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MG Midget and Sprite Technical - Engine out for camshaft change

Hi,
I currently have 1380cc engine, 286 camshaft, high rise rockers and a single hif44 carb.

Generally all runs well, but no fun for normal driving.

I plan on changing the cam to something a bit more suited to everyday use, probably a 276

This is the first time I'll be taking an engine out and would welcome any comments on what else I could be doing when the engine is out, besides cleaning areas I can't normally get too.

Thanks
Bruce Burrowes

You might want to check the condition of the engine and gearbox mounts. They can be unobtrusively knackered. If you have a carbon release bearing it's a good time to consider a roller bearing. Also check the various pins and their holes in the clutch release assembly, which can wear out of round. Check the clutch itself of course.
Depending on what gearbox you have fitted (and whether or not it has a drain plug) you have a golden opportunity to drain/flush/refill your box. Also a good time to arm yourself with a new gasket and pop the water pump out. Give it a good coat of looking - at and while it's out is a good chance to flush your engine coolant galleries. Oh - and a remote clutch bleeder.
Just a few easy tasks - I'm sure the clever people can think of more things.
Greybeard

Hi. Last year fitted a 266 cam with 1.5x rockers. Very pleased with its road manners and performance.
M Guest

Bruce,

We have a 1380 with a 276 and 1.5 rockers, also an HIF44 in our Minor. We find it a bit peaky, but it was better when the cam was advanced by 4° (it is now timed 'straight-up').

We have just had a track day at the Bedford Autodrome, and the 276 is fine there, but for normal everyday it does need to have at least 2,500rpm to pick up well.

We are thinking of going back to a 266, which was very drivable, and still produced 100bhp on Aldon's dynamometer, with >=100 lbf. ft. torque all the way from 2,500 to 5,200 rpm.

A lighter car may well be fine with a 276, and it could be worth timing it 4° advanced, to give a bit more grunt at lower revs.

The picture below shows the characteristics associated with changing the cam timing from the standard 'straight-up'

Richard



Richard Wale

Depending on when it was last rebuilt its an opportunity to refresh the engine. eg bearings oil pump, timing chain/gear. Its a good idea to renew the cam followers if changing the cam. Also check core plugs(two inside the gearbox bellhousing). If you do take off the head (you can just remove the rocker shaft to get the pushrods out) then possibly a valve grind and ring renewal.

A compression check before you remove it will indicate the condition of the valves and rings.

Is the gearbox ok? no excessive whining or selection issues.
Bob Beaumont

Tried to add a comment, but was timed out.

The 266 produces more torque and power up to 3,500rpm, and after that the 276 wins out. I would imagine that a 286 would only take over even higher up?

Richard
Richard Wale

As I understand it, increasing the bore, therefore capacity, effectively "softens" the cam. On my 1330 motor with a 266 there is not the slightest hint of lumpiness. It had oodles of torque and revved to 7k if needed. As and when I pull this out of the corner I'd be heading towards a 286 but it would be interesting to hear what result you get with a 276.
Clive Berry

Bruce, if you've not removed an engine before and need advice on the process, then you could start a separate discussion on that. There are a few issues to consider.
GuyW

I’d say keep the 286, my 1380 feels a bit boring since going back to a 276.
I might even go wilder next time the engine is out.

But that is not what you asked.
Check the bearings
Change the oil pump
Renew the clutch plate
Check the timing chain/gears
Change the timing cover oil seal
O K

Many thanks for all the comments, very helpful.

I've also spoken to Kent and they said changing the followers was a must as they wear and create their own profile.

As suggested I will also start another thread on engine removal.
Porridge

so got cam swapped over with new cam followers.

Ive attached a picture of the old cam followers, in a right state..


BruceBurrowes

Also attached a picture of the cam itself showing the two worst marked lobes, see bottom and top.

It still feels very smooth (unlike the followers) but there are some marks.


BruceBurrowes

Jeez I've never seen followers as bad as that! What got in the engine to cause it? I'd be worried about the general state of health of the complete unit...
David Smith

"It still feels very smooth (unlike the followers) but there are some marks"

bit of an understatement on the condition of the cam---that's stuffed

Looks like a lube issue to me
What oil are you using
and
have your new followers got the side oil holes in them and do they spin freely in the block

willy
William Revit

thanks for the comments.

followers are stuffed, but the cam?

oil used is standard, (20/50 i think). any suggest I use anything else?
BruceBurrowes

Currently putting the engine back together, nd had to order some new bolts due some user errors, but finally got there..

I just need to confirm if I have the time the distributor incorrectyl.

Attached are the pictues of the distributor "socket", the rockers and the markers onthe timing cover.

Does anyone know if this is correct? Im concerned I've got 180 degrees wrong..



BruceBurrowes

the distributor socket..


BruceBurrowes

and the rockers....


BruceBurrowes

Haynes says..

make sure No 1 piston is at TDC at the start of the pwoer stroke. This is when inlet on no4 cylinder is just opening and exhust is just closing.

The front of the engine is on the left and im not sure if the pistons are 1 to 4 from the left, or 1 to 4 from the right of the picture....

Im sure i read somewhere in haynes where the cylinder counts starts but cant seem to find it..

BruceBurrowes

No 1 is at the front of the engine - left in your photo.

Dissy drive looks like it needs refitting. It should be more like the red marks on this amended pic.

Also, check your distributor to see whether the larger segment is on the top or the bottom when the rotor arm is pointing in the direction of the yellow arrow, which is where No. 1 lead should be.


Dave O'Neill 2

Bruce, just in case its not obvious, to get the dizzy drive orientated as Dave illustrates - don't rotate the engine! Leave the engine crank as you have it.

Lift the dizzy drive pinion up slightly with your long bolt screwed down the centre and "feel" it lift pastjust one tooth of the camshaft before easing it back down. It will turn as the spiral teeth re-engage. If that doesn't align it, then repeat, just one tooth at a time until it sits at the correct orientation as in Dave's marked up photo.
GuyW

thanks guys,

i've already rotated the engine as im setting the valves...

but i understand the point guy, I havent had to use the long bolt, as the sump came off to chnage the camshaft, so not needed.


Dave, when you mention " the larger segment" I asusme you mean on the rotor arm itself..
and this should be on the top right, so where the larger of your red boxes are?

btw many thanks, these answers are invaluable.
BruceBurrowes

no, not the rotor arm. If you look at the drive closely you'll see that the slot is offset to one side a little, leaving the drive on one side of the slot a bit larger than the other. It's that larger side which should be at the top. This is after returning to TDC. TDC occurs twice in a cycle so make sure you get the one with no 1 valves having clearance and no 4 valves rocking.
Paul Walbran

On the topic of your stuffed followers etc:

Zinc content of oil is important, make sure you use one with the right levels of zinc for flat tappet cams.

If you haven't already received the advice with the replacement camshaft, starting procedure for a new cam is to run it immediately to 2500 RPM and hold it there for 20 min. This speed relieves some of the spring loading from the nose without generating excessive inertia loadings in the initial/final open/close stages. Doing this procedure allows the cam to bed in and extends camshaft life.

Your old camshaft looks stuffed from what I can see, simple check is to measure the lift at each lobe: zero the calipers at right angles to the lobe then get the max reading in the line of the lobe. Do each one individually. 10 thou down is enough to condemn a lobe, even by that stage the shape is so (subtlely) altered that odd accelerations are occuring in the valve train that will cause increasingly rapid wear and earlier valve float.

Not uncommon for the followers to be like that in B series, though by no means always for it to be all of them. B series has the same size followers, late B series the exact same ones (2A13), so presumably it's the loading from the extra mass, higher rocker ratio, and necessary higher spring pressure that makes the difference.
So I think we can safely infer that it won't take much additional loading for the A series to be running near the limit of what the system can handle.
(I suspect B series also have a lube issue around the exhaust lobes as they seem to go more than the inlet).

Factors which increase the loading on the lobes are:

Roller rockers (increase the inertia loading on initial/final open/close) and compress the springs further (for a given set-up)
Valve spring rate. Fairly obvious.
Driving style. In particular, using a set of serious race springs on an engine which is used primarily on the road at speeds usually under 5000 RPM. (The odd spirited squirt above this not being enough.) Or the other way round of course, road springs when racing.

The other thing that can cause excessive follower wear is valve float.
As engine speed increases, valve train inertia reduces the contact pressure between follower and lobe during the negative acceleration phase over the nose section of the lobe (in which the valve train has to be slowed to a stop then accelerated in the closing direction). Eventually speeds are such that there is so much inertia the follower starts to lose contact in this nose phase. When this happens, contact is regained further down the lobe, in the decelerating closing phase. The valve train crashes to a halt, sometimes surging off for a second bite if bad enough, and generating excessive loadings between follower and lobe, and (once severe enough) a lot of noise which we recognise as valve bounce.

Race springs are intended to keep the follower in contact with the nose of the lobe at very high speeds and avoid this. However, to do so they need to be much stronger than standard springs so at low engine speeds (where there is not as much valve train inertia) the net loading on the nose of the cam is very high. The result is a high wear rate at low speeds. For a race engine this isn't an issue as most of the time it is 5000+, but for a fast road engine it is the reverse situation.

A good example: We recently had a case of a race MGB, mostly retired from the track other than the odd sprint and used mainly for (quite a bit of) touring. It was detuned via a fast road camshaft but otherwise unchanged so still had the seriously strong race springs which had been installed in the car's competition career. End result was that the camshaft had worn badly, with two of the inlet lobes having only 1mm of lift. Owner was most relieved to find this was the reason he couldn't keep up at a recent club sprint rather than it being the driver losing it!

The moral to this is to use springs suited for your intended use.
We have battled with this on our own midgets, all of which run high lift rockers and high lift road cams. We have replaced a few worn camshafts along the way, and discovered that a key limitation is the available room in which to fit valve springs. Standard springs work worked fine with this combination up to about 7000 RPM, which is an aceptable compromise for cars used mainly on the road and a bit of competition work, provided that the key-on-brain-off mode of competion is kept under sufficient check as to observe the rev limit imposed by the springs.
However, standard springs have a marginal issue with coil bind, which is too close for comfort when used with high lift cams and roller rockers as (compared with race springs) they have an extra coil to reduce the spring rate so the compressed length is longer. Hence initially we used heavier springs, shorter at coil bind. And hence we have replaced a few camshafts despite using oil with the right levels of zinc. So the solution has been to recess the seat in the head by 1mm and generate a suitable clearance to avoid coil bind.

Ironicaly we haven't had the same problem with B series running the same cam profile despite even higher rocker ration and large (and therefore heavy) valves because there is more room available for the valve springs, thus enabling a better selection to match spring rate to intended use.



Paul Walbran

Very interesting description, Paul.
Given the very wide range of things that impact on the camshaft wear, approximately what sort of mleage might one expect on an A standard spec series used for normal road use.
GuyW

Standard spec A series - completely standard?
A series seem to last very well usually, certainly better than B as a rule.
Paul Walbran

Yes, I guess it's rather a silly question - too many variables. We know a standard A series is certainly good for at least 120,000. Probably more with good maintenance.

More to the point perhaps, is how much more quickly does, say, a fast road cam wear than a standard? At what sort of mileage should one start checking for valve lift variations? Presumably one cannot expect them to go for as long as a standard, especially being maybe less tolerant of a gradual loss of performance if you have chosen to fit a non standard in the first place.
GuyW

When used with standard springs we've had good life out of them even when used with 1.5 roller rockers. Undoubtedly they would wear faster than standard, but certainly long enough to feel like it's fit and forget and not bother counting. 10 years+ and still fine. The only trouble we've had is engines where we have upped the springs when using high lift cams (~320 thou at lobe) and roller rockers, and still used the car mainly for touring.
1275 has the unique problem (in BMC engines) of having to remove the engine to check/renew followers. All other can be done easily and quickly with the engine in situ thans to the access covers on the side. This means that the followers can be checked and renewed long before they get to the stage of beeing abrasive on the lobes.
Paul Walbran

Paul
I've got a little theory running loose in my head I'd like to run past you
I noticed back in your writeup that there was mention of a possible lube issue with the B series exhaust followers- I'd noticed in the past that the exhaust followers seem to cop it more than the inlets-
My theory on this ended up being that the inlet valve ,when it's opening, already has the exhaust valve open at the same time so theory would expect very little pressure in the cylinder itself, but , when the exhaust valve opens it's at the end of the power stroke and there would have to be residual pressure in the cylinder to push against causing a heavier load on cam follower
Also the head diameter of the B valve being larger than an A would make it harder to push open against cylinder pressure
Probably wrong, just a thought
William Revit

Hi paul,

i think your explanation of the cause is correct, as it fits the actual use of the car.

"For a race engine this isn't an issue as most of the time it is 5000+, but for a fast road engine it is the reverse situation."

The reason Im doing the work is to fit a camhaft better suited to how the car is used, so fast road.

Bruce
BruceBurrowes

Willy, I think you are dead right. Funny, I had two bits of evidence and hadn't joined the dots. Worn exhaust cam followers, which I had thought to be a lube issue, maybe masked by the crank webs was all I could think.
But in experimenting with greedy cam forms and carbon pushrods, I have also bent/broken the odd pushrod over the years, and it has been the exhaust one which failed first each time. Puzzled by the lower mass of the exhaust valve, I did some numbers and the residual pressure at valve opening, especially with early opening profiles, is significant and more than accounts for the difference in inertia between the two.

So why didn't I connect the two observations before now? "Boy look" my wife would say :-)

Interstingly, once the exhaust valve has failed to open, then it's all over for the inlet as well, as the burnt gas gets retained and then re-compressed, the inlet pushrod hasn't a hope of coping with that by the time it gets near to TDC.


Bruce - what springs & camshaft are you running?
Paul Walbran

Hi Paul, sorry for the delay. The camshaft was a kent 286, with high rise roller rockers from minispares, not sure about the springs, but will look for the old engine spec.
BruceBurrowes

One of our cars has the same cam and rocker combination, and has had the same issues.
As per earlier post we solved by machining 1mm out of the spring seats to allow the use of less aggressive springs. We have found the standard 1275 Midget double springs to work well up to 6500 - 7000, which is an acceptable compromise for a car used primarily on the road with the odd competition outing.
Paul Walbran

This thread was discussed between 17/03/2019 and 04/06/2019

MG Midget and Sprite Technical index

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