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MG Midget and Sprite Technical - Valve Clearance

I have just adjusted the valve clearance as per the handbook. All gaps were to tight (small) according to my 'new' - hardly used guages which are mm so therefore adjusted to my 'feel' of 0.25mm. Now they are a sliding fit.
Before I give it a run can someone tell me the effects of too tight (too small gap) a clearance as opposed to too slack (too large gap) which I think is more likely as I seem to remember I always errored that way. Like all these things its been 30 odd years since the last go at this and I can't remember and am out of practice anyways.
I also cannot see any settings for tightness of lock nuts so presume its the usual turn with spanner until its 'seated tight'.
Dave Squire - Notts

If they're way too tight then when it get's hot, you might not have fully seated valves and you'll burn the seats. Too loose retards the valve timing and too tight advances it. A little either way has little effect on a road engine. Some people prefer a little sloppy.
Lawrence Slater

Thanks Lawrence, sloppy is 'normal' for my use of feelers 30 years ago and I think its the same now. So should be OK. When battery charged will be away for a tester.
Dave Squire - Notts

Starts fine and sounds much better and smoother. Less lumpy on tickover.
I find that just amazing and feels just great! (seeing as I am 30 years out of practice and my legs were starting to ache).
Thanks Lawrence your description fits with lots of old carbon inside rocker cover (removed with petrol wash). Any suggestions for removing carbon around valve springs etc. or should I just run it until the carbon goes into the oil and just change the oil regularly? Say in 3000 miles time as its the first clean oil for 10 years in it at the moment?
Dave Squire - Notts

I've got a lot of baked on carbon inside my rocker cover. I've tried soaking it in petrol and paraffin, no joy. I've tried hot air gun and scraper, but with only limited success.

Anyone recommend a suitable solvent to try and dissolve it?
Dave O'Neill2

**&&&%%

just lost my post so I'll just put

now check points, plugs, timing and mixture

for good info and background on valve clearances there's this John Twist video - 10 MG Valve Adjustment - http://www.youtube.com/watch?v=fezXUwVfH7U

I'd change the oil and filter after 500 miles now so that the existing oil acts as a flush - thoroughness is probably often more important than time - drain oil as hot as possible and leave to drain as long as possible

the less miles, frequent or regular use the more often you need to change your oil

as far as your engine goes don't neglect your air filters and importantly the cooling system and antifreeze/coolant

&&8**$$ Firefox, the &&**%$
Nigel Atkins

There is one other downside of clearances being too loose - risk of the take-up being past the end of the opening ramp, resulting in higher wear rates on the valve train. (For those not familiar with the ramp, it is a part of the cam lobe during which the lift speed is low and constant-ish to allow the clearnace to be taken up without hammering). Opening ramps on original BMC cams usually finished at about 11 thou on the cam side, that's 13 thou at the valves on the std rockers. After the ramp, acceleration is rapid so the further you are off the end the worse the effect gets.

If you have a sliding fit on the right size feeler but can't get the next one in, you'll be safely on the ramp provided your rockers don't have any wear indentations where they contact the valve (most feelers are wider than the indentation and so bridge it, giving a false reading).

I have generally set clearnces on road cars a thou tighter then spec so that any gradual valve train wear (up to a couple of thou of the recommended interval) between adjustment checks won't take it past the ramp. I have not observed any ill effects on valve or seat life on the engines I've had long term, including those with fast road cams.

There can be a small performance upside of running extra clearance, but for longevity of the valve gear (sepecially the 1275 cam followers which require engine removal to change) I have always opened clearnces a couple of thou just for specific events when every little bit helps then reset afterwards.
Paul Walbran

Paul, I get the ramp idea and the way that wear can occur due to excess force during the wrong part of the cam rotation but am still struggling with the idea of timing in relation to clearance so when you say - 'If you have a sliding fit on the right size feeler but can't get the next one in, you'll be safely on the ramp provided your rockers don't have any wear indentations where they contact the valve' what do you mean? I would have thought that messes up the timing effect of the clearance from cam to cam.
It just seems a bit strange to have valves set at different clearances on the same car? Do you mean all of the inlet valves set the same and all of the exhaust valves set different? Or just a general mix up? Could you either clarify or extrapolate please?
I am interested in getting it straight in my head and even though the car is working better than it was I would still like to understand this part of its operation in function (schematic / academic / physical) terms for my own underlying peace of mind.
Dave Squire - Notts

Some say "a noisy tappet is a happy tappet" - ie it is better to be loose than tight.

Doug Plumb

But noisy tappets wear out the valve train faster. The noise level is directly proportional to the hammering effect. The harder the hamering, the faster the wear.

Dave - sorry, I must have been a bit vague in what I wrote. I wasn't meaning to have all the clearances different. So, use the same feeler for all (though some engines - not A series (or B,C)- are specified with different exhaust and inlet clearances)

What I was trying to express in the reference to another feeler was an idea of the tolerances when setting tappets: if you set the clearance (12 thou in A series) with a sliding fit on the blade, it shouldn't be so loose that you can get the next size feeler in (13 thou).

the reference to indentations in the rockers was to indicate that their presence can make it impossible to set clearances accurately with a feeler no matter how careful you are as the feeler blade rides in the edges of the wear dent.

The timing effect of small variations in clearance isn't big. While there might be a difference in seat timing (ie from valve actual open to actual close) the timing figures were specified at .021" clearance. Vary this a thou either way and there's not that much difference.
Looking at it another way, within 10-20 degrees of the point of opening/closing there isn't much pressure difference across the seat to generate flow, and if the clearance is a bit tight, having the valve a thou or two off the seat will see very little transfer of gas one way or another across the valve seat in the extra couple of degrees or so duration concerened. (The opening ramp lifts at about 1/2 thou per degree)
Paul Walbran

Hi Paul, yes, woke up this morning realising what you meant. I agree completely about the wear of the rockers and therefore the necessity to account for it.
I appreciate the help; hadn't thought about wear at the tappet but obvious now you mention it.
The engine sounds a lot better in the workshop and I intended going out today for test drive but its snowing and freezing here at the moment so becoming treacherous. (Day car sliding this morning on snow covering slush that is beginning to freeze).
So it seems to me that general rule of thumb is no ticking even if a bit slack = OK. A noisy ticking tappet = some wear (more than acceptable) in the valve train somewhere so tighten up (go a thou at a time until ticking gone)?
This puts the pressure on the correct part of the cam keeping cam wear down. (now as I see it thats important).
Dave Squire - Notts

On the cars I've owned in the last few years, I've always used 12 thou for the inlets and 15 thou for the exhausts - A series, 1098 and currently 1275. Not too tappety and no danger of burnt exhausts on high speed runs.
D Excell

Dave - In general that's right, the exception is when cam follower(s) get worn and the worn shape makes it impossible for the ramp to do its work. You need to maintain minimum clearance, I wouldn't go below 10 thou.
Paul Walbran

Thats been really helpful, thanks Paul, weather improving so may get a go out in a while.
Cheers, Dave
Dave Squire - Notts

Cheers Dave.
D Excell - 15 though is factory recomendation for competition work. It is just off the ramp, but the take-up speed is within the parameters of that used by some modern A series profiles. The main issue is that any wear will open the clearance past this acceptable stage immediately; simply avoided by checking clearnces more often.
Paul Walbran

Gottcher, not just the once since original engine build like mine looks like. Its in the calender for regular check and in the brain if ticking starts.
Weather worse and so busy with work, started car this morning in garage, still sounds good. Will go for run when I can get away from this desk and the weathers OK. (2 morrow according to forcast)
Dave Squire - Notts

Paul,
My Kent cam recommended 16thou valve clearances which is what I have always used. It isn't unduly tappety once warmed up. So would they grind a longer ramp on their cams to allow for this?
Guy W

in a previous thread about or including setting the tappets the over and under sized feeler gauge method was discussed, also the method of using additional feeler gauges instead of the clicstep(?) device, I might be wrong but I thought there was a JT or someone else video showing it
Nigel Atkins

Soak carbon in coca cola for 24 hours.... or cheap alternative.... it will flake off and a nylon scouring pad is all thats needed after that.

Unless you are rebuilidng your engine I wouldn't worry about it - your engine is always going to be full of carbon. If its on your valve springs etc then it will be all over your piston and valves themselves and thats where you could most do with getting rid of it.

You can rent special machines which are meant to chemically remove coke build up. I watched Ed china use one on wheeler dealers once... but I'm not sure how much they work and how much is hype.
C L Carter

Question is, does Coke remove Pepsi?
Lawrence Slater

Is that what's meant by a de-coke?
Chris H (1970 Midget 1275)

Guy
I was referring to the ramps on BMC cams. My comments above also assumed normal roadwork, not competition.

Other profiles have the ramps in different positions at different rates, so in the absence of any other data you have to be guided by what the camshaft manufacturer specifies. Generally, there is a small performance gain by setting the clearance just off the end of the ramp - witness BMC's spec of 15 thou for competition work. On the rolling road there have been useful gains by going 1-3 thou further than that.

However, the further it's opened the faster the wear rate. It's the old story with competition engines - a bit more go at the expense of a bit more stress and shorter component life. In the case of valve clearances, if you know the clearance puts take-up just off the ramp then you need to check the adjustment more frequently so it doesn't get too agressive after wear. At 20 thou clearance the take up speed has doubled, and is still accelerating hard.

The biggest risk with higher wear rates is the wear on the cam followers. This has two effects: one is that it changes the follower profile to concave rather than slightly (.002") convex which has a major (bad) effect on clearance take-up speed (a noisy tappet when set to the correct clearance is a sign this is happening); the other effect is that once through the hardening the pitted surface of a worn follower grinds away at the cam lobe, eventually destroying it. Not a happy job in a 1275! Other engines are not so bad as the followers are more easily removed for checking and replacement if necessary before it gets that far.

For myself, I keep it on the ramp for road work and then open up to just off the ramp for competition. This is less work and expense than replacing followers and cams!

It's easy to find where the ramps are using a dial indicator and degree wheel, and it's the first thing I do after fitting a different profile. They usually extend over about 20 degrees or so, so are easy to find:
I usually do this by sitting the dial indicator probe in the top of the pushrod before fitting the rocker gear and reading cam lift from there. However, if the engine is already assembled it can be done by removing setting negative tappet clearance (ie valve held open a little) and reading off the valve spring cap, remembering that the motion is different from the pushrod end by the amount of the rocker ratio.

Zero the dial indicator with the valve gear on the back of the cam (usual rule of nine - opposite valve fully open)
Rotate the engine until the indicator starts to move.
Continue to turn the engine 10 degrees at a time, noting the lift from the DTI every 10 degrees until reaching a cam lift at least 50 thou.
Calculate the lift in each 10 degree interval. (ie lift at the end of the interval minus lift at the beginning of the interval)
Plot these "lift per 10 degrees" figures on a graph (makes them easier to interpret)
Typically, the graph will show a small rise, then go flat (this is the ramp - some profiles climb slightly rather than go flat.) then rise rapidly as the opening acceleration kicks in.
The turning point between the flat bit and the climb is the end of the ramp. Note the crank degrees reading at this point of the graph then return the crank to that position. The dial indicator reading is the lift at the end of the ramp. (Alternatively you can read this off the lift figures you initially noted)

BMC opening ramps lift 3 to 3.5 thou per 10 degree interval at the cam, or 4 thou per 10 degrees at the valve for std A series rockers.
Performance opening ramps can be up to 5 thou per 10 degrees (at the cam).
After the end of the ramp the lift speed increases rapidly over the next 30-35 degrees to a figure a bit over 30 thou per 10 degrees - a 10-fold increase. For standard cams the figure is about 13 thou per 10 deg, for performance cams it's about 33 thou per 10 deg. The max lift speed is dictated by the follower diameter, too much and the lobe is lifting on the corner rather than the face. At 33 the contact is nearly at the edge, at 36 it is on the corner.



Paul Walbran

Paul thanks very much for that explanation. I have copied and paste that into a word document to save for future use when I next rebuild this engine.

My Sprite engine has covered some 70,000 miles since I built it, with a Kent 276 cam. I initially set the valves at 12 thou but in use (road, not comp!) I found that the valves were binding after a few minutes on long uphill climbs at full throttle. I was advised to open up the tappets and sure enough, that cured it.

I don't know what sort of life expectancy a camshaft has but so far mine seems to be OK. I fairly regularly check the valve clearances and also check the valve lift with a dial indicator at the same time. I did have one camshaft (on a V8, not an A series) on which just one cam lobe wore to a near perfect circle with consequential zero lift!
Guy W

Ditto Guy.
Lawrence Slater

This thread was discussed between 13/01/2013 and 15/01/2013

MG Midget and Sprite Technical index

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