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MG MGB Technical - compression

what should the compression be in a 18v engine from a 78MGB. Thanks
Mark Neisser

Mark. Opinions vary. Your exact figures will depend on how old the engine is. Most basic references say that the cylinders should all read withing 10% of the highest reading. Some other sources say that there should be no more than 20 psi variation between the highest and the lowest reading. Much depends on how original the engine is. Some high performance cams will actually show a lower compression reading than the factory cam, but the performance will be better. As a general guideline, you should have at least 100 psi on each cylinder and the greater the variation is, the more unbalanced the engine will run. On a recently rebuilt 18V engine, I am getting right at 150 psi on each cylinder. Les
Les Bengtson

I was just looking at that last night. The manuals indicate that early MGB low compression engines should have 130 psi on each cylinder, high compression being 160 psi. The 18V engines (both high & low compression) should nominally have 170 psi. This could be due to different valves, camshaft, timing, etc. However my rebuilt engine with low compression pistons, Kent 714 cam and a Mike Brown head generates 145 psi. Your mileage will vary...
SteveO

Thanks, Les and Steve. Must be a problem with my gauge. Its showing 75psi on an engine that doesn't burn a drop of oil and runs like a charm. Time for a new one.
Mark Neisser

Mark,
It may be time for a new gauge. 75 PSI all around should leave you wondering why you're having problems out-accelerating a wheelchair. Definitely get a decent quality unit that screws into the spark plug holes. The other style (rubber end one that gets pushed into the hole) can give somewhat mysterious readings, and if your lovely assistant isn't anywhere nearby, you can still do the test by yourself with the screw-in type.
SteveO

Mark. I had a professional shop tell me that my Bronco engine could not be tuned for emissions because it had only 60 psi on two cylinders. Both of my compression gauges showed 120 psi with less than 10 psi variation between cylinders. As Steve suggests, a new gauage may be in order. I have both the press in and the screw in types. Both show within a couple of psi of each other when used properly. The basic rule is "if the engine is running fine, the compression is sufficient". A radical cam will show lower compression readings than a factory cam, but will provide increased performance. From what you report, I would not worry about what the gauge is reading--your engine is running fine and not buring oil. Drive it and enjoy it. When you have the time and money, purchase another compression gauge and see how it compares to the one you currently have. When an engine is running poorly, you will know it. If the guage says the compression is bad, but he engine runs well, go with the engine every time. Les
Les Bengtson

I recently purchased a compression guage. It worked great the first two times I used it last week. Now, it shows 0 compression on an engine that runs great. Now, if I can just find my receipt.
Greg Bowman

This is a little bit off-topic but would anyone care to suggest an inexpensive (but not "cheap") compression gauge and dwell-tach that would suit the casual mechanic? Thanks. Greg
Greg Smela

The problem is that these are so common and plentiful, you probably couldn't list the number of manufacturers in a month of Sundays. The good news is that neither of these are very intricate instruments, so very few that you'll find are trash.

I'd try a Sears Craftsman. Price is usually reasonable, quality is good and they have a good warranty. AutoZone/Kragen or Napa will probably sell you a K-D which is also a reasonable brand.

Beyond that, in any medium sized auto parts store, you should find three models of either guage. Avoid the cheapest one. Whatever they have for the next step up should be fine for home use.

I bought my first of both of these guages at Montgomery Ward in the mid 1970's. The tach/dwell died 6 or 7 years later when it drowned in a flood. The compression tester was also under water, but survived and lasted about 25 years.

Now when you go to buy a torque wrench, that's the time to invest some money for a good one. But that's another topic, isn't it?

Matt Kulka

Thanks Matt.
Greg Smela

Compession guages have a shrader valve in them. if aliitle junk gets in there they will read zero when you stop cranking the engine. have someone else crank it and you watch the guage. if it is going up and down then the shrader valve may be bad. do not replace it with a tire valve. parts places sell them for compression guages.
My bgt reads 160 psi
steve

thanks for the info. i'll get a new one or just forget about it. the gauge is a "snap on" so its no piece of crap. at least it shouldn't be!!!
Mark Neisser

Mine is the NAPA brand and it's only a few years old. It consistently shows 105 on all 4 and the MGB runs good. On my son's 1970 midget it shows 120 on all 4. Even though the Midget smokes like crazy and leaks a lot of oil, it runs better than the MGB, which leads me to believe the gauge is correct.
According to Bill Wardlow, our local expert, you should count 7 beats of the compression stroke to make sure the test is accurate. Then after the first test, squirt some motor oil down each plug hole and test it again. If the pressure goes up, the rings are worn. If the pressure stays the same, the rings are okay.
Cheers,
Ken T
Ken Thompson

Mark,
I use a Sears Craftsman screw in compression tester. One thing I do is check the compression by cranking the engine over 7 times, as Ken suggests. Then I retorque the head and adjust the valves. Then I recheck the compression. Nearly every time the compression is higher and more consistent. For example: 1st compression check 145 - 130 - 135 - 140. 2nd compression check 145 - 140 -140 - 140.

I don't know if the actual reading is as important as having good consistancy in all for cylinders.

Dave
Dave Kalp

Mark,

If I remember correctly, all Snap-On tools are guaranteed for life. You probably paid a pretty penny for the guage, and it would be great to have another new Snap-On guage, especially if you don't have to pay for it!

Matt Kulka

This thread was discussed between 28/07/2002 and 02/08/2002

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