinder appears to be within about 20% of the others (averaged) which isn't good, but it's still perfectly tolerable until you get around to fixing it, I would think. My brother's '80 B has even lower compression in one cylinder, but we're not fixing it until it gets the whole restoration. It vibrates noticeably at 1000 RPM, but other than that it's fine.

Also, you can try to put a few tablespoons (1-2, not more or you might break something) of motor oil in the cylinders and try it again. Since WD40 evaporates so easily, it's not a good tool to use in compression tests. As I'm sure you're aware, if the compression doesn't go up measurably, its not the piston rings. Another comment on compression testing: Make sure you open the throttle all the way. More than one time I've forgotten to do this, and I test my compression more than I probably should ... The results are usually off by about 75PSI, making them totally worthless.

 

[Dave Russell]

Go here for how to build a simple leakdown tester. https://www.britishcarforum.com/ubbthread...true#Post139216
D

 

[Matthew E. Herd]

Dave,

Perhaps that's something we could put in the knowledge base ...

It's general purpose, useful to just about everyone.

 

[Dave Russell]

Hi Matthew,
If you or anyone else wishes to put it in the KB, be my guest. I can't get anything into it from my MAC without going through format changes & other gymnastics.
D

 

[Matthew E. Herd]

I'll try to get around to it ... unfortunately I'm a busy school student

 

[Geo Hahn]

I do a crude (no numbers) leakdown using the hose from my compression tester... one end is a spark plug thread, the other a quick-connect for an air line (have to remove the valve core in the tester hose).

It's a crude test but you already know you have a problem in #2 so this will confirm rings or tell you a valve is not closing 100%. Pump up each cylinder at TDC (car in gear, brake on) and listen where the air goes (crankcase, exhaust, intake).

FWIW I drove my TR3A for many years with poor compression in one cylinder. Knew it was an exhaust valve and it kept getting worse until finally I relented and did the head. Never could tell that cylinder was down when driving.

Yes, do not spray WD40 for a wet test... apart from being too thin & volatile, if that red tube pops off and disappears into the cylinder your whole day will be ruined.