Detecting worn Cam followers

[David Morris]

Hi All,

I learnt recently that flat-bottomed cam followers and their associated push rods are, certainly on the US cars used in the examples, designed to rotate during operation. This applies to solid or hydraulic cam followers and they look very similar to those used in the UK.

The vehicles dated from the late 50's to mid 70's, so were certainly near in period to Javelins and Jupiters. I have seen, as most will, how the base surfaces of our followers get worn and pitted and hence show their age.

Apparently, it is comparatively easy to detect a worn cam follower. The article showed that you remove the valve covers and put a mark with a felt-tip pen on the side of the push rods. With the engine ticking-over, you can see whether the push rod, and its associated cam follower is rotating. Non-rotation apparently indicates that the cam follower is worn out.

I am probably telling grandmother how to suck eggs, but this tip was new to me?

All the best,

David

[Keith Clements]

I think the test tells you if the follower is rotating , not that it is worn. You can tell a follower has not been rotating by the concentric circles on its face, either not being there or there is a shiny worn band across the diameter of the cam, indicating it has not been rotating. The cause of non rotation can be a tightly fitting follower in the block possibly caused by dirt in the oil or that the cam lobe is not contacting the follower slightly off centre.

viewtopic.php?p=46944#p46944

[David Morris]

Hi Keith,

I agree that there may be other reasons why a cam follower isn't rotating, but the point being made was that the basic reason why a follower remains stationary is that a dimple has been worn into the contact surface of the follower, preventing rotation of both the follower and it's push rod.

I suggest that any follower and associated push rod that looks like it doesn't rotate is suspect and needs to be inspected.

The test I have suggested is simple and very straightforward.

All the best,

David

[Forumadmin]

A non rotating cam follower will quickly wear the cam. This will cause a loss in valve lift and hence loss of power as well as increase in tappet noise if they are not adjusted.
So a good indication of wear is increasing frequency of valve adjustment.
It is quite difficult to measure valve lift unless you have a dial micrometer, and even then it is difficult because of sloping rockers.
So without taking the camshaft out, you can do it by relaxing each tappet in turn until the valve does not start to open at the apex of the cam. Then measure the gap to the valve after rotating cam through 180 Deg. (One turn of crank).
Philip did some good research into the bending of the camshaft in the block and it's effect on valve lift.

[Forumadmin]

A non rotating cam follower will quickly wear the cam. This will cause a loss in valve lift and hence loss of power as well as increase in tappet noise if they are not adjusted.
So a good indication of wear is increasing frequency of valve adjustment.
It is quite difficult to measure valve lift unless you have a dial micrometer, and even then it is difficult because of sloping rockers.
So without taking the camshaft out, you can do it by relaxing each tappet in turn until the valve does not start to open at the apex of the cam. Then measure the gap to the valve after rotating cam through 180 Deg. (One turn of crank).
Philip did some good research into the bending of the camshaft in the block and it's effect on valve lift.
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[RichardMc]

As time progresses and technology changes some things that were once widely known by car enthusiasts and engine builders seem to slip from general understanding. One of these bits of knowledge seems to be related to pushrod engines, which isn’t surprising since in the UK almost every engine from the last 25 years has been overhead cam. In America, from reading the engineering forums, the subject is well understood because most big V8 engines have pushrod valvetrains.

There are two very important things you must take note of with any pushrod engine.
The cam follower must be ‘dome’ shaped
The camshaft lobes must be ground at an angle ‘slope’

When building a Jowett engine searching out an engineering service that has experience of the BL A-Series engine would be an advantage. The good ones can grind cam followers correctly, whereas a general motor engineer will possibly just grind them flat. The same goes for the camshaft, if you just give the engineering firm a profile to work with there is a very high chance they will grind the lobes flat. This is because 99.5% of their work will be cams for OHC engines which are flat.

Get either of these wrong and the engine will eat either the cam or followers or both in a few hours.

10 years ago the respected race engine builder, Dave Baker, wrote an article on this very subject where he eloquently explains what is happening and why, I’ve re-posted it below for anyone who is interested.

Cam Followers 101

The key thing about a round follower is it has to rotate to survive. This evens up wear and allows the cam lobe to bed into the follower and let both items work harden.

Pushrod engines have followers that tend to be in the 3/4" to 7/8" diameter range. Cam lobes are generally half an inch wide or so to generate sufficient stiffness. To rotate the follower the cam lobe has to operate on one side of it. With a follower 3/4" diameter and a lobe 1/2" wide even if you position the lobe right on one side of the follower some of the lobe is acting on the opposite side of the follower and trying to spin it back in the opposite direction.

To counter this pushrod followers are made with a very small convex radius on the top surface. Generally about 1 metre radius of curvature or 2 metres diameter. This gives the follower a dome about 2 thou high which can only be detected against a good straight edge. To run against this the cam lobe is ground with a very small angle from one side to the other, also about 2 thou high from side to side. This angle runs against one side of the dome of the follower so that any part of the cam lobe that extends to the other side of the follower doesn't contact it. This ensures all the forces make the follower spin in one direction.

With overhead cam engines the followers can be much larger in diameter. Normally between 25mm and 40mm diameter. A normal sized cam lobe can sit entirely on one side of the follower. This now means a flat cam lobe can spin the follower one way only if it's positioned entirely on one side of the follower.

So overhead cam engine followers are dead flat and can be resurfaced on a grinder or flat plate with a sheet of wet and dry paper if they have any wear. Pushrod followers are not repairable which is why they have to be replaced with new ones every time you change the cam.

Finger followers have a hard life and unless made of carbide or chill cast iron they tend to wear out steadily over time because the cam lobe wipe area is basically over the whole area of the follower. They also have to be replaced with the cam every time a change is made.

A couple of points about grinding cam lobes for domed followers. Firstly it's obviously vital that the small angle ground on the cam lobe runs the correct way in relation to the dome on the lifter and this depends on which side of the lifter the cam follower sits, at least from the perspective of the guy grinding the cam and which way round he's got it held in the grinder. So he has to know the layout of the components in the engine block and for every type of cam he grinds he'll have a spec sheet which says either no lobe angle for OHC engines with flat lifters or it'll say lobe angle runs left to right or right to left and specify how much angle is to be used.

Secondly how does one actually grind an angle across the surface of a cam lobe? That's really easy. You grind the angle into the cam grinder abrasive wheel first. The grinding wheel is dressed with a diamond tipped tool and on a cam grinder the angle at which this traverses the wheel face can be adjusted from parallel for flat tappet cams to sloping L/R or R/L by varying amounts for domed lifters.

© David Baker - Puma Race Engines c/o Pistonheads.com 2015

[Forumadmin]

Brilliant, thanks Richard. It is certainly worth anyone contemplating rebuilding an engine to check this and tell the mechanic and parts supplier.

[David Morris]

Hi All,

I think Richard's post above is very useful and nicely wraps-up this situation. He makes the valuable point that you need to check the followers for correct rotation, and checking the associated push rods for rotation probably gives peace of mind.

All the best,

David

[PJGD]

Some further comments on this topic:

From the JCL crankcase drawings, the Jowett tappet (cam follower) is 0.8125” diameter [20.64 mm] diameter, and the cam lobe width is approximately 0.455” [11.55 mm] wide, so falls within the typical range quoted above. The one meter [39.3”] radius on the tappet contact face is also important, although if you are a pedant you may want to adjust that value depending on whether your tappet is cast iron or steel and how strong your valve springs are.

Where the problem comes, is in grinding the slope angle on the cam lobe. To implement this strategy efficiently you would want the offset of the cam lobe to the tappet bore to be all in one direction — all to the left or all to the right of the tappet centerline.. That way you dress the angle on the grinding wheel once and do all the lobes in one pass.

Unfortunately, I doubt that Jowett adopted this strategy because as illustrated in my sketch the offsets are staggered with the exhaust lobes requiring the slope one way and the inlets the other way. There is no way in production (or even in a typical reconditioning shop) that the wheel is going to be dressed that often. I am pretty sure that at the factory, all lobes were ground flat across.

From the JCL crankcase drawings, the tappet bore center-to-center spacing is 1.0625” [26.98 mm] while the cam lobe spacing is hand measured at 0.94” [23.87 mm] resulting in the staggered offsets. At the moment, I can’t think of a reason why one could not make a new camshaft with lobe spacing that matches the crankcase, and then one could adopt this sloping lobe strategy. Even better would be to go to roller tappets which is a much more robust solution.

Cam Lobe to Tappet Relationship.png

Jowett Camshaft.png

[RichardMc]

Super star! You have just saved me at least two days’ work. This is exactly what I was thinking, the inlet and exhaust slopes go in opposite directions due to the offset. I’ve got a few cams and they are all different, some do have a slope but not all. The problem is I have no idea where they came from or if they have been ground in the past, one is supposed to be a special factory grind but came from a Jowett person, who is no longer with us, many years ago. I need to get them in the lathe and dig the DTI out.
Thanks for the tip on follower radius and valve spring tension, hadn’t got that on my radar. I’m looking at beehive springs instead of duals but that’s a whole new journey.
Any chance you did this in CAD? If so, can I be a cheeky blighter and ask a copy of the file?

[PJGD]

Richard:
Drawings sent via email in DWG format, but if you want them in DXF or another format, let me know.

Beehive springs and retainers are good for saving valve train weight, so a move in the right direction. If I were still in the UK, I would go to G&S Valves (I had a batch of Jowett exhaust valves made by them in the past - most of which went to Dennis Sparrow), and get some hollow valves made — that would save even more weight.

http://www.gsvalves.co.uk/assets/g-s-te ... mation.pdf

[RichardMc]

Thanks for the G&S info, I got the email but not the files, just the file paths:

/Users/philipdingle-2/Documents/Jowett Files/Jowett Camshaft.dwg
/Users/philipdingle-2/Documents/Jowett Files/Jowett Cam-Tappet Relationship.dwg

Any chance you can email them directly? (remove the spaces) richardianmcauley @ gmail .com

Many Thanks

[Srenner]

Went down this path not too long ago when I wanted the regrind a handful of cams and check a new one sourced from Bill Lock. I was interested in what the Lock grind was and if any small improvements could be made. The idea was to find a master lobe close enough to the factory profile and use that for the regrinds.

In the US, there is a device called a Cam Doctor which is used to check all the dimensions. Not all cam shops have one and typically they only check one intake and one exhaust. I wanted someone to check all the lobes, even on the Lock cam.

I sent my stuff to an oldtimer with over 50 years doing cams. We had long discussions (one-sided with him lecturing me and sending reading material an inch thick).

Here is the link in JT that has pictures illustrating some of what Phillip has stated in the above.
viewtopic.php?p=46943#p46943

I think some of the cam/tappet problems I have seen are related to the regrinds putting the lobe taper all in one direction causing the cam followers meant to rotate in the opposite direction to simply ride in one spot.

Time to check all those reground cams I have!

[David Morris]

Hi All,

Wow! I have learnt a lot from this post and all the interesting associated posts from knowlegeable folks. It looks like we need to make really sure the oil we are using has lots of zinc! And, of course, making sure our cam followers are rotating.

Very instructive!

All the best,

David

[RichardMc]

Thanks for the link Scott, interesting observations, amazing how much info is available on this forum if you dig deep enough. I would be interested in what you found out about the cams you tested (without naming names) and how much variation there was.
Thanks to Phil too, the CAD files worked via direct email.