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Author Topic: Engine Compression  (Read 766 times)

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Engine Compression
« on: February 14, 2020, 03:17:07 PM »
I've been watching some of your awesome MG repair videos on youtube.  First of all - thanks so sharing so much great content.

I have a question regarding a project a friend and I just picked up.  We have a '70 MGB with virtually no compression in front cylinder and low <90 in the other 3.  We pulled the head and first off noticed a crack.  The engine has been rebuilt as it has +.030 pistons in it.  Unsure how many miles it has on it since the rebuild.  The crack doesn't really explain the low compression especially in front cylinder.  We will be pulling the valves to see what lies underneath soon.

Anyway - what would be your recommended course of action in this situation?  I assume at the very least we will need to source a good replacement head.  We are trying to minimize cost but still want to build a quality motor.

Let me know your thoughts.

Thanks so much
Steve Betzhold


  • AMGBA Club Tech Staff
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    • University Motors
Re: Engine Compression
« Reply #1 on: February 14, 2020, 03:29:55 PM »
A:   Back up a couple of steps.  When an engine comes to rest, two valves remain open.  If the engine sits for a period of time –months / years – then those two valves develop some rust on their faces and cannot perfectly seal.  So, before I determine compression, I run the engine for ten minutes and then check the compression.  If something’s amiss – a variation of more than 10% cylinder to cylinder, then I adjust the valves (0.015” in your case), and check the compression yet again. 

If the compression is extremely low in one cylinder, the problem could be the valves, the piston/rings, or the head gasket.  In ALL these cases the head needs to be removed to fix the problem – so I never squander time trying to figure it out from above.  Once the head’s off the problem is usually easily seen.

Support the head upside down on the bench and fill each of the combustion chambers with fluid.  Gasoline is handy but dangerous.  Fuel oil / mineral spirits are effective.  If one of the valves is leaking, you’ll see the fluid drip out of an inlet or (more likely) an exhaust port.

Position the pistons at the same height.  Pour that same fluid into the cylinders – say one inch above the surface of the piston.  Over the next fifteen minutes look at the fluid height in each cylinder.  If the rings are faulty in a cylinder (or worse) the fluid will escape down the sides of the piston and into the sump more quickly than the others.

Cylinder compression – the raw number – is a function of the valve lash, the integrity of the valves/rings/gasket, the heat of the engine, and the speed it’s spinning over.  I’ve never noticed a difference in compression by the position of the throttle.  The important thing is that they’re within 10%.  Of course, a true figure of less than 90 psi indicates something amiss (list above) or cam timing.

There are cylinder heads available on eBay and at swap meets.  For example, the Chicago Area swap meet every late winter, early spring always has cylinder heads there.

Hope some of this helps!
John Twist
Safety Fast!
John H Twist
University Motors Online
100 East Beltline Avenue SE
Grand Rapids, MI  49506


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