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 MGB Technical - Camshaft Wear

A month ago or so, it was time to do some minor adjustments to my 72 B and I wanted to better ensure that the carbs were balanced, so I started with the usual adjustment of the valves first.

Now I've had the valve cover off many times for valve adjustments, and even had the head off a couple years ago for a valve job. So I've needed to adjust the valves a few times. And I've been looking closely enough that I should have noticed this before, BUT!

When adjusting the valves this time, probably less than 6 months from last time (and only 2,000 miles or so), something was obviously going wrong.

One of the rocker arms was sure not moving like the others. Upon closer comparison, it looked like the valve was bearly moving up and down. No I haven't measured the distance.

We watched all the valves and rocker arms for a comparison, and the one valve is definetly not rising as much as the others. The others appear to be moving the same, just this one isn't.

Our first thought is, the cam lobe for that valve must be really wearing out fast. We didn't notice that the push rod was bent, but I should go back and really compare all the travel distances, and this one push rod.

But if it is the cam lobe - why just one and to me it sure happened quickly. It is a very obvious difference in travel from the others.

Is this the dreaded issue of not having the right additives in the oil and cam wear was inevitable? I've been running Castrol 20w-50, and was adding 1 quart of synthetic 15-40 in an effort to slowly clean the engine out. I have no idea how many miles on engine, but when purchased with broken odometer it read 90k+. Right now I have all 15w-40 or 50 synthetic in the engine.

The thing is the car is probably running the best it ever has, and my gas mileage is finally into the mid-20's, and last tank was 27mpg. It starts Great, runs so smoothly, and responds so well - I hate to touch anything. If it wasn't for the visual of the rocker arm bearly moving, I would think everything is fine.

But I know something isn't right, and a cam replacement may be down the road soon.

So, questions are: Why so sudden, why to just one cam lobe; and anything to watch for or consider when I go to buy a replacement cam? I'll be doing the work myself. This is not a restoration project, it is my daily/weekly driver, not for show or purity of parts.

Your thoughts. & Thanks!
R.W Anderson

You are well past the usual cam wear out point; it is not uncommon to replace cams on engines with 50,000 if oil changes were neglected. I've never seen a BMC engine that needed to come apart that did not need a cam and followers. It will not be one lobe, just that one is worst. The lobe and follower surfaces are hardened; it takes a long time to wear through, and then wear is progressively faster as it works into softer metal and the follower disintegrates, forming a sort of cutter to further eat the cam. At this point,the followers are bad enough that it would not be unlikely for one to simply fall apart, with serious damage to the block, so fix soonish.
Stock early MGB cam or mild "fast road", good followers, timing chain and tensioner, check that sprocket teeth are not worn.

FR Millmore

As an approximate 'rule of thumb', would I be correct to say that if one is replacing a rocker shaft assemby, then one should consider the rest of this mechanical train? i.e pushrods, cam followers and indeed cam? My engine has gone past this 50K miles marker. Seems to be running alright, and I am unsure as to the past history, but definitely some rocker shaft wear evident. Mike
J.M. Doust

Mike if you doubt the cam measure the lift at the push rod. It should be .265" (although some books give .250") I replaced my cam at 70,000 miles with a performance cam and it was still like new however the followers were just starting to show their age. As FRM states the minute rubbish from wear embeds into the lifters and accelerates wear on the cam. Thats why you always replace the lifters and cam together and you buy good lifters with a hardness of 60 units. I use APT from the states (Moss also sell APT) Although I have also used ones from Mini Mania with good results. Denis

Looking at the push rod with the engine running, it should rotate. If it doesn't there's wear! Lift the tappet out with your finger. You'll soon notice the wear. If you have to replace the cam and tappets, i would replace, at least, the chain and tensioner. Also when installed and timed don't forget to run it in with cam lube, at above 2000rpm for at least 10 minutes.
Allan Reeling

I think I'll do the actual measurement to record actual lift to see just how much wear, and whether or not it is at all valves, and more at the valve in question.

Some friends have suggested that if I'm going this far and doing the cam, and associated parts I may as well rebuild the whole thing and be done with it.

I'm thinking I should open my garage door and drive the MG out and drive a Miata in. Yeah, I know I should escort myself to the wood shed for a lashing having suggested that!

I'm also thinking that if it is true the engine may self destruct from metal parts falling into the oil and circulating from the worn cam and rockets, I may as well drive it until it does self destruct, as it sure is running the best it has in 3-4 years.

Thanks for feedback.
R.W Anderson

John Twist and camshafts
Geoff E

Now that is one good video. And current too! A shame UnivMotors got hit by space junk.

Well, it looks like I need to pull all the pushrods to see how they look. Very good video. I liked the explanation that one owner felt his car ran well, yet the cam barely had a lobe left to it. Gees, does that sound familiar.

So its off to the garage in the days ahead to check pushrods and measure rise and fall. Wouldn't it be nice if I too only needed a pushrod.

Thanks a lot for the video link.
R.W Anderson

When replacing a cam due to exesive wear just think for a minute.
The grit grinded off has by now gone through:
The pump
The oil galerys
Some bearing surfaces

Yes the magnetic sump plug and the filter get a lot out but not all of it.

Cam wear means change
Oil galery plugs (incluing cleaning them)
Chain & tensioner
Oil pump

And this is just when you discover no hidden surprises
Onno Könemann

Excellent comments folks. I have just addedd some more to my list of thinks to do, Ah, life's pleasures. Mike
J.M. Doust

Okay, last evening I measured the distance the valves rise and fall on my 72B. All to determine which if any valve was not opening as far as the others and thus indicate cam wear.

I did this by standing a wide ruler on the machined surface of the head (where the valve cover would set/seal) and noted how far the top of the spring rose and fell. I did this for every valve at each spring (engine off, plugs out, car in gear to rock car to alternate valves).

All valve spring tops came up to the same high mark and dropped to the same low mark, showing a 1/4" (0.250) rise and fall. This visual measure was within 1/32nd of an inch or less (0.031).

I also checked to see that each push rod spun by hand. Which they did.

I was relieved to see that whatever wear there is, it was at least uniform.

I also recalled the above posting that indicated the specifications distance was either .265" or .250". So I thought I was in the ball park.

Then I looked up the specs in a Haynes MG book and saw the distance for valve rise and fall was .365". The above spec implies I'm close, the book implies the cam has worn down 0.100" or so. Indicating the valves are not opening far enough.

So now I'm either fine given the .250/.265 posting above, or I'm worn down given the .365 spec in my manual. Am I mixing my specifications?

Discussing all this with my neighbor last night has concluded that since it is going to be in the 80s most of this week (50s for the 6 a.m dark drive to work) I should drive the car and wait for something to break; then worry about it.

Your thoughts! (And as always, thanks for all the comments.)
R.W Anderson

Does the Haynes specify lift at the cam or at the valve? If at the valve, then you must consider the rocker ratio which is IIRC about 1.4:1, therefore lift at the valve will be about 1.4 x lift at the cam, or 0.250 * 1.4 = 0.350 -- near enough 0.365 given I don't know the exact rocker ratio! ;-)
Rob Edwards

Interesting. Forgot all about the lever length and fulcrum effect on measurements. I just assumed the fulcrum point of the rocker arm was at the center of the rocker arm.

I believe the text referenced "valve" lift, but now I'll have to go back and look at book, and dig out my Proper original Manual too.

Does this suggest I need to go back and measure the rise and fall of the pushrod instead?

All I know is my rise and fall at the valve is .250" approximately.
R.W Anderson

If you measure .250 at the valve and haynes says .350 at the valve there is no need to remeasure.
Either haynes is wrong or your cam is worn
Onno Könemann

My factory manual (1972 year) has VALVE lift (inlet and exhaust) at 0.3645 in. or 9.25mm for the 18G/GA engine with no variation on that for engines up to 18GG.
Roger T

I agree with Onno. If you're only getting .250 at the valve, then it's time to go into the engine. I didn't read closely enough to see that you were measuring the valve spring -- .250 sounds like cam lift so that's what I assumed you were measuring.

I found a quote online stating that MGB rocker ratio is 1.445:1. So if the valve lift is 0.3645, then the cam lift is 0.252 (or maybe 0.267 depending on whether or not 0.3645 includes the 0.015 valve lash).

However, it does seem a bit odd that: !. all the lobes are worn evenly, and 2. the wear is an amount such that the valve lift numbers end up looking like the cam lift numbers. Seems like a lot of coincidences. 0.100" is about 3/32". How certain are you that your measurement technique would let you see 3/32"? Can you get your hands on a dial indicator? Got a Harbor Freight nearby? ;-)

Rob Edwards

I measured at the spring. I measured the difference between compressed spring and uncompressed spring. Gauge had 1/8" indicators, rise and fall was 1/4". It was easy to see 1/16", more guessing to see a 1/32" difference. So I'd say I may be off by 1/32" not 3/32".... But you are right, a dial indicator would have been better. Yes, there is a HF around here somewhere. Off I go to buy more Chinese stuff.

I too am suspicious of my measurements, that imply a perfectly uniform wear on the cam.

In the meantime I'm driving and driving in 80 degree unseasonable weather, waiting for the snow to come in the few weeks ahead. Then it will be too cold to worry about all of this until Spring.

Thanks again.
R.W Anderson

Rob, I would think the valve lash would have been deemed apart from the actual valve lift at the spring ie treated apart as the gap between the uppermost point of lift of the valve by spring movement and the lowest set position of the rocker face.
Roger T

Hi Roger,
Not sure I quite follow you, but I've been reading quite a bit about cams recently because my TD needs one. The only consistent thing I've found out about different cams is that there's no consistency about how manufactures specify them! :-)
Rob Edwards

When the OE camshaft came out of my car at 85000 miles to make way for the Oselli 270 it was fine. You could see it had been in a car but it was ready for it's next 85000. BMC engines were designed for a 50000 mile service life but oils have improved out of recognition since the 1940s.
My BMC manual gives cam lift as 0.25" 6.35 mm. These numbers agree with each other. The ratio is indeed 1.4:1 from the same source with valve lift .3645 (9.25mm) These do not quite line up so the rocker ratio must be an aproximation. You can change the cam (for a mildly tuned one?) with the engine in the car then get on with driving and enjoying it. Neither my MGB or MG TF are trouble free, but they have soul, the only Mazda's I own are light bulbs
Stan Best

"the only Mazda's I own are light bulbs"

But unlike our cars they don't have a 'flicker' setting :o)
PaulH Solihull


You can borrow my dial indiactor if you want, but I think you already have answered the question. You have uniform cam wear. It is probably affecting the breathing of the engine quite a lot.

Dave Braun

Aw shucks, just drive it. If you wanna see, spend an hour pulling the carbs/manifold, chest covers, and pull out the lifters and eyeball thier surfaces. If they look like the surface of the moon, or a teenager's face, you're done, ez peezy....just enjoy the ride till you can't as you're gonna have to rebuild sooner or later........ With all the springs compressing equally, according to your eyeball and yardstick, I'd buy that $30 dial indicator & mag base from HF and confirm...or just drive it. Valveolene Racing oil, 20-50, has the ZDDP or you can add it at oil change: it is required for our flat lifters since synthetic and most organics no longer feature this saving grace. Of course if your lifters are pepperonied, it is too late. Cheers Vic
vem myers

This thread was discussed between 29/09/2011 and 06/11/2011

MG MGB Technical index

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