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MG MGA - Head off..and ...

Some time ago I reported on cylinder compression tests on my 1600...worst one ( no3) was just over 100psi and down over the last few years...
This is the first time I have had the head off in 20 years and the seats are the original steel i.e. no inserts,
Results were good...
..Pic below shows one of the cylinders before valve removal/clean.
..Pic on next post is after clean up and lapping.
..Loads of hard black removed ..also a very hard white compound on the exhaust valve . I presume this is a result of the use of lead substitute additive ..any ideas ???
.. No lateral movement of valves in sleeves. All the exhaust valves had bronze sleeves).
...No evident recession additive seems to be working ..the exhaust valves are still proud of the chamber by about 27thou (0.07mm). The seat width is about 2mm . Lapping was sufficient to get good surfaces.
..The poorest exhaust valves/seats were in cylinders 2 and 3.. with latter worst . Not surprising given the combined exhaust for these two.
...I noted resistance in all the exhaust valves in their sleeves., especially cyl. 3, was due to a material build up on the stem at cyl. end . ..removed and all moved easily.

Now await some parts( incl stem seals ) before reassembling.
I also did a check on the bores with my newly acquired bore gauge . ..and no significant ovality or taper. No bore damage evident.
Pistons had no lateral and could not get a 2 thou feeler down side of piston .
Any ideas how you can get carbon off concave piston heads without removal and without leaving a mess on top of rings.....
Looking forward to getting this all back together and seeing the new compression levels.

Neil Ferguson

after clean and lap ...

Neil Ferguson

The white looks very much like mine (now years back)after 150K with leaded petrol.
FWIW Roger
Roger Walker

"Any ideas how you can get carbon off concave piston heads without removal and without leaving a mess on top of rings....."

Yes, a liberal application of grease to seal the space.

MAndrus ...bit wary of that as material would get stuck in the grease ...but on the same theme a narrow bead and wedge of hard plastic sealant around the top edge of the piston may do the trick. It can easily be pulled out as well .
Neil Ferguson

how bad were the valve surfaces and do you think the drop in compression was entirely due to them?

Also, are not just a little bit tempted to drop the sump and then remove the pistons so that you can check out the rings and little ends? It is not a massive job at the stage you are now and you could simply glaze-bust the bores and re-ring if the rings are worn.

Colyn Firth

Colyn..the worst seat and valve surface was the exhaust on cyl 3 and the next worse the adjacent one in cal 3...which is consistent with the compression results . They also had the stickiest stem to sleeve movement . I used coarse then fine paste and kept going at the surfaces for some time and they came good . pleased with result.
I would drop the sump if the cyl bores had shown problems but none....oil pressure is very good, no real oil consumption and pistons solid so not tempted . If it ain't broke etc...
Incidentally I did report some excessive oil consumption a few months ago. When I installed the judson with a SU HIF 44 I hooked up the sump breather to the carb connection. I reverted to the original air velocity ambient vent a few months ago ..after that no oil real consumption . I drove 1400km just a few weeks ago and used just a litre...phew!
All good..but waiting on parts...
Neil Ferguson


I have found the HIF float chamber spacer. If you email me your address I'll post it to you

I am also now informed that a VZX1095 inlet valve and needle are beneficial to prevent fuel starvation at high running speed / load - these were used in the Turbo applications of the carb. While you are ordering that, you need an extra float chamber gasket as the spacer kit is missing that part

dominic clancy

This thread was discussed between 29/05/2016 and 30/05/2016

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