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MG TD TF 1500 - Compression Test Evaluation

My car ran very smoothly and with good power prior to my restoration project. The engine has been stripped for painting and accessory overhaul, so I've done a cold engine compression check. I checked each cylinder "dry" and then squirted oil into the each cylinder to check "wet" compression.

Results (psi) are as follows:
No. 1 150 dry and 200 wet, increase - 50
No. 2 60 dry and 100 wet, increase - 40
No. 3 150 dry and 200 wet, increase - 50
No. 4 120 dry and 180 wet, increase - 60

Comments and recommendations please.

Corey Pedersen

Number two is low. The increase with oil shows rings are worn, but 120-150 is not too bad.
I'd say number two has either stuck rings, a broken ring or a bad valve. If it were me I'd mix a 50-50 mixture of ATF and acetone and pour it in all four cylinders after moving the engine so all the pistons are half way up the bores. Let it sit for a week, turn the engine over without the plugs and recheck. If it was stuck rings, this may fix it. For piece of mind you may want to pull the head and do the valves and look at the bottom end from the top end. A long gouge may be in the cylinder wall of number two.
D. Sander

I noted that the wet compression increased equally in all cylinders. That suggests to me that the rings are all OK (or all equally worn) and No. 2 has a valve fault.
Corey Pedersen

A compressed air leak down test will pinpoint where the leakage in number two is going. You may want to at least do the valves anyway. If a valve breaks you are committed to a full rebuild, 4-5k
D. Sander

The timing chain in my '77 F-100 302 broke at 96K miles causing the valves to crash into the pistons and requiring a new 302 crate engine (only thing not original on the damned truck). Does the XPAG also result in valves crashing into pistons when a timing chain breaks? God know how old the chain is in TD2/25404 (1953). Should I be worried? No noises or other obvious signs but then the Ford 302 gave no warning either - Just came to a rapid stop.

J K Chapin

BTW, if this cosntitutes a hijack, I apologize and will start a fresh thread. You guys are Tony Tiger
J K Chapin

Jud - I forget the term used for engines like on your Ford, but it is not a worry with the XPAG engines. Cheers - Dave
Daved DuBois

I have always heard these engines referred to as Interference engines. If not timed properly or if the timing chain/belt breaks valves and pistons meet. This is why changing timing belts on time is so critical.
Norm Peacey

The compressed air test will tell the tale. Air pressure is applied via plug hole adaptor. It will either be leaking out from a valve (likely exhaust) or into the crankcase. I would bet on a burned exhaust valve, or broken ring. That is way too low, time to explore and fix with the engine out. George
George Butz

I don't understand the purpose of a compressed air test. No. 2 cylinder appears to have cylinder head fault, so the cylinder head has to be removed, at the very least. I need to determine if the rings/cylinder walls are faulty as well to determine if the engine has to be removed. How do I determine that without disassembling the engine?
Corey Pedersen

Air hose from compressor is connected to an adaptor, screwed into the plug hole. With valves closed, turn on the air. You will hear a hiss whereever the air is leaking out. If no air/ hiss from the valves (remove the intake/ exhaust manifolds first) and it is hissing out through the bottom end crankcase, it is a ring/piston problem. Yes, either way the head will have to come off, if there are scores in the cylinder (or the top ring land is broken on the piston, etc) then out with the motor.
George Butz

Thanks George. Where can I obtain a plug adaptor?
Corey Pedersen

Internet magic! I found them.
Corey Pedersen

Dave and Norm, thanks. I'll drive with one less worry.

J K Chapin

Folks. Don't overmechanic this one. I would NOT tear the head off until you really identify the issue.

Car ran fine prior to storage and now has low compression in #2. Sounds like a stuck valve. I have had this happen in a single winter storage. If it is stuck just near close, it could have some compression.

I think Occam's razor applies. 'the simplest answer is most often correct"

Air test should give the answer an if it is the valve, a little penetrating oil around the seat can loosen it.
Bruce Cunha

Bruce many thanks for your insight. I will apply some penetrant to the valve stems and crank the engine. Then I'll visually check for valve movement and do another compression check of No.2.

The car idled very smoothly at a very slow idle speed and I find it hard to accept that there is a significant compression problem.

I had drained the oil from the engine and have been cranking it with only the residual oil for lubrication. I didn't fill the crankcase because I thought I might need to do engine repair/rebuild after compression testing. Any stickiness in the valve guides would therefore be exascerbated. I'll also add engine oil before doing any more testing.
Corey Pedersen

This would be a good time to do a valve job. If a valve brakes off, it will cost between 4 and 6 k. The valves fatigue with age, (something about being hammered open and closed for years.) You will get no warning when this happens. This would also be a great time to examin the cylinder walls.
D. Sander

David, good input, thanks. I've just had the diff modified for 4.3 gearing. A fresh valve job and a lower engine speed should provide a very long lasting engine!
Corey Pedersen

A valve job would be money well spent. New valves, valve guides and seals won't break the bank. With the head off you can get a great look at the cylinders. I'd put a mixture of ATF and acetone in number two if the valves and the cylinder walls look good. You could have a stuck ring. When a valve snaps off and falls in to the piston it immediately becomes a very expensive total engine rebuild. I've seen it happen a number of times.
If you have good oil pressure and the cylinders look good, you should be all set with a valve job, assuming your camshaft and tappets have been getting along, and are not all chewed up...
D. Sander

Before this subject gets put to bed, I decided it might be the best place to make a realtively free offer to any of our regular readers. It would be on a first come first serve basis if there are any replies.
A neighbor gave me a combnation compression/vacuum gauge. I checked it out and dial responds to air pressure but does not hold the reading after the pressure is removed. I looked on line for internal valve disc parts but found none.
My offer is that I will send it to the first/any taker for the cost of a small flat rate box at $5.35.
It seems too good to pitch in the trash.
Photo of the dial part is attached.
Replies to mgcarnut at sbcglobal dot net

Jim Merz

In view of my potential need for a cylinder head rebuild, can someone please recommend a machine shop in my area (Hebron, CT).
Corey Pedersen

Hugh Pite has spoken for the vac/press gauge.
Jim Merz


My mechanic Mike at Stonebridge Classics rebuilt two TF 1500 engines in the past year, plus works on a Vintage Racing TD for another customer and two MG race cars (MGA and MGBT-V8) for Storm Field the now retired NY weatherman. He works alone in Bethel, CT. He works with a machine shop in the same town that does top quality work. He builds to racing standards as that is all he knows as anything less is unacceptable. He holds the track record in class at Lime Rock is his Datsun 510 so he understands the importance of quality work. He is not cheap and don't expect it back quick, but the job will be well done.

Mike has brought my TF back to better than factory condition over the past 8 years I have owned it. Mike has fixed the efforts of many past mechanics who messed up everything you could possibly do to an MG. I bought it from a dealer in Maine w/o a prior decent inspection. He dropped it off at my home and the next day on the way to DMV the whole dashboard went up in flames! Mike told me I was lucky to be aline, so we went from there. I also added a top flight fire extinguisher!

BTW, Mike will only do a leak down test to determine how efficiently an engine runs. My rebuild was based on the results of this test.

JSL jeffrey

Check valve clearance for one excessive - that one is sticking. Set them, and check if one changes either way - that one is sticking. By all means spray penetrant on the stems and repeat.

!!!!!!!NEVER crank without oil!!! NEVER!!!

All that said, valve jobs before it really "needs it" save a lot of parts. Once a valve burns you have to grind a bunch off, before it is just touchup.

FR Millmore

I purchased a leakdown test kit and tested the engine after lubricating the valve stems with penetrating fluid and hand cranking the engine. All cylinders read the same (in the "moderate" range of the leakage gauge). I listened for the source of the measured air leakage. There was no audible leakage from the manifold ports. All of the audible leakage was coming from the crankcase, and sounded the same for each cylinder. In view of the prior good running history of the engine and the lack of any blue smoke out of the tailpipe I'm now convinced that the engine is healthy.

George/Bruce, many thanks for your guidance in this matter.
Corey Pedersen

This thread was discussed between 30/01/2013 and 05/02/2013

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