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MG Midget and Sprite Technical - Sprite in peril of the scrapper !

I have had enough issues with the Sprite - the engine borer supplied the wrong big end bearings for the ground crank so I spent a year trying to get oil pressure, finally got it running and started to run it in, then fitted the much nicer twin box system only to hear a "funny breathing noise from the engine" I wondered about head gasket, but the good weather ended and I stuck the car in a carcoon, the weather is back and felt able to tackle the Sprite once again - so checked compression - cyl 1 to 3 145PSI, cylinder 4 - zero PSI, I have checked that the rockers have clearance which they do - so it must be head gasket or bore/rings - any suggestions before it's sold as a project or just scrapped in shear annoyance - thanks

Malc

btw 1275cc A series
Malc Gilliver

No compression?

1. Valves
2. Head Gasket
3. Piston/Rings/Bore

Probably a head off job to check any of these, unless you have an in place valve spring compressor and a boroscope (for the valves)

Sounds like valves are always open...
Tim C

If you really have zero compression I think it must be a stuck open valve.
Rings/bores would just give a low reading and a gasket would need a big blow by for zero.

Take out the plugs and turn it over by hand watching that no 4 cylinder valves go up and down fully.
richard b

I would think not rings as a neighbour had all 8 compression rings broken in his Ford Xflow Morgan and that still had about 1/2 normal compression.
David Billington

"Take out the plugs and turn it over by hand watching that no 4 cylinder valves go up and down fully."

Agreed

The boroscope was to check that neither of the valves had a big chunk missing!
Tim C

Tim / Richard / David - Agreed your thinking 100% matched mine

I went back at the car the following morning, I put the car in gear, rocker cover off and first visually checked the adjustment looked about the same and no lock nuts were loose, then pushed it down the driver to watch all the valves move and at TDC had clearance on inlet and exhaust rockers to valves.

This took me back to why has cylinder got 0 PSI ???

So I checked cylinder 4 first and sure enough 145 PSI !!! Then checked the others and had 145 PSI on all of them.

I even sat down and drew out the firing order and still can't see why - but in the end I have to conclude that there was something else - maybe I didn't tighten the pressure tester enough, maybe I didn't crank quite long enough, maybe the compression tester has a fault, maybe the car just likes to "f**k" with me !

But I did make it to the "pub meet" at the only Sir Christopher Wren designed C of E School I know that has a cellar bar - https://www.sirjohnmoore.org.uk/

Thanks for your suggestions
Malc Gilliver

Only just picked up this thread. I was doubtful about head gasket. In my experience they always fail between cylinders, so you get no compression on 1 and 2 or 3 and 4, most commonly the latter. I note that this issue arose after a storage period. I had terrible problems on one engine with sticking valves. This was on a newly rebuilt head that I bought from a very well known tuner whose reputation was beyond reproach. I lost count of how many times I honed the guides. They would stick when the engine was fully hot and cruising. Eventually I solved the problem, but not many years later the head cracked. This was a CAM4810 casting, replaced with another one that has been 100% reliable.

I would say that you just had a sticking valve that has corrected itself, so just keep using the car.
Les Rose

Les,

I have discussed the issue with more people and the most suggested issue was a sticking valve, I guess the engine stopped holding 4 open and then it just sat there.But took time to wake up.

My head came from Derbyshire, btw.

Mal
Malc Gilliver

Doubt it was a stuck valve, your rocker clearance check would have shown that up----more likely a piece of carbon or something on a valve seat, it doesn't take much----or same,the schreider valve on the comp.gauge sticking or contaminated with a piece of carbon etc.
William Revit

I get that, I'd just like to know tbh
Malc Gilliver

This thread was discussed between 23/06/2026 and 26/06/2026

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