Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - Cylinder Pressures , Tappets etc

I did a cylinder pressure test 2 days ago and got the following results...

Cyl.1 ...103psi, , Cyll 2 ...95 , Cyl. 3...90, Cyl. 4....95

After Injecting oil into the cylinders..

Cyl.1 ...132, Cyl.2...145, Cyl.3.....137, Cyl.3....133

Figs . gave me some concern re ring condition ..especially in cyl 3

Today I took valve cover off and found the tappet clearances less than 15 thou ( by up to a few thou..not a lot ) so I adjusted them all back to 15.

Redid the cylinder testing without oil and got..

Cal.1...120, Cyl 2...117, Cyl.3...110, Cyl. 4...123

pleased about improvement but three issues remain in the grey cells..

1....Does the reduced tappet clearance indicate seats are recessing ? I have original seats but use a top end lead substitute ..or is the wear normal ?Last time I checked was about 5 years ago.

2...Can such a slight adjustment in clearance really make such a difference in cylinder pressure?

3.. Do I still have a ring wear problem ..especially in cyl.3.

Car drives well ...

Neil Ferguson


Are you deliberately running low compression with your supercharger set-up? Otherwise all those figures look low to me. This is what Barney has said in the past:

175 or so is high compression likely to have flat top pistons or a severely milled head.

140-145 is generally good.

125-135 is functional.

110-120 is low, but it still runs. Under 100 on one or more cylinders will probably misfire on startup, then run with reduced power.

Steve Gyles

Forgive me if this is a stupid question Neil, but did you have the throttle jammed wide-open when you checked the compressions?

A friend of mine ( a trained mechanic too) checked a customers compression and got some low readings on a couple of cylinders. He recommended an engine rebuild which he subsequently carried out, then when he re checked the compressions he found the same low readings on two cylinders!
He told me about it and I asked him if he had checked the readings with the throttle wide open.

I could tell by his face that he hadn't thought of this and he told me some time later that the compressions were actually fine. Oops!

Colyn Firth

I'm with Colyn on this, throttle must be open to get proper readings.

Colin Manley

"throttle must be open to get proper readings."

Begs the question! Why?

Barry Gannon

The throttle was not open .....and it makes sense to open it to ensure cylinder can suck easily. Will check again and advise ( Colyn...the question was certainly not stupid .....thanks ) .
Neil Ferguson

Another question, did you run the car after the first compression test? If not you may still higher compression due to the oil you put in the first time.
Neil MG

with the throttles closed the engine can only draw in the small amount of air the it uses at tickover. Wide open, it can draw in the maximum possible volume and therefore give the proper compression readings.

Back to the story of my mechanic friend, I suspect that he probably still charged his customer for the unnecessary work, I suppose he could truthfully say that the engine now had excellent compression!

Colyn Firth

Followed the advice..throttle locked wide open and engine cold like first time...engine turned over about 10 times..

Cyl.1...133, Cyl.2 ....120, Cyl. 3 ...105, Cyl.4...122

Did the test on 3 twice.... still low.....and difference between 1 and 3 is over 20% ..a concern.

Put oil in ..
Cal.1...145, Cyl.2...145, Cyl.3...125, Cyl. 4 ...162 ( wow!)

Did 4 twice to check the wow. and same.

Took the car out for a 40 min spin up and down some hills locally at speeds up to 100k and performed well...pulled strongly up hill etc.

A wee bit worried about 3 ..but bumf I read say don't panic until under probably not going to disembowell yet.
I have noticed that when I start from cold it runs on 3 for a time ( 5 to 10 sec) before the 4th when I blip the throttle .consistent with a weak cyl.. (I have read )

I had the thought that maybe the Judson compressor is restricting natural airflow in and depressing the figs..( but does not explain 3). I also had the thought that the low pressures might be the reason I can't get boost above 0psi .

If there is so much leakage past the rings should I not see venting from the crankcase ...rocker cover.

Comments ..incl. how worried should I be ?

Neil Ferguson

Neil MG..yep..did run the car ...and generated a good cloud of white smoke as well.
Neil Ferguson

Valve seat recession I would suspect. Time for unleaded inserts. Simple check with head removed, but valve gear still in place is to turn the head upside down and fill each chamber with fuel. If the level drops quicker in one of the chambers (No. 3 in your case) then it will confirm the recession.

Steve Gyles

Found my cyl. pressure teass of about 3 years ago oil used ..
Cya.1...130, Cyl,2...123 and 125 , Cyl.3...118 and 123, Cyl,4...126

With the exception of Cyl 3 these are very comparable to current figs..
So There is an issue with deterioration of cyl 3 from 118 ( already lowest then ) to 105 now.
I decided to do the 20 psi CYL. leak test as described by Barney but had no kit so had to make something .
I obviously have a pressure tester so made up the hose/fittings as shown on pic below. It consists of the compression test hose clipped to a clear hose ( radiator overflow hose ) .
To the clear plastic hose end I fitted a tye air pressure fitting ( with internal check valve ) .
The brass fitting at the plug hole end of the test compression tester hose has an internal check valve pointing the wrong way (for obvious reasons ) and I needed to take it out of action for current test so I put a small screw in the adapter (the one that fits in the head plug hole ) which when the adapter and hose fitting are brought together holds the check open.
Worked a treat and I I top centred cyl 3 and tested it... loud leak hiss and seemed to be coming through the sump up to valve cover . Tried a few times and got the same result. So most likely I have ring deterioration in shown by this test and the 20 psi increase in pressure test between dry and oiled .
Leak tested cyl. 2 for comparison and no hiss audible,

Having made the hose assembly shown below I used it to 10 psi pressure test the cooling circuit as it still loses too much . All I did was take off the clips holding the clear hose to the test hose and connected the end of the clear hose ( with tie wrap ) onto the radiator overflow nozzle. I inserted a part disc onto the fill nozzle shoulder to keep the lower cap seal off its seat...worked well and found a big leak from one of the hoses ..the most awkward of course.

reasonably chuffed so got all the kit back ready for a test drive ..and the starter drive flew off its shaft.... too much hard work for it obviously ( time to strike ) ..onwards to another day. I seem to have a non standard drive assembly ....the drive mechanism is held on by a split round ring in a groove ( like MGB?)..the MGA standard seems to be a threaded shaft and nut and split pin .
Neil Ferguson

pic for last post

Neil Ferguson

The last time I rebuilt an MGA engine (a 1622cc Mk II) was in 1975 when the big ends went exactly 4 days before I was due to go on a camping trip to Switzerland towing a trailer tent.

As I had to fix the engine as cheaply as possible I just fitted new big end and main bearings as the journals looked ok to me at the time. (It was a decision driven by a combination lack of funds, ignorance and a lack of common sense! :-) )

I also decided to emery the glaze in the bores and fit a set of new Cord 3-part piston rings to give the compression a bit of a boost.

The oil pressure was back up to 60 psi and the engine was definitely producing more power when I had finished the job 2 days later.
I even lifted the engine by hand, from the workbench down onto a wheel barrow so that I could move it out to the car! I was much stronger, fitter and very young and foolish back then!

I wouldnt do any of it like that today.

The Cord piston rings made a surprising improvement in power output and could be a quick fix for you Neil
1- they are still available.
2- They would be strong enough to cope with your supercharger.

Colyn Firth have lost me . I have only ever once replaced rings on a car ( a spitfire in about 1967 ) so not clued up.
What was special about the cord rings ?
How can a ring boost compression ratio?
Why are vulnerable to low pressure boost ?

I presume I have to pop the Pistons and check bore and groove sizes etc before I order rings ...
Neil Ferguson

Cord rings were specially designed to compensate for bore wear.
When I used them back in the 70s they were triple rings, each single piston ring was replaced by three individual very thin rings in what appeared to be stainless steel.
The 3 rings move in and out in the piston grooves to slide over uneven bore wear and maintain contact with the cylinder wall much better than a single ring could.

With four ring grooves you would therefore end up with 12 piston rings per piston instead of 4 and this created a much better seal. So you can see why this would restore compression on a worn engine.

(Yes you are correct, it doesnt boost the compression ratio.
As for a supercharged engine, I dont know if the Cord ring design would be tough enough to withstand the increased pressures it would be subject to, you would have to check with Cord )

It looks like Cord still exist,
I just checked out their website

Colyn Firth

What you have shown with repeated tests is low compression with improvement when you put oil into the cylinders. Additionally the low compression varies from cylinder to cylinder.To my mind your numbers suggest ring and bore wear, worst on #3.

I would not have thought that valve clearances only a few thou narrower would cause a major problem.

I don't know enough about the induction effect of a blower to know if that would change measured compression figures artificially.

I admit to almost total ignorance about blowers, but I would expect that you should be getting some measurable boost pressure from it. If the pressure is zero (ie ambient) doesn't that mean that it isn't doing anything?

If the engine is old enough that bore wear is affecting compression it's likely that there is also significant wear on other components, including the head.

I agree that if you are getting ring bypass that should be pressurising the crank case but whether it would be grossly noticable or not who knows.

As to what to do... In a perfect world we'd all drive cars with brand new engines. Reality means that this is rarely the case :-)

Without knowing about the history of your motor I'd be assuming that it was tired. Running it with a supercharger will probably accelerate the amount of wear that it would get if it was not blown.

So it's a balance between being prepared to spend time and money on a complete rebuild vs risking a catastrophic engine failure that does irreparable damage that prevents you rebuilding your current engine.

I just did a complete rebuild on a 1622. I spent about $3k on comprehensive short motor machining, $1800 on some pretty fancy head work including a lot of porting and about $1k on parts.

With your blower you get more bang for your buck. You can use stock low compression pistons and don't need silly headwork. You could probably end up with a completely rebuilt long motor for a couple of grand if you have the right machine shop.
A Bowie

Colyn....thanks for advice re rings. I have been consumed with getting the starter out and fixing the drive ( retaining ring popped out of worn shaft groove ).
I don't believe my car has ever had bore work done ( I've also had it for 27 years ) and It will be a good few weeks before I get to working on the engine as I will be moving house . Do I need to wait and see bore condition before I order rings . I note the rings I looked at on net quote oversize 30 thou etc.. I would prefer to have rings to hand when I pop the Pistons so I can close up right away but is it practical?
Should I replace all rings on all Pistons ?
Even though no indications of problems would you replace big and little ends whist doing the work?

Neil Ferguson

Andrew.....thanks for detailed comments .
Engine is a wee bit tired but I am not pessimistic .most vital signs seem OK .
Cyl 3 needs work and I probably will do all cylinders and the ends when I open up. ( see comments to Colyn below )
I will also examine head and if seats need work then also.....also guides. I may also do a wee bit of flow improvement work on the passages...but only basic stuff.
Re Supercharger comments...
I have a vacuum gauge in the car and can see the charger working/pumping so it is getting more air into the cylinders and doing work.
I also believe, after much reading on the subject ,that a low pressure charger is not stressing up the engine fact possibly the reverse .The engine is not 'blown' in the sense of big boosters .
Neil Ferguson

the Cord ring solution would probably get you by for quite a few years ( I ran my 1600 MkII hard for 4 years as my everyday driver after I fitted them), but obviously a rebore with new pistons would be the ideal solution. It depends what suits your budget.

If your oil pressure is still good then the main bearings are probably ok, (it is just possible to change them with the engine in the car but it would be much easier to take the engine out to do this)

When I fitted the Cord rings I took the glaze off the bores with emery cloth to let them bed in better.

I ordered the Cord rings to fit the piston size, (your pistons will have "STD" or "+30" etc oversize stamped on their crowns) so you wont know what size you have until the head comes off (unless you can get hold of a good fibre optic gadget that will let you look through the plug hole).

You should fit them to all four pistons.

I took the head off and decoked the engine, reground the valves, fitted new inlet valve stem seals and cleaned all the carbon from the ring grooves.

All the work was done with the engine in the car, I dropped the sump, took the pistons out and changed the big end bearings from below.


Colyn Firth

With the degree of disassembly you will need to do the rings you may as well do the bearings too. A set of new shells costs maybe $100... Again, a new timing chain would be cheap and sensible.

Were it mine I'd strip the whole short motor and give it a really good clean as this always pays dividends on reassembly. Getting someone to tank the block and even grit blast the inside of the water jacket may pay dividends in terns of cooling.

I don't think you should contemplate replacing the rings without first measuring the bores and pistons to ensure that what you have is within spec. Again measuring the crank journals would be wise too. If everything measures up OK you can reassemble it all with confidence.

WRT the head porting, presumably the money is in improving exhaust flow given that your induction is forced rather than relying on good flow characteristics and vacuum?

Looking forward to seeing how this progresses.



A Bowie

Andrew and Colyn two guys have been extremely helpful ...many thanks.
I move house 700 km about mid April so not going to disembowell before then ..and it will take me a few weeks to get set up in new place, hoist will be a month at least before I get going.
My scope based on your comments..subject to reality when exposed...
1...Off with head and strip...check valves and the exhaust passages mildly to get rough out.
2...When head off check piston top and bores condition . Order rings to suit ( or panic !)
3....Drop sump off and get Pistons out ...check Pistons condition (replace replace little ends ....and consider big ends ( my oil pressure is very good ..always above 60 ).....
4 ..Hone/emery the bores
5....Reassemmble and zoom away with sprightly re-energised mga .
Neil Ferguson

5....Reassemmble and zoom away with sprightly re-energised mga .

A Bowie

re one of my earlier comments that the starter drive ( bendix ?() had come off the shaft.
Upon examination the round split ring was sitting in a shallow shaft groove and also recessed into the end spring holder/washer. I suppose the idea was that the recess in the washer would hold the split ring in the shaft groove and prevent dissemble under load ..and it did for at least 27 years.
The shaft ring groove was bruised, extended and rounded it came under load..
I dressed up the groove with a file to make a new groove shoulder reassembled with the old split ring and put another standard retaining ring on to take up the slack and hopefully sit against the shoulder I made ..put it all together , put oil filter back on ..Engine rotated three times and off it all came again.
Next try followed me drilling a hole near the end of the shaft and assembling with the old ring and then inserting a split pin in the new hole abutted against the old split ring..
It seems to have worked....hopefully for a long time . ..such fun!
Neil Ferguson

This thread was discussed between 21/03/2016 and 27/03/2016

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.