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MG MGB Technical - 'L' Cylinder heads, Leaded or Unleaded?

I fitted a large valve head to the 18V that was rebuilt for replacement of the original engine in the 1970 B. It was my understanding that the large valve heads came out after unleaded fuel had become the standard.

At 15K miles, the engine idles roughly under a load (with the A/C on) and compression readings are 100, 90, 78, 90 PSI from front to rear. I am wondering if this head was set up for leaded fuel and now has burned n° 3 exhaust valve. I had the head gone through prior to assembly of the engine and the machinist didn't find any valves that required replacement.

The engine never had a cylinder that exceeded compression of 100 PSI. The midget is usually in the 145 PSI range so the lower compression in itself has seemed to be an anomaly. On the other hand, V engines had a lower compression ratio when the lead came out of fuel in 1973/74.

Any comments on the "L" had would be appreciated. I am getting ready to pull it.
Glenn Mallory

Sounds like your car has the lower compression pistons ,which would explain the lower comp. pressures compared to the Midget, but they vary quite a bit
May be first ,check your valve clearances---
William Revit

Glenn-
An Original Equipment specification an 18V BMC B-Series engine in good condition that is fitted with 8.0:1 compression ratio pistons should have approximately 140 PSI (9.84 kg./cm.2) of pressure on all of the cylinders when the pistons are at 0 degrees Top Dead Center. The pressures should be within approximately 10% of each other, that is, within 14PSI (0.984 kg./cm.2).
Stephen Strange

Firstly if the compressions have never been higher than 100 then I'd be trying another gauge. Maybe the cam timing is off, or your method of adjusting the valves is way off. If the inlet gap is too small it closes later in the compression stroke and reduces compression. Mm vs thou?

As far as I'm aware big valve heads and unleaded don't go together, big valves came in around 72 until 74 when they reduced again, unleaded from 75.
paulh4

Thank you for the observations. I did replace my compression gauge and readings are the same. Valves have also been meticulously adjusted, checked and rechecked.

I believe that Paul is correct about the big valve head not being unleaded. Burned valves are probably accountable for the deterioration in compression readings. The engine has only be used in long-haul, sustained highway use with overdrive engaged (extra load). The rebuild has about 16,000 miles on it, almost exactly what it took to ruin the valves on my 1971 Midget when leaded fuel was taken off the market.
Glenn Mallory

Glenn,
are you wedging the throttle wide open when you are doing the compression tests?
If you don't do this you will get substantially lower readings than you could because the closed throttles are not wide open and allowing the maximum possible air to be drawn in.
(apologies if you already do this)

Cheers
Colyn
Colyn Firth

Glenn, I was too late to correct my last post.

Sorry, the last line should have said, "Throttles are NOT wide open to let the maximum possible air to be drawn in"

In other words, if the throttles are not fully open then you will not get a full charge of air into the cylinder which will cause lower compression readings.

colyn
Colyn Firth

Borrow or hire a leak down tester & compressor to see where you are losing compression. Or get a garage to do it for you.

My guess is that if the seats were recut / lapped in the recon job then the unleaded fuel use will have burnt the valves / seats.

I wouldn't even hesitate to pull the head and the sump to extract the pistons to check rings etc.
Chris at Octarine Services

I removed the large valve head and replaced it with a rebuilt small valve head that is original to the car. I had viton oil seals installed which greatly limit oil consumption.

When the "L" head was removed, there were signs of compression loss through the valve side of the head to the outside. Probably not torqued properly when installed (by other).

The machine shop found that the valve seats on the "L" head were loose. So new seats are being installed for next time.
Glenn Mallory

Thanks for reporting back Glenn
You were lucky you got the head off before a seat got loose enough to get out or break up and cause some damage
Good result all up---
William Revit

This thread was discussed between 25/01/2020 and 14/03/2020

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