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MG MGB Technical - Valve clearance


Last year i brought a 1968 B,the engine was rebuild (by the last owner )and had 12000 miles on it The engine was tikking, so i adjusted the valves, 0.13 cold, after a few miles about 100 or so , the tikking was back, i noticed that the NR 6 valve had a lot of clearance, adjusted them again, and about another 100 miles the same problem, this went one a few times , till i pulled the nut for locking on the rockerarm verry hard.
I drove then about 3000 miles without a problem, and now the tikking is back, again on valve NR6, i adjusted them, and pulled the nut realy hard on, but the tikking didn't went away.the valve nr 6 is the only one that is tikking, i put an feeler gauge (0.03)between the valve and rockerarm when the engine was running and the tikking was gone.
Where should i look, at the camshaft, the pushrod, the rocker arm,tapped, or something realy different.
thanks serge


It is probable that the cam lobe and / or follower are wearing badly on that one valve - you will need to remove the manifolds to get at the rear tappet chest cover and release the pushrod by undoing the adjusting screw and sliding the rocker arm to one side.

Then you can lift out the no 6 tappet using a pair of thin nosed pliers.

Wear on the bottom of the tappet will be obvious and if that is badly pitted and concave then the chances are that the cam lobe is worn too.

Replacement of the cam is possible without taking the engine out but you will have to drop the sump and take the oil pump out as well as the distributor drive etc.
Chris at Octarine Services

You might like to check cam/valve lift across the engine to make sure you do have lobe wear before bothering the cam-shaft. Follower wear will result in increasing clearances that you have had, but not necessarily cam wear i.e. the lift will still be good. Cam lift i.e. at the push-rod end of the rockers is 0.25" or 6.35mm, valve lift is 0.3645" or 9.25mm.
Paul Hunt

Checking the lobes for wear is well worth it as they can sometimes be worn beyond simple repair. All the ex-factory cams that I measured when I bought new ones in the '70's were 0.268" lift from the base circle. This would make the cam lift listed appears to be at the BMC timing clearance (not running clearance) of 0.016" cam side/0.022" rocker side.
Paul Walbran

The cam may have suffered a premature death due to lack of ZDDP...
Glenn Mallory

A pals newly-rebuilt engine only did 20k before the cam and followers were knackered, he wasn't aware of ZDDP until I mentioned it. The garage he had it serviced at (who were also the rebuilders) had always used a semi-synthetic 10W/40 at API SL, which is a low ZDDP oil intended for modern engines with catalysts. They weren't aware of ZDDP either. He now uses a formulation for 'older' engines with higher ZDDP, but hasn't done enough miles yet to see if that was the problem, or just that modern cams and followers aren't case-hardened as well as they should be.

Even though I have an 'old' engine I use a high ZDDP oil (Comma Sonic) for insurance reasons as much as anything else, i.e. same as Castrol Valvemaster.
Paul Hunt

When you purchase a new crate engine with a flat tappet cam from Chevrolet, they provide you with the high ZDDP oil for the initial breakin. They also void the warranty if you don't continue to use an oil that has a ZDDP level that meets their specifications. The friction point, between the camshaft and lifter, is the highest inside a flat tappet engine. RAY
rjm RAY

I don't know whether I'm super talented or just lucky, but I've swapped a cam without either sump or oil pump removal. Just wind it in.
If there is a hardness discrepancy between the cam and follower,( it happens), a soft follower will damage the cam less and you may get away with just a follower replacement, use cam lube and a running in additive. It's worth a try. Also check the condition of the rocker pad if it's wearing fast it will chew up your feeler when you slide it in with engine running, and the rocker shaft under that rocker( slide the rocker back and feel the shaft or use a mirror). Also pull out the push rod, I've seen them bent and tapping on the head, but that wouldn't cause the clearance to keep increasing!.
Allan Reeling

Allan - you can change the cam in later engines without dropping the oil pump but the early engines need the extra clearance afforded by dropping out the oil pump and its drive gear.
Chris at Octarine Services

Chris, I'm interested in where you've found the change point to be. As far as I can work out that change was somewhere during 18GG, as I have found the early 18GG engines (1970) generally need the sump dropped but 1971 models with the 18GG engine usually don't.
(And of course we are talking 5 bearing engines here, the 3 bearing engines don't have the intermediate web in to block that the cam fouls on)
Paul Walbran

The changeover point is the introduction of the horizontally split conrods in June 1970.
Chris at Octarine Services

hi Paul

I cannot say which ones let you remove the cam easily without sump and oil pump removal....doesn't seem any logic to us, some come out some don't. Mind you I don't suppose many internals match the external number anymore, if they did in the first place!

Peter Burgess Tuning

I've yet to find an 18V which required the sump to be dropped, or an 18GB/GD onew hich doesn't, but 18GG I just cross my fingers and hope :-)
Paul Walbran

Serge, I noticed you wrote valve adjustment 0.13 cold. Should it not be 0.15 cold (I pressume you're measuring in thousands of inches using a feeling gauge, not metric)?

That's an intersting one. Yes, the book does say .015 cold. But the Midget book give the figure as .012 cold, which translates to .0135 by the time the difference in rocker ratio is factored in. Although the camshaft is different between the engines, they share the same opening ramp form. For those not familiar with the term opening ramp, it describes a part of the cam form where the lift velocity is held at a low constant speed - typically for about 20 deg or .006" of lift on BMC cams - so that the valve train can take up without hammering.

As the opening ramp ends at .015", it means that wear in the valve train between tappet-clearance checking results in the take-up occuring past the end of the ramp, when the valve train has high acceleration. This will reult in shortened valve gear life, acceptable for racing where increasing the clearance can be beneficial for performance but not good for longevity on road car engines.

Because of this I have always gone with the A-series version and set B series clearances to a tad over 13 thou. Some have asked whether the extra time off the seat would affect the life of the exhaust valves due to them running hotter, but the extra is not even as much as a mild fast road cam.
Paul Walbran

So setting to 0.13 thou cold is suggested then! I have tried on my engine (spec 846F) but it seemed 'tight'. Indeed less rattling of the valves but I was concerned.


This thread was discussed between 14/08/2013 and 07/09/2013

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