Yoink the head off there in situ, only takes a few minutes, and then jack her up and take the oil pan off and you can get access to the top and bottom of the pistons to do things like pop them out, hone the bore and re-ring the pistons. Once you pop those pistons back in, refit the crank end bearings and torque those caps down and you can have the head back on in a few minutes. I can say this with such confidence because I'm not doing the job... If this was my car I would use that as an excuse to get this 1275 I have in my garage ship-shape. Hopefully you won't be out of commission for long!

Ian - so sorry you’re having these  problems. What a pain to have to pull the engine and work with the rings to solve the problem. I hope the down time doesn’t cancel any of your plans for car shows you wanted to attend.

I had a very similar problem with the 1960 Bugeye that I raced back in the 80’s. I always built by my own race engines and, for the most part, did a nice job. I always used Hepolite pistons and rings. But one year I decided to try a Deves steel ring set because I was told they were better rings. But I installed them without reconditioning or cross-hatching the cylinder walls. In the next race the cylinders were getting blow by and was blowing all the oil out of the engine. On hard braking or sharp turns on the track, I observed the oil pressure drop to zero! I had a choice to make: pull into the pits and shut the car off or continue racing and hope the engine held. I was in first place in my class and the runner up was right on my tail. It would be my first win in the car, so I decided to stay out and just hope the engine stayed together. On that engine, I was hitting as high as 6500rpms which, as you know, is a LOT for A-Series Austin engines. The engine did hold up and I got my first win! But after I got home from the track I tore down the engine and got those steel rings out of there!

So, I know what you’re experiencing. Since you had your motor rebuilt by a shop, I’m assuming they did the proper job of honing and cross-hatching the cylinders. Since your compression problem is confined to cylinder #4, your theory about the heater valve is probably right. The curious thing, though, is that most of us that raced Bugeyes removed the heaters and capped off the hole in then head where the valve sat. So, I wonder why we didn’t have that same problem on race motors???

Couple of points here Ian. The engine really should be warm for a compression test, so that the pistons and rings have expanded to allow maximum sealing - and - you need to hold the throttle open at the same time so that the engine can draw air in without obstruction. At present I have exactly the same problem as you do on my Frog, except that in my case it is no.2 cylinder that is down on compression at 130psi (the others are all around 180psi). I'm about to check my valve clearances just in case one of them (in cylinder 2) isn't being allowed to fully shut (which would be the easiest - but most unlikely cause of the problem). However, there is some hope for me here because oil in the cylinder makes no difference to the reading, so rings are unlikely to be the cause. I've never had the cylinder head off this engine, so that will likely be my second job to see that no.2 cylinder's valves are properly seating. I have long experience with engines, from WW2 Jeeps to Ford and other works rally team engines; what is odd in this case is that the engine currently pulls like a train and shows no sign of sickness in the way that it runs, nor does that spark plug look different to the others. Time - and investigation will tell. Good luck with yours.

Ian, great info to know... You should be pretty good at pulling the engine...Making it much easier to reinstall... Always finding new things on the bugeye and fixes.  Thanks for sharing...

that is a big difference in compression. Gosh I always turn off my heater valve in the summer. Maybe I should stop doing that. Maybe do a leak down test before you pull the engine? Or pull the head off?

Interesting scenario Ian but I question your assumption that the rear of the engine is running hot due to the heater tap being closed. I'm not sure about the situation in North America but in Australia heaters were an option and certainly not all Sprites were fitted with one. If there was no heater the opening in the head was fitted with a diamond shaped blanking plate. If your assumption is correct there would have been a lot of Sprites (and other A series engine equipped BMC offerings) running around with similar problems to yours. I have no idea of the history of your engine but I would suspect that something to do with its rebuild could be the culprit. What I find puzzling is if your car was burning this much oil you would expect to see clouds of blue smoke out the back but apparently there isn't such a thing. Before pulling the engine out I would just remove the head or even better, get yourself a bore scope device - they aren't expensive these days.

Hi Ian. So sorry to hear about the Bugeye troubles but I have complete faith you can take care of it all!
I have a question, where can I find the proper valve setting tool (for correct spacing)??
Thank you

My engine over heated at end of may and boiled over after 3 years of running after a complete rebuild, had my local garage collect the car, compression on cylinders 3 and 4 were very poor even with the heater tap left open.
I eventually had a call to say head gasket had failed and head had cracked, it's currently being worked on, I'm taking it back to standard 948cc after going large with the bores as I feel that may have contributed ro the issue, I hope to see how you get on with your repair.

If you wanted to keep the valve open but still avoid getting unwanted heat in the summer, you could swap in a hose to bypass the heater core.