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MG TD TF 1500 - Flat Faced Cam Followers

These questions are directed to the experienced XPAG/XPEG engine builders out there.
1. Do flat faced tappets (as fitted to the T Types) actually have a slightly radiused top?
2. Are the cam follower block castings slightly off vertical to assist in cam follower rotation?
3. Is the cam lobe slightly off centre in relation to the follower and is it this that induces cam follower rotation?
4. Should new cam followers and the cam be dressed to match each cam lobe?
I’d appreciate answers from knowledgeable engine builders please. Cheers
Peter TD 5801
P Hehir

Peter
I feel you are overthinking this-----

1--"radiused top" All flat tappet camfollowers should be ground with a slight convex finish on the bottom, at the top where the pushrod fits it's concave to suit the radius of the end of the pushrod

2--Not sure what you mean by block castings but I presume you mean the follower bores, they should be dead square to the centreline of the engine (cam)

3--The cam lobe is central to the follower bore
BUT-all flat tappet cams have the surface of the lobe ground on a slight angle to the centreline of the cam to promote proper rotation of the follower

4--'new' followers shouldn't need grinding
William Revit

Thanks Willy. I do tend to do that. By the top I did of course mean the interface with the cam so I obviously should have said the bottom. With question 2, I was referring to the bores. Reason for asking is that I was informed recently that the bores were cast slightly off vertical for the reason stated, i.e. to promote follower rotation. Your answer to 3 is clear. There was also talk of the follower being mated to the individual lobe following a regrind for two reasons. One being to ensure the lobe and follower were a matched pair and the other being that quality control on new followers, especially those from the subcontinent, isn't what it should be. Clearly based on your answers rotation of the follower ONLY occurs if the face that mates with the lobe is as you describe. Thanks for the clear response. Cheers
Peter TD 5801
P Hehir

I'm not a rebuild expert, but I can answer the question. Original XPAG cams and lifters are indeed flat. The lifter bores are offset horizontally to the cam lobes to promote rotation. Modern cams have slightly tapered cam lobes and rounded lifters faces to further promote rotation. Crane for example, tapers the lobe 0.0011", meaning it is higher on one side than the other by that amount. They lifter face must be crowned as well, 0.0005 to 0.0008". You cannot mix a tapered cam with a flat lifter. No additional "dressing" is necessary. They will break in to each other on their own, primarily during run-in.
Steve Simmons

Steve can you explain in a bit more detail what you mean by 'The lifter bores are offset horizontally to the cam lobes to promote rotation.' Does this mean that if you were to draw a vertical line down through the centre of the follower it would NOT align with the centre of the lobe? This is how I'd always understood it to be.

The statement 'modern tapered lobes and rounded lifters' suggests that if I were to have an original TC/early TD cam reground it would be reground with a taper and that the followers would then also have to be rounded to suit? It seems that with a regrind there is the option to go either way? If that is the case then 'rounding' the new followers to match the now tapered lobe would be essential? And that a tapered lobe/rounded follower would outlast a flat lobe/follower set up and so be the better option? Cheers
Peter TD 5801
P Hehir

Peter
Just had a look down the lifter holes on a motor I am just about to send off, If anything some lobes are 'just' forward of the centre of the bores but only just and on a few I'd say zero without measuring them but just possibly forward
just by eye
William Revit

I had a Crane cam installed and Crane lifters. When I had a rocker chip out I replaced it and then Crane push-rods also. My lifters rotate continuously.
PJ Jennings

Peter, that's correct. The lifters have to be slightly offset in order for them to rotate as the cam slides along their face. Think about it like sliding your hand across a record on a turntable. Sliding across the center will do nothing. Sliding off to one side will rotate it. You don't have to regrind with a taper but it's considered a good idea, and how all modern cams that I'm aware of are set up. You can't use a flat tappet on a tapered cam because the edge will dig in. If you use a crowned tapper on a flat cam, it will wear faster than normal.

There were variations in some castings where the lifter bores were more or less offset over the cam lobes. You can correct this by shimming the cam, or probably better by over-boring and sleeving the offending lifter bores. I don't recall the actual amount of offset but it's small.
Steve Simmons

Again thanks for the replies. My questions 2 and 3 were an either or. I'd always believed that the rotation was caused by the method described by Steve as 'spinning a record' and was seeking confirmation that the rotation was caused by an offset but vertical bore rather than an angled one. The only cam resurfacer/profiler here in Sydney stated that original cams had flat faced lobes and followers and were always ground flat, without a taper on the cam or rounded lifter faces on the followers. He was extremely reluctant to regrind both a taper on the XPAG lobe and rounded faces on the lifters, saying 'we don't do that'; but did agree that the cam followers these days were not anything like good old British engineering using quality materials and processes. When pressed he stated that if I wanted a taper I'd need to turn up a dummy follower with a point, insert it into the block and mark the precise location where it touched each follower; which all sounded a bit too hard. To use Steve's analogy surely to potentially cause an issue this 'contact point' would have to perfectly coincide with or cross the centre line of the follower, where it might then be working in opposition to the taper? This vertical point of contact between the lobe and the follower is obviously critical to the efficiency of the follower rotation and the longevity of both cam and follower. The block is being tanked at present so it's moot anyway. He did ask that I bring both sets of followers in, the modern unused set purchased 10 years ago and the ones that were fitted in the 60's and I suspect were only in use for about 3 years; and he would dress these. There seems to be only one cam regrind facility left in Sydney now, leaving no other local option or second opinion. Cheers
Peter TD 5801
P Hehir

This thread was discussed between 29/06/2020 and 30/06/2020

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