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MG MGA - Valve timing check

The book says to check valve timing with #1 intake clearance @ .060 and with this clearance it should open @ TDC.
Can it be checked the other way, that is when clearance goes to zero and then measure around the pulley from TDC?
Art Pearse

It can be, but then you are measuring from a difficult and not necessarily accurate portion of the cam, namely, the opening ramp. The movement here is very small per degree, and you add in the problem of correctly calculating and measuring on the circumference. In contrast, the movement where the book says to do it is about the maximum per degree, and you know (we hope!) exactly where TDC is.
FRM
FR Millmore

Art the way the book says is very easy to do. Set your clearance of number one inlet valve as accurately as you can at .060, turn the engine in the direction of rotation while spinning the push rod. If the cam timing is correct the push rod will stop turning at TDC.
Denis4

Did the 60 thou book check. Checked the feelers with a mike and found that 20+25+12+3 = 63! Cut out the 3 and got .060. inlet opens slightly after TDC, by .18" on the pulley rim. This is 3.7 deg late. I then set to TDC and there was a .010 looseness in the valve, so put another way, the .060 check would work fine at .050. I repeated this for No 4 intake with exactly the same result.
Is this check close enough? I think if I were a tooth off the sproket alignment it would be much more out than 3.7 deg. the pulley is an old MGB harmonic type - maybe the rubber has permitted some movement?
Art Pearse

Art It is not unusual to find the timing marks out a few degrees so it pays to check its accuracy (the pulley marks)first. Use a piston stop that will hit the piston just before TDC, turn the engine (by hand)clock wise then anti clockwise marking the pulley at both stop positions. TDC is exactly half way between the marks. You can buy piston stops but I made my own by removing the center of a spark plug, tapping a 3/8" thread inside the body and screwing a short stud into it. It wants to protrude 3/4". Round off the end so it wont mark the piston. You can screw it in and out to get a very exact position as close to TDC as you can. Just be a bit gentle you wont hurt anything. I have found even solid crank pulleys are not always dead accurate, often 2-3* out. Denis
Denis4

3.7 late isn't bad, but it is enough to feel if all else is right.

As per Denis +
Timing chain wear or
various indexing errors in manufacture of any involved parts.
That's why you degree cams if you really want it to run "RIGHT"
I degree 2 advanced to allow for chain break in on new ones.

On the TDC stop, it is better = more accurate if the stop points are further away from TDC, so make your stop longer than Denis says. This due to the relationship of piston travel to crank rotation at reversal points.

FRM
FR Millmore

OK, I will fab the piston stop and check the mark. I re-used the sprockets and chain as they did not seem all that bad. The new tensioner is about 30% out.
Art Pearse

Art I just measured my piston stop and the stud protrudes 7/8" exactly. At that length you can adjust the depth by screwing the plug in and out. Of course you dont have it right at TDC, I find about an inch away gives a good positive stop and easy to measure. Even one timing set to another can be different with cam timing. I just fitted a new cam in my car and while waiting for my new chain set I fitted the old set and measured the cam 3* retarded and I reasoned the new set would be just about right. What do you know, fitted the new set (same make even) and it was 6* retarded, so then had to wait for a 6* offset key. I came out spot on then. Denis
Denis4

I made the piston stop from an old spark plug, welded in an old 5/16 stud, filed the weld spatter off the end threads and rounded the end spherical and smooth. 7/8 projection per Denis rec. Worked a treat!. Discovered the pulley mark was 0.12" early. This is 2/3 of my error, so I am only 1.2 deg late. Good enough for me! Very easy to detect the stop position. This has been a learing experience for me! Thanks to you all. I will file a new TDC notch.
Art Pearse

Hi Art,

If you are using a crankshaft pulley with a rubber center (harmonic balancer) you should give up using the pulley mark for TDC. After a bit of use the outer rim of the pulley will often move in relation to the center.
This is a common problem with Triumph engines and customers are always complaining of poor performance or inability to start after setting their ignition timing.
Always use the method described by Dennis. On Triumph engines we bore a hole in the rear engine plate and mark the flywheel. This allows strobing a running engine.

Mick
M F Anderson

Art,

Further to your previous post, I would not file a new TDC notch on a harmonic type pulley. You should use the method descibed by Dennis each time you set the ignition. The outer rim of the pulley cannot be trusted on the harmonic type pulley. It can move more than once.
Use a drop of white paint on the TDC that you have found and strobe the engine. Put on a new paint mark next time if you find that TDC has changed.


Mick
M F Anderson

Mick's correct, but I recommend marking the center part of the harmonic damper with reference to the rim. That way you can still use the pulley marks, but you also know if the damper is moving and therefore going to come apart as time goes by - which can be extremely unpleasant! I think ALL dampers should be marked this way, without exception.

FRM
FR Millmore

This thread was discussed between 19/06/2009 and 26/06/2009

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