Welcome to our Site for MG, Triumph and Austin-Healey Car Information.


MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB GT V8 Factory Originals Technical - weird test results

I ran the second compression test last night after squirting oil in the cylinders and got more unexpected results. Maybe somebody can suggest the meaning of the following numbers. I'll give the number for the dry test followed by the one with oil in the cylinder:

#1 112/117
#2 107/120
#3 95/117
#4 114/134
#5 112/117
#6 123/123
#7 100/115
#8 105/115

I didn't expect to see the numbers come up that much or be that consistent on this test. Is it possible the head gasket is now sealing after sitting a week or so? Or, do I have a ring problem? Or should I just wait another week and run another test? And then there's #4.

Shouldn't matter, but I had just switched to Mobil 1 before the drag strip runs. Anyway, maybe I'd better start preparing the spare motor.

Jim Blackwood

Hi Jim,

Definately looks like a piston ring problem.
The results certainly indicate that.
Could it be possible that the Mobil One synthetic was not splash lubricating the rings enough which caused excessive wear ? ie: Perhaps too thin compared to your more thicker previous conventional oil.
I have heard several cases where engines were ran on conventional oils then changed to synthetic after about 50,000. The tolerances were used to the conventional thicker lube and excessive wear in all areas resulted.
Don't get me wrong, When used in a new/rebuilt engine Synthetic oils are far superior in lubrication and breakdown.
I've used Mobil 1 in my vehicles since new -its great and expensive !

Perhaps the head gasket only leaks under running conditions or during boost when at higher RPM ?

#4 may have a slight carbon build up resulting in a higher compression ratio.

Good luck


1969 MGB 302 V8
Pete Mantell

I have been told on many occassion when rebuilding "old" engines that you should NOT use synthetic oil for the initial bedding in. The reason I have been given is the the rings won't seat as the oil is "too slippy" I have always been told to use something like 30 weight, no detergent oil for the first 300+ miles, then move over to synthetic. I don't know if this is just an old wives tale, but it looks like the new rings haven't bedded in.
Tony Bates

Let's not jump to conclusions, the car had 1500 miles on that engine using regular Valvoline 10w40 before changing to the Mobil 1. If the rings were going to seat in they would have by then. By all indications they had, although I had not run a compression test before the drag runs.

The other symptom was apparent pressurization of the coolant system, blowing out a rear freeze plug.

Jim Blackwood

Jim, do a leak down test it's more accurate.
#3-#7 is low and indicates a ring. These pistons are in the same bank but not next to each other, that eliminates the poisibility of a blown head gasket between cylinders.

Did you do the test while the engine was hot?
Gaskets could leak when they are hot and not when cold.
Bill Guzman

Jim . . . if you want to hook air up for a differential compression test, you can verify where the problem is by listening for air. If you hear air at the carb, it's intake valves, if you hear air at the exhaust pipe, it's exhaust valves leaking, if your hear or feel air at the crankcase vent, then it's rings. You can also look for contamination in the radiator coolant and oil if you suspect head gasket. As Bill said, a hot engine can give better ring sealing, but may show a bad gasket that wouldn't be obvious cold.

Gordon Elkins

Good suggestions guys, I hadn't thought about the temp. I'll try the differential leakdown if I can rig the test setup, should I be using some sort of flowmeter for this? I should have what I need for instrumentation. Both tests were run cold, but it was pretty clear that I was getting combustion gasses into the coolant system whenever I was in boost, as all the coolant was blown out very quickly. I had removed the radiator cap on the drive home and could see a plume of white smoke coming out when I hit the gas. That big hole in the hood does have some uses.

Jim Blackwood

There's an article in the latest Street Rodder magazine about making a leakdown tester. Can be done pretty cheaply.

Wayne Pearson

Your figures look as odd as mine, but they were odder when fully up to temp (and cam/tappets clattering) than warm but before the clattering started. Did you have the throttlew wedged wide open in the tests? Did a top-end rebuild and found all valves were gas/petrol tight except for a very slight weeping on one, the bores still had the honing marks over 80% of the surface, rockers and shafts were perfect, inside was as clean as a whistle, with at least 80k since the last rebuild. It was the coolant pressurisation that triggered the rebuild, being blown out of the overflow, although not as severe as yours. BTW, the cap should release excess pressure, I suspect the freeze plug was coincidental, and if one has gone the others could be weak. After the rebuild the coolant problem was solved :o) runs much cooler than before :o) but still clatters when hot :o(

I found two problems in the cooling system - a very slight leak past the gasket at the front left hand passage between head and inlet manifold, and a scaled-up bypass pipe inside the inlet manifold.
Paul Hunt

Paul, did you find any evidence of leakage past the head gasket?

Jim Blackwood

True the cap does release pressure, but I've seen too many instances (mostly Fords) where the heater core blows during a drag race. In a Ford engine, the heater supply hose is on the intake manifold right before the thermostat. So under high rev conditions, the engine builds up more pressure than the pressure cap sees - probably because the coolant flow can't get past the thermostat fast enough.

Wayne Pearson

No. I used a block tester ( UK site but US made kit) several times under various conditions but it always came up clear. Head gasket was mine and everyone else's first thought.
Paul Hunt

Yes, I remember that now. Did you ever figure out the cause?
Jim Blackwood

See above, no coolant loss or pumping up now. I had a pressure gauge on the cooling system during diagnosis and after repair. Before it would slowly rise to the cap pressure whatever that was. Now it runs at only 3psi at 70mph on the motorway, and cycles between 5 and 10 as the fan cuts in and out when idling, even in temps of 30C we had recently.

The other interesting diagnostic was the temp gauge. Beforehand, even before it developed the problem, the gauge would always rise slightly past N when warming up from cold, then slowly oscillate about N on a reducing basis until it settled on N. As the (4-cylinder) roadster has always done exactly the same I took no notice. But now the needle rises fairly rapidly about 2/3rds the way to N, at which point the stat opens, then rises very slowly after that to the left-hand side of the N, only getting above that when the airflow reduces for whatever reason.
Paul Hunt

Right, so no indication anywhere of a head gasket problem I take it. Very odd indeed. Well it won't be long before the heads come off I suspect, though I will try to cobble up a leak down tester befor doing that.

Jim Blackwood

The absolute best way to test engine integrity very quickly is with an exhaust gas analyzer and vacuum transducer coupled with an oscilloscope. I use these on a daily basis and they just cannot be beat for accuracy. On any vehicle that has overheated or runs rough, I take off the coolant cap and replace it with a strong plastic bag held on with a rubber band. I then run the exaust probe into the bag via a small hole and watch the exuast readings. I disconnect the ignition to load up the cylinders with fuel and any leak will result in increased hydrocarbons read. The smallest of leaks will show up as the machine reads in ppm. If the bag pressurizes with no increase in hydrocarbons or carbon monoxide then the overpressurization is most likely the water pump drawing in air and pumping it through.
I use the vacuum transducer to watch the valves open and close. The transducer will produce a sin wave representing the vaccum/pressure pulses in the intake as individual valves open and close. Transposed over ignition events you can "see" an individual cylinder's valves open and close. Mechanical problems such as burned valves, vacuum leaks, slipped timing belt/stretched chain, broken rocker arms etc, will ea ch have a unique effect on the pressure waves on the manifold. Most people don't have these machines but alot of modern shops do. It might be worth checking out.

This thread was discussed between 28/07/2003 and 03/08/2003

MG MGB GT V8 Factory Originals Technical index

This thread is from the archives. Join the live MG MGB GT V8 Factory Originals Technical BBS now