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MG Midget and Sprite Technical - Cam Check Height

I am measuring my cam shaft in the engine and it would be helpful to know whether the standard quoted stock A series oem cams are degreed at 0.050" or a different value. It obviously makes a considerable difference to the read-out on the protractor.
Graeme Williams

Graeme
I believe the standard is to check at 0.017in.
A.C.Dodd will be giving tutorials on the subject on the Masc Stand at the Practical Classics Show on 1/4 & 2/4. Come along and bring your notepad.
Alan
Alan Anstead

I found something he had posted on a Mini forum and he mentioned 0.017" However, Piper use 0.050" which seems to be a standard quoted in a number of places. The opening and closing angles that Adrian uses are the same as Piper, but with those differences in check height, the angles should differ considerably. Hence I am (easily) confused.
I wonder what the standard was in "the old days"?
Graeme Williams

It doesn't seem to matter
Take a protractor reading at a point on the up. Say 0.017".
Take a protractor reading at the same point on the down.
Add your two readings and divide by two.
Vizard says at 0.003" but then goes on to say it can be 0.010" or 0.020".
Alan
Alan Anstead

What are you measuring - full lift degrees, lift height or duration?
Dave O'Neill 2

Dave: I am measuring the opening and closing angles for exhaust and inlet valves. I have been working on an initial lift (and closing lift) of 50 thou ie taking the degree reading when the push rod has lifted that much. However, I am wondering whether 50 thou is a widely used standard (I know Piper use that) or whether Mini Coopers and the like worked to a different value?
Graeme Williams

The Spridget "Special Tuning" information I have gives checking clearances for various camshafts, standard and modified.

The majority - including standard 1275 - are listed as .016" valve clearance. If checking at the pushrod, you need to divide by 1.25, giving .0128".

I have used this checking clearance on my race engines in the past and it has produced the expected results.
Dave O'Neill 2

Graeme
My two bobs worth
There are several checking clearances as you have found
Mostly because different manufacturers use different specs-----like why aren't all oil filters the same
Most aftermarket camshaft now quote two different specs

There is the duration at .050"method which is more acurate 'IF' you have the duration at .050" specs as these are measured well past all the quietening and progression ramps of the lobe--This is the camgrinders prefered checking method and is measured at zero clearance directly off the camfollower so it will be at exactly .050" of lobe lift-not valve lift

Second method is called- Advertised timing or actual timing---This is the actual timing that your engine sees
It is quoted at normal running clearance and measured when the valve just leaves the seat--Some makers quote this at 0 valve lift, some .003" and others at .005"
hence the confusion-but
You can measure this with a dial indicator on top of the spring cap or if you havn't a dial indicator it can be done by setting the valve clearance to normal spec and just go by feeling the rocker clearance closing to 0
Or set the clearance say 10 thou wider and then use a 10thou feeler gauge to determine when it tightens up and you loose clearance

willy
William Revit

Thanks for the help. I have been able to measure the opening and closing angles of the camshaft and to identify it as a 286. I had some paperwork which came with the car relating to a 286 but the p/o said he understood it had been removed. The next step is to decide whether to leave it in or swap it for something more mild and drop the c/ratio at the same time.
Graeme Williams

Workshop manual gives .021" at the valve as the timing check clearance, this equates to .016-.017 at the cam lobe.
Paul Walbran

Talk about a can of worms.

I've just checked my WSM - it gives .021" for 948 engines. For 1275s it says .029" for timing (see image)

Dave O'Neill 2

...and if that wasn't bad enough, the 948 & 1098 Special Tuning booklet lists various camshafts and gives timing clearance as either .019" or .021", depending on which cam it is.

Dave O'Neill 2

...then the 1275 Special Tuning booklet says either .016" or .019", depending on cam.

On my 1275 engines, I have followed the ST booklet.

Dave O'Neill 2

Ha! I hadn't spotted that. A clue on the 29 thou might lie in the MGB manual: Timing clearance is 55 thou and this figure is the lift at TDC.
The timing figures for the 948 and 1275 are both the same (5/45/51/21) suggesting the cam form is the same ... until the valve lift is compared (.312 v .316). But then, the 1275 rockers were different.
Can of worms indeed!
Paul Walbran

This thread was discussed between 20/03/2017 and 25/03/2017

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