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MG MGB Technical - Tappets


Hi I would be grateful for some advice.My tappets have clattered for some time despite being adjusted several times. I have set them at 13 thou but still noisy. I have just used an old feeler gauge while the engine was running and the noise stopped while slipping it under no 3. Do I tighten this one down further or is there some other procedure?


Rod
Rod Merrall

I would be a bit weary of having the tappets too "tight". 13 thou, seems a bit close and could risk of burning a valve. Much discussion has been posted here over the 20 years that I've been with this forum.
The general opinion seems to be 15 thou according to Chilton's Repair Manual and Haynes Repair Manual.

MG's have been known to be a bit "chatty"

Cheers

Gary
79 MGB
Gary Hansen

As well as worn rocker tips that result in a greater clearance than the feller gauge might suggest there are also cams that give an inconsistent clearance round the backs of the lobes - mine and others I have worked on.

I always used the strict 'rule of nine' point but could never get the same clearance twice on some valves, which turned out to be because the gap was changing as it passed through that point. Checked all of them with a dial gauge and only one gave the greatest clearance at the rule of nine point, the others could be before or after. Since logging the point of greatest clearance and checking/adjusting them there not only are they repeatable, but also they haven't changed i.e. haven't needed readjusting and they are slightly quieter.

Doing it with a running engine has always seemed a bit Mickey Mouse to me.
paulh4

Tappets that remain noisy when set correctly usuallt indicate wear in the cam followers. If left long enought they in tern qear the camshaft.
Paul Walbran

I too use the Rule of Nine, but as PaulH4 suggests that isn't necessarily the widest clearance. So are we saying that they should be adjusted at the widest clearance, irrespective of whether that meets the exact RoN criteria? I hope so, because that's the way I do it, and as Paul says I find it repeatable and possibly a little quieter.
Peter Allen

Perhaps not surprisingly I see no reason why not. But if you choose to use the Rule of Nine a Rule of Five is even easier.

For some time I'd seen reference to adjusting them 'rocking' or on the rock' mentioned but no explanation of what that actually meant. Long story short it seems to refer to the partner valve being somewhere in it's travel from just started to go down to just before it stops coming up, which gives a far wider range of adjustment positions and engine rotation, rather than when it is fully down and about to go back up again which strictly speaking is one particular engine position. Take that a bit further and if you use the point at which both partner valves are partly open at the same time, then you can check/adjust two valves in one go. Hence Rule of Five counting cylinders rather than valves. In theory it makes no difference other than needing to turn the engine less often as both lobes of valves being adjusted should be somewhere on the back when both partner valves are partly open.

Of course it's no help where the clearances are changing round the back of the lobe.
paulh4

John Twist has recently made an excellent new utube video about checking 2 valves at the same time on 2 July 21 about this.
Cheers
Charles
Charles9

Yes, it's a less thorough repeat of his video number ten (a nice easy number for me to remember) with white board explanation. Because he's so used to doing it I don't think he explains the practical explanation good enough for the likes of me.

I've found confusing and contradictory info on the order of the valves opening (or closing) and those used to it just do it without keeping record.

10 MG Valve Adjustment - https://www.youtube.com/watch?v=fezXUwVfH7U





Fancy starting a sentence with the word because and on the anniversary of Thatcher's war.
Nigel Atkins

Rod, I had a problem with a noisy No2 valve, which turned out to be because of a problem with the alignment between the rocker and the valve stem. Paul Hunt (thanks Paul) helped me with the alignment problem and I managed to be able to slacken the rocker cover stud nut first, then the head nut at the rear of the pedestal and slightly move the pedestal to realign the rocker with the valve. More info on these two websites:

http://www.mgb-stuff.org.uk/enginetext.htm#rockers

http://www.mgb-stuff.org.uk/rockers.htm#1

Moving the pedestal cured the problem of the noisy valve, which I was pleased about, as my next course of action was to replace the cam followers and possibly the cam itself.

These B series engines have never been the quietest, but carefully adjusting the valve clearances using the rule of 9 to 15 thou did reduce the valve train noise.

Andy
Andy Robinson

I've never bothered learning the order in which the valves open and close. I just look at them all as I'm turning the engine and check the appropriate valve according to what the others are doing, then they can all be done in just two turns of the engine rather than working along from one end to the other. For those with poor short-term memory if you always start with No.1 you should be able to remember when you get back to that one.
paulh4

Yes, I do them too as they come up as otherwise you're having to roll the car to and fro too many times. This is the drag with doing this job with the B series engine in MGB configuration in that we don't have a starting handle. Pushing the car back and forward in top gear is a bit of a faff. On my Riley RM with handle I can do the job in a jiffy. Same I guess would apply to earlier MGs.
Peter Allen

I use a tick sheet as I wouldn't remember if I'd done no. 1 or not, and then I have to remember to tick the sheet each time.

I'm terrible at setting any gap but I've found I'm not the only one. I've found it's more about consistency of setting rather than the exact number, and if you keep checking too much you'll always find the need for some small adjustment.
Nigel Atkins

Thanks for the comments. I haven't had time to get back to it yet but will let you know how I get on. A friend suggested adjusting hot; is this a good idea?
Rod Merrall

Rod, if you adjust them hot they'll really rattle, if the spec is to adjust them cold. Some cars, like my Riley, for example, is to adjust them when they're hot. Logic, one might think, would be to specify the latter method, but maybe it's a more consistent figure when adjusted cold.
Peter Allen

Rod,
personally I think you're overthinking this a bit.

If it's the gap that's making no. 3 noisy then adjusting it should help but you really want the gaps set the same for all otherwise there's no end to it.

You've not got a track car so adjusting whilst hot doesn't seem worth the effort and anyway how hot is hot (and come to that how cold is cold). I can't remember but how much does the metal expand at what temperature? And is it enough to make that much difference to 'hot' or 'cold' adjustment on a standard factory street car.

Cold to me is the engine hasn't run that day or for a good few hours and it makes more sense to do any work in comfortable ambient temperature and not to have oil going everywhere and chasing moving parts.

You don't want things too close or too loose but there's probably a margin to the particular gap number most pick with these standard engines.

As I put before, you could start the engine stop it and check and might have to reset some for infinity perhaps. Gap them, check them, leave it at that, just be consistent.

Once you've adjusted the valve clearance you want to check and if required adjust, in order, points, plugs, timing and carb mixture.

Nigel Atkins

Hot is 13 thou in the WSM and 15 thou cold. However they have that the wrong way round for the 18V846/847 engine, and although only cold is given for 1976 on it states 13 thou!

The only advantage I can see to doing it 'hot' is in the middle of a freezing winter, and it's going to be cooling all the time you are doing it.

If you think they are noisier than they should be find the biggest gap for each valve, set them there, and see if that makes any difference.
paulh4

At the risk of causing a stir, I'll just leave this here and step away...

As has been said, B's can be a bit on the noisy side but they also suffer from wear in lots of the valve train. This can be particularly pronounced on the rocker arm pads. The only way to check them is to remove the rocker arm but as a short term "fix" you could use "Click adjust" which will account for the wear https://www.ebay.co.uk/itm/160896147443?hash=item2576285ff3:g:fX4AAMXQ2q9RZ9xa

best of....
MGmike
M McAndrew

Just to stir the pot. I use the rocking method - when the exhaust valve is closing and the inlet valve is opening that cylinder valve gear is “rocking”. If it is No 4 cylinder set the clearance on No 1 (both inlet and exhaust). If it is No 3 set valves on No 2. If No. 2 set No3 valves. If No 1 set No 4 valves. That easy, you don’t have to turn the engine of but a couple of times. Use a bit of paper to mark down what you have done (if you are over 70 like me)!
If the back circles on your cam aren’t consistent like Paul has found find a new camshaft supplier/grinder.
I set the clearances warm - run the engine up to normal engine operating temperature, then set the clearances as nominated on my cam specification sheet.
Mike
Mike Ellsmore

Here is a plot which shows why valve gear starts to hammer when the tappets get even a little bit loose.

The plot is of valve train velocity on the pushrod side. Y axis units are thou per 2 deg of crank rotation, X-axis is crank rotation.
It is a real plot of a BMC camshaft, it wobbles around a little by amounts representing measurement resolution limits of about a hundredth of a thou. Which is the same order of magnitude as a micron.

I have marked it up with some arrows.
From the LH red arrow to the blue arrow is a long period of relatively constant velocity. At around the point of the blue arrow the valve train starts to accelerate rapidly. By the time it gets to the second red arrow it is accelerating very hard and ultimately reaches a maximum velocity which is a tad over ten times that of the initial value between the first red and blue arrows.
The faster it goes, the more noise there will be when the clearance is taken up. Hammer hammer hammer.

The other thing that can affect this is the shape of the cam follower - it should be a couple of thou convex (they look flat but if you put new ones face to face you can see centre cotnact and daylight around the edges,) For the purpose of this post, we are working with followers of the correct profile.

The blue arrow represents the ideal clearance from an engine longevity viewpoint, by the time the green arrow is reach velocity has increased about 50% and at the second red arrow the valve train has approximately doubled in speed since the blue arrow.

Now for some clearance figures:
The yellow arrow is at 13thou tappet clearance
The blue arrow is at 15-16 thou.
Green is at 19 thou.
(all translated to measurements at the rocker arm)

As can be seen, a little bit of clearance increase results in a sizeable jump in velocity at the point the clearance is taken up, which is why rockers with indented faces where they contact the valve have so much effect.

The problem with setting the clearance exactly at the blue arrow is that even minor wear/clearnce increase means that the valve train is moving signifcantly faster at the point of take-up.

Setting the clearances where the yellow arrow is - around 13 thou (hot book figure as per Paul's post above) - gives a couple of thou wear before getting to the start of the rapid acceleration (15 thou, blue arrow)

Setting at 15 thou hot would mean frequent checks of clearance to avoid getting into the high velocity clearance take-up, and would shorten the time the valve is off the seat by only 10 deg. For comparison, fast road cams ahve their valves off the seat for 20-ish more degrees than the standard cams, and race cams have timing which is 50+ degrees longer off the seat and don't usually suffer from valve burn, so the gain of opening the clerance up to reuce seat-off time is not signicant and in my view not worth the hassle of frequently checking clearances (or the cost if they aren't checked).

Race cams usually specify clearances which are a little past the blue line as (depending on cams profile) it can make the performance a bit crisper. The trade-off is check more and faster wear, but hey it is a race engine and a bit more wear is often acceptend in the interests of going faster.





Paul Walbran

Mmmmmmm, fail on the image, try this

Paul Walbran

Paul, can you please explain the reasoning behind increasing valve clearances for high ratio rockers i.e by dividing designated clearance by the standard rocker ratio and multiply by increased ratio rockers.
My 1.625:1 Titan rockers require 0.021” clearance as per the cam specification.
Mike
Mike Ellsmore

While the take-up point is specified at a certain lift off the base circle of the cam lobe, the easiest place to measure the clearance is at the valve.
But the valve the motion is scaled up by the rocker ratio, so if changing the ratio you need to work backwards to find the lift off base circle at the lobe (= divide by old ratio) and then calculate what that lobe lift translates to at the valve with the new ratio (= multiply by new ratio).
For the 1.625 roller rockers it is 2-3thou more clearance than std rockers, actual amount depending on original clearance.
Paul Walbran

This thread was discussed between 07/08/2021 and 16/08/2021

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