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MG MGA - Valve Stem Circlips
|I thought I would give my 1500 engine (5,000 miles only since a total rebuild) a check over as it has been sitting on the garage floor for 14 years.|
When I came to dismantle the cylinder head I was bemused to find that only 6 of the 8 valves had the circlips around the cotters. There is of course now the question where did the other 2 go!! But that got me thinking as to what practical purpose do these circlips serve? The cotters are kept well and truly in place by the strong (double) valve springs, so is this a case of 1950s belt and braces over engineering?
|Interesting thought. It might be something to do with valve bounce at high revs Steve. The circlip would keep the top of a collet tight into the valve stem's shoulder should it somehow pop out of the valve cap. I bet its there for a reason or it wouldn't be there.|
I spoke with Bob West, asking for a couple of clips. He does not do rebuilds anymore. He outsources to the likes of Peter Burgess and others for engine work and cylinder heads. He says the cylinder heads rarely, if at all,come to him now with those clips. He said to leave them off.
|I always seem to be coming in with tales of woe but this is my biggest ever which would make me very wary of leaving the cir-clips off.|
In my high mileage days of 1971 I had a 1600 Mk 1 engine in my 1500 car. I did a decoke over a weekend, as you did in those days. After driving around 100 miles on the M1 for work I suddenly had no power, lots of noise and the smoke coming out of the back that was like the worst of F1 engine blow ups. Once I was home and stripped the engine what a mess. It appeared to have dropped a valve that had hit the piston which was in what looked like aluminium gravel bits in the sump. The small end had gone through the bore leaving a 2 by 0.5 inch hole, the con rod was bent, one bit of the piston gravel had managed to get itself into another bore where it jammed in a open valve. I never did find the valve, presumably it was in bits in the sump with the piston gravel. Not surprisingly I binned this engine. The point is I have always blamed this failure on my decoke of the previous afternoon, and thought/think it was caused by the cotters coming adrift possibly due to me not getting the circlip on properly hence I wouldn't gamble on this. On the other hand I hear/respect what Bob West says.
The non technical bit is I bought a Gold Seal 1500 engine in Birmingham. My employers hired me a Morris 1100 so that I could do my job. One evening I took the front passenger seat out and replaced it with my new 1500 engine and drove it to the garage near Nottingham where the RAC had taken it. I didn't go through the floor but bringing the old unit home was worse as it was a dirty engine this time. Anyhow I got away with it. C'est la vie.
This 1500 engine has been with me ever since until I replaced it with an 1800 unit this Spring.
|My goodness - why leave them off? It's not that difficult to put them on, even if your Sykes-Pickavent spring compressor gets in the way - which it does.|
When Peter Burgess did my Sprite head a few years ago, he put them on. Unless someone in his shop sneaked them on when he wasn't looking.
Ah - it's a weight saving. Better accelleration.
|Nick and Cherry Scoop|
It's like finding half a caterpillar in your lettuce. where is the other half?!! All very well having these clips in place, but what if 2 are missing as in my case? Potential for a bit of damage somewhere in the system.
I don't have a particular opinion on the for/against. I am more interested in the why.
From what I can see in diagrams, the earlier XPAG did not have these clips.
|I've done head jobs on a few cars in my time and the only time I've seen the clips is on MG!|
|You're quite right, Steve. The truth is, not being an engineering type, I'm a bit scared of the whole valve and spring assembly.|
Coiled and waiting to spring, as it were.
I wouldn't dream of leaving the circlips off.
|Nick and Cherry Scoop|
|After my experience clearly there is no way I would leave them off. Interestingly though according to the Moss catalogue they are not fitted on Mini, i.e. A Series, engines. I suppose this enforces my view that someone in a design office decided to fit them for a reason on A engines (or is it to all B series?) hence I wouldn't change it without understanding why they were fitted in the first place. My incident happened shortly after joining a motorway so was probably the first time I went to higher revs. |
I suspect the design reasoning was that in certain rare circumstances the closing valve can get ahead of the springs hence allowing the cotters to be momentarily slack This would allow the cotters to jump out of position. What I can't really see is what are these circumstances although one ideas would be in valve bounce, although I don't think I was going fast enough to get bounce. I wish I could remember whether it was an inlet or outlet valve that went, although my gut feeling is that it was an inlet one.
The theory is fine but don't gamble your engine on omitting this small!
|The patent describing the valve assembly is online at http://www.google.com/patents/US2705483. It dates from 1952 and is assigned to the Austin Motor Company.|
If I understand the description correctly, and it is written in a sort of legalese, the idea of the retaining clip is to maintain the hold of the two cotters onto the valve stem if the valve spring breaks. This will then stop the valve sliding down the guide and falling into the cylinder.
|After some research I found the following note on a Mini (A-Series engine) specialist website: "Note also that while the early large groove collets were originally secured together with circlips, around 1969 these were judged to be adding nothing to the security of the collets and were discontinued."|
|Malcolm. Interesting. However, that would require both springs to break.|
|Having read the 1952 vintage patent researched by Malcolm; the comment by the A-series specialist; and the information gleaned from Bob West, I am coming to the conclusion that it is a belt and braces modification for a one-in-a-million potential failure of a double spring in an A/B-Series head. In my humble opinion there is more of a chance of the relatively frail circlips parting company than the double failure of valve springs - I seem to have one such head to prove it.|
I am going the way of omitting the remaining 6 circlips. I have to remove the sump next to retrieve the distributor drive that I managed to drop! I will be interested to see if the sump contents also reveal bits of circlips!
|Only one valve spring on the 1952 A-Series engine, so it's a possible.|
Nice research, Malcolm.
|Nick and Cherry Scoop|
Might have been the case in 1952, but all MGA B-Series are double spring. Reversion to single spring came later (cost saving) with some 1800 variants. Sounds like a classic case of old practices being carried over from one engine design to another without someone asking WHY.
|I must be that 1 in a million with my valve drop leading to to disastrous engine failure!!!!|
Valve bounce does not allow the cotters (keepers in the US) to become slack.
Valve bounce at excessive RPM is when the springs cannot close the valve quickly enough to keep up with the cam lobe moving away from the tappet.
All that happens is that the valve clearance is momentarily increased above the correct gap.
On some engines a valve may be hit by a piston.
At all times, during normal running or at valve bounce, the spring is always in tension closing the valve.
|M F Anderson|
|I suppose all I know is the very dramatic way my engine blew up and the resulting damage as outlined in my first email in this thread. As most of my long held theories are being dismissed does anyone have any other ideas what was the cause? I suppose the valve that 'disappeared' could have been damaged during the decoke although the failure was only a day or so after the decoke and I would have suspected this if anything untoward had happened to the valves while they were out. It was an expensive incident as I had bought the 1600 engine for £50 not that long previously and I had to spend another £50 on the replacement, all on a car that had only cost £100.|
This thread was discussed between 21/06/2014 and 25/06/2014
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