Cam and Cam Follower (Tappet) Geometry and Function?


(Roger McWilliams) #1

The cam followers (tappets) on the 3 1/2 litre engines have a convex surface at the side engaging the cam lobe. This characteristic “mushroom” shape has about 0.001" droop at the tappet radius, i.e. when a tappet is placed with mushroom top down on a flat surface the tappet stem can rock back and forth because of the convex tappet surface.

I have the impression cams are designed to follow the convex cross section of the tappet, i.e. the cam lift changes along the cam surface in contact with the tappet. This sometimes is called a linear cut and would have about a 0.001" difference from one side of each cam lobe to the other side so as to match the mushroom convexity?

A couple questions arise.

A cam lobe offset from the tappet center line would induce tappet rotation whether flat or convex?

What is the function of this convex tappet surface and the corresponding cam grind? Both surfaces require extra machining and cost, so it seems there must be a reason. Would the angle of the convex connection create sideways force on the tappet surface to offset the torque-induced bend from the vertical tappet motion and thereby reduce or eliminate tappet guide wear?

A spare used camshaft on the shelf shows uneven wear on the lobes, each lobe with one end shiny and the other end dark. Methinks this camshaft was ground straight and not fit to the tappet convex shape at one time long ago.


(Rob Reilly) #2

I have a spare cam but it is still in the block. I guess I could pull it out if this discussion goes into extra innings.
I think the idea is to avoid the edge of the cam wearing a groove in the tappet face. So a slight profile on the cam makes sense to me. If it was offset even a slight amount it would cause the tappet to rotate. Kind of like a ball bearing in a race, or billiard balls colliding at any angle other than straight on, or the rails and wheels on a rail car being slightly tapered inwards, so the rail car finds a natural centering and neither flange rubs against the inside of the rail.


(Peter Scott) #3

Hi Rob,
If you are rebuilding your 1938 engine and your spare camshaft is a MkV one you might find it useful to substitute it. It gives slightly more torque and doesn’t lose much in top end power and probably makes more sense for the these cars are normally used.

Peter


(Rob Reilly) #4

Part no. C.2133 for both 2.5 and 3.5 Mark V, so it ought to fit in a '38 2.5, replacing part no. C.474, right?
Thanks for the tip. :+1:
Hope it didn’t get wrecked by the flying pieces of piston and block.


(Ed Nantes) #5

Generallly people here prefer the earlier camshaftwith the wider lobes. Although the defining factor is somethimes the condition of the gears
Just looked at an LSeries block and see that it has the oil holes to the cam blocks .the centre one looks bigger dia than the fr and rear.


(Peter Scott) #6

Yes, I’m running mine with the MkV shaft.
See:
www.nostalgiatech.co.uk/New%20Page%2027.htm

Peter