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MG MGB Technical - New camshaft or Regrind?

My new camshaft just arrived today, but I suspect that its a regrind. I know the lobe height is supposed to be .250", but what should the base circle be if it isn't a regrind? That is, what is the measurement from bottom of the base circle to the peak of the lobe?
Kent Daniels

Kent. What makes you think it is a regrind? Where did it come from?

I do not have a new cam at hand to check. The "base circle" extends about one half of the way around the lobe. You can measure the diameter of base circle, with the top of the lobe pointed upwards, then, measure the distance from the bottom to the highest part of the lobe, and determine if there is a .250" difference. If not, you can contact the organization that sold the cam to you and discuss the matter with them.

Les
Les Bengtson

The cost would probably be an indicator of a reground or a new cam. If it's considerably less the $200 it's probably a regrind. I have a used cam that I don't think has been reground. The base circle is approximately 1.120". The stock operating lift is .250" but the cam lobe should measure .265". It operates at .250' due to the valve clearance. The lobe lift would not change if the cam is a regrind with the same grind as a new cam. Only the base circle will change. The method suggested by Les is correct for checking the cam lift.

Clifton
Clifton Gordon

I bought a standard camshaft about 2 years ago. I seem to remember it was £65 and made in India. It looked induction hardened to me rather than Nitrided or nitro-carborised, which I think is better (greater depth of hardness). It generally looked well made and hasn't given any trouble. I fitted new tappets, push rods and rockershaft at the same time. Still, the B series is in production in India.
Paul Hollingworth

Clifton,
Pure pedantry on my part but just in case anyone has their calipers out, won't the lever ratio of the rocker arm (1.42 nominal) mean that the 0.015" clearance at the tip of the arm becomes something like 0.010" at the cam for an expected unworn lobe height of 0.260"?
Steve Postins

Steve. Your information is correct, I used the figures that are posted in the shop manual and those listed here.
http://www.teglerizer.com/mgstuff/mgbcamspecs.htm
My intention was to point out that a reground cam with a stock grind will have the same lift as a new stock cam, but a reground cam will have a smaller diameter base circle, I have a feeling Kent may be concerened that a reground cam may not have the same lift as a new cam.

Clifton
Clifton Gordon

Clifton,
No the lift dimension is OK, but the base circle may be a bit smaller, which means a faster ramp and faster wear. I think knowing the base circle dimension will tell me what I've got.
Kent Daniels

The base circle being smaller will not effect the lift profile at all, so you won't get faster anything, but if the base circle is say 0.5 mm small then that comes off the radius of curvature everywhere thus increasing the contact stress. This would be a problem on the standard profile, but if the period is extended it makes it easier for the cam designer to achieve a larger nose radius.
Paul Hollingworth

More of a question on my part. Aren't MG cams steel with chilled cast iron lifters where as most cams are cast iron?

The grinder could theoretically weld up the lobes and regrind without affecting the base diameter much if at at all.
Robert McCoy

i would stick a piper 270 cam in and if you were going to change the head fit a piper 285 .daz
daz

According to the quote below from Flatlander racing it would cost $250 for welding 8 lobes, at that price it's best to buy a new cam.

http://www.flatlanderracing.com/

"If your cam needs welding there will be an extra charge of $40.00 net for the first lobe and $30.00 net for each additional lobe."

Clifton
Clifton Gordon

This thread was discussed between 16/02/2008 and 18/02/2008

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