MGB & GT Forum

Finding out more about this 67 GT that I bought a month ago. I recently did a compression test on all cylinders and all are within 3 lbs of 100. The engine runs well, doesn't blow smoke, has no blow by, starts instantly. The PO told me there is less than 700 miles on a rebuilt engine, The engine tag is missing, but the clock stamp indicates the engine is 1969. The head serial number indicates it is from 1972. Im interested in boosting the compression a bit for more lively performance, 100 seems rather anemic, but again, it doesn't run bad. Has the petronix electronic pick up and a weber two barrel.
So what would you guys suggest as my next step in figuring out whats up with this engine? Shouldn't my compression be much higher? Would the mating of a 72 head on a 69 block cause low compression?

Are you measuring in PSI or metric? My first (idiot) mechanic gave me the numbers in metric, which were about 100.

Readings are in PSI, but two things have occurred to me. Is my Harbor Fright compression tester accurate, and do I really have a 1972 head? I was going by the patent number which I am now finding out isn't an accurate way to determine which year head I have. I'll pull the valve cover and find the part number.

3 words: Harbor Freight gauge.
This is a common problem with $30 compression test gauges.

---

jeff@advanceddistributors.com

Usually that's a go-to recipe for a high compression engine. The static compression ratio works out to about 9.5:1(vs. 8.8:1 on a 62-71 engine and 8.0:1 on 72-80) assuming you have shallow dish pistons as the block would have had originally. When I first put a 12H2923 head on my engine years ago, I recall getting around 140 psi static.

With that said, do you know exactly what this "rebuild" involved? Did they toss a 12H2923 head with a valve job on it and call it a rebuild? I've read several stories here of folks who did exactly that and found pretty quickly that the added compression put a lot of extra stress on a tired bottom end. Anecdotally, about 2 years after I did mine and probably a couple thousand miles without me checking the compression. Long story short, I ended up with all of my cylinders low, and one in particular at around 70psi. With the head off, that piston could be rocked back and forth in the bore with light finger pressure. A full, proper rebuild later and I am running about 165psi flat(D9 cam, and static compression around 9.7:1 thanks to overbore, block finishing, and a very nice head that had a little extra taken off partially to equalize chamber volumes). Whether the two were actually connector or not I don't know, but I do know that my car was right around 100K(odo reading 96K and not working when I bought it...) and seemingly never had any engine work prior to my ownership. I also know that I had good compression-130psi or so-on the original head before I went on the hunt for more power.

Before I dug too deep into it, though, I'd retorque the head, check and set valve clearance, and if numbers still seemed off I'd be tempted to try a different gauge first. Even if you have to go out and spend $30 on a new one...the last I looked that's about what head gaskets were running these days so at worst you'd have spend the same as if you pulled the head just to go hunting for problems.

Cast number is more telling than age of the head. 12H2923 and CAM1106 are the late, small chamber heads(38cc nominal). I think your 68 would originally have had a 12H1326, which is a large chamber(42cc) non-smog head. I suppose a 12H2389, large chamber smog head, wouldn't be out of the question.

You guys are great, i'm always learning. I took off the valve cover and according to the time stamp, this head was built on Dec 30, 1968, #12H2389 which matches up with the engine time stamp of Jan 2, 1969. My bad about the incorrect info stated in OP. i will adjust valves and retorque head while im in there.
As to the extent of the rebuild, I don't know. I have no contact with the PO and im not sure I would believe him anyway, as he neglected to tell me about the faulty transmission. All I am certain of is the engine is clean and repainted and things are clean under the valve cover.



Edited 1 time(s). Last edit at 2024-04-25 08:20 AM by Dontbsilly.

One other factor, a PO could have installed a longer duration/higher overlap cam that can cause static compression readings to report low.

It could be a PO installed the late deep-dish pistons, but on a recent rebuild a would expect something in the 130 range. The large chamber head could affect that as well.

Possibly the cam timing is slightly off? This is easy enough to verify with a dial indicator on the #2 valve after verifying TDC.

A cam that's installed off a tooth will drop readings by 30-35 psi. A fresh HC engine will be 150+ psi on crank-over new and those numbers should go up around 20 psi as the rings seat.

The cheap gauges are notorious for giving 100 psi readings. Borrow a high end gauge from one of your local auto parts stores. Most have a free loaner program. Retest and crank until you get the highest readings, which may take 5-10 revolutions.

---

jeff@advanceddistributors.com

Clay, this is a distinct possibility that the car has an aftermarket camshaft. The petronix and weber and intake seem new, along with many other parts (coil, alternator, starter, fuel pump, radiator, oil cooler and vacuum advance) It doesn't seem like they were trying to save money. Also the car is a bit "jumpy" at idle.

This IS a 12H2389. What does that mean, Smog head? They made low compression "smog" heads in 1969? Would this explain 100 ft pd readings?

Don, the 2389 head in original configuration on a 69 ( GH ) engine is considered high compression at 8.8/1 . The head will have provision for
Secondary air injection and the holes in the head on the right side are probably closed off with small plugs. As others have said, source a good quality gauge and retest .
RudyB

---

I know enough to know that I don't know enough.

If the women don't find you handsome they should at least find you handy. Red Green

To all who suspected a faulty gauge, you were correct. Went to Auto Zone and their gauge read 125 on all cylinders plus or minus 5. Thank you for the information and the helpful advice from all. I will switch my attention to refitting the twin SU's that came with the car.

Here's the thing with compression testers.
The fitting that screws into the spark plug hole must have a Schrader valve. If it doesn't have one then the volume inside the entire length of the hose becomes part of the combustion chamber volume and it reads low. In that case just be concerned whether al cylinders read at or near the same.

Example. 1/4" ID hose 24" long adds almost 20cc to the combustion chamber volume.

One other note which may or may not be relevant depending on whether new rings and pistons were part of the "rebuild", 700 miles is not yet broken in. 500 would be a standard minimum break in period with break in oil, but rings won't be fully seated until 1,000 miles more or less. Without being able to verify that you do have new rings, I would treat the engine as if still in break in, just in case. I would also not use synthetic oil at this point for the same reason. Check compression again at 1,000 and see if there is a change.

If you have access to a bore scope you could take a look at piston tops and cylinder walls. Pistons should be quite clean at such low miles if new, and the bores should show signs of honing.


GMc

---

Wait for me at the gate Tuffy, because without you they're never gonna let me in.

SDCH WTCH-X Twin Oaks Tuff Nutt
2008 - 2022