Cam follower/Tappet position relative to cam lobe centre 3

[NickJ67]

I'm building up a Triumph I6 engine for a TR6. I've done a few of these in the past, but this one is more modified than any previous ones and has an aftermarket camshaft.

This is a 60's vintage OHV engine with the cam in the block. It has flat tappets.

I understand that the tappet centerline is intentionally offset longitudinally relative to the lobe centre to promote rotation. With the original, stock camshaft the lobes are visibly offset, most to the rear of the engine, but 3 or 4 to the front. All remain well within the diameter of the tappet.

Stock cam, lobe 1. Moderate offset to rear


Stock cam lobe 10. Moderate offset to front


The aftermarket cam has greater offsets, (except for one that has very little) visually and some of the lobes look to be very close to the outer diameter of the follower, if not actually overlapping in one case (pic, lobe 10). This may be partly due to the aftermarket cam having slightly wider lobes, though the width is variable on both stock and aftermarket as both are ground on pretty rough-cast blanks.

Aftermarket cam lobe 1. Larger offset


Aftermarket cam lobe 10. Much larger offset - looks as though the lobe will overhang the edge of the tappet.


In general, there have been quite a few instances of rapid failure of cams and followers in these engines in recent times. This is mostly put down to poor quality followers (too soft), reduced ZDDP levels in oil, or incorrect break-in procedure. I'm wondering whether this offset could be a factor as well but struggling to find any information on what good design practice is reckoned to be.

Anyone know, or know where to look?

Thanks & Regards

Nick

 

[Tmoose]

Slightly crowned "flat" tappets running on slightly tapered cam lobes are pretty popular, but not universal.
I don't know what is standard for that Triumph engine, but if a cam grinder assumed incorrectly about tappet crown or cam taper it could be a problem.

FWIW when Buick introduced their V8 in 1953 they chose flat lifters and zero cam taper over the "conventional" crown plus taper claiming "consistently longer life."
The drawing on page 488 (Fig 29) suggests little or no offset between lifter and cam lobe centerline.
Pages 488 and 490 here.

http://wildaboutcarsonline.com/members/

... _Buick.pdf

Note with a crowned lifter and tapered cam lobe the contact point on the tappet will remain the same as the lifter or lobe CL is moved axially, but the contact ~line on the cam will shift, so could eventually run off the lobe.

 

[tbuelna]

The amount of offset between the follower bore axis and the axial center of the cam lobe face is only part of the equation. What matters more is the offset of the center of contact between the tappet face and lobe face, and this depends on how much crown is applied to the tappet face, how much taper is used on the lobe face, and how much combined angular error there is between the tapped bores and cam bearing bores.

The tappet bore offset, lobe face taper, and tappet face crown are used to promote even wear of the tappet face, and also to ensure a consistent contact geometry between the tappet face and lobe face.

 

[NickJ67]

Thanks for your replies.

This is a flat tappet engine.

I take the point about offset being only one factor. I need to try and measure the lobes for taper. All the OE cams I have are well used - not sure what will show there!

My main concern was that although the OE cam has a pretty consistent offset, even though some are ahead of the centre and some behind (should the taper be ground the other way to match this?), the aftermarket cam seems to be much more varied with larger offsets in both directions (which match the OE directions), but also a couple of lobes with really very little visible offset. Difficult to see how the geometry can be working properly on all of them!

The aftermarket cam has wider lobes (13 - 13.5mm vs, 12mm OE) which is probably not especially relevent.

Regards

Nick

 

[tbuelna]

NickJ67-

Attached is some general design information regarding lobe taper and tapped face crowning for OHV engine valvetrains.

Hope it helps.
Terry

 

[NickJ67]

Thanks for that Terry - that's very interesting information.

Gives me some ideas for discussion with the cam supplier (who is also the maker). Will be interesting to see what they have to say!

Happy New Year to you!

Regards

Nick

 

[tbuelna]

NickJ67-

If you are putting the engine back to stock, then it would be best to use the OEM specs for tappet face crown, lobe taper and tappet bore/lobe offset. The information I provided recommends an offset between the tappet bore center and contact point on the cam lobe of 1.0mm to 3.0mm. The only purpose of this offset is to promote a small amount of rotation of the tappet to help equalize the wear at its face, so the offset dimension noted seems reasonable.

The face crown (or crown drop) of flat tappets is normally quite modest, usually around 0.0005" at the outer edge of the tappet working face. And obviously the taper angle ground on the cam lobe faces must also be quite small and precise to ensure the contact point is properly located on the tappet face. It is standard practice to grind all the cam lobes with the tapers angled in the same direction so that the thrust forces do not cause the camshaft to shift back and forth axially.

With regards to premature wear of tappet faces and cam lobes, with older engines this is often the result of these parts being "re-manufactured" a number of times, where re-grinding can remove the case hardened wear surfaces. Make sure your "new" camshaft and tappets are actually new parts, and also that they are the correct materials with the correct heat treatment.

Good luck with your project. It sounds like fun.
Terry

 

[PackardV8]

It is standard practice to grind all the cam lobes with the tapers angled in the same direction so that the thrust forces do not cause the camshaft to shift back and forth axially.

I'd respectfully dispute this. A glance through a cam grinder manual will show a dizzying variety of cam lobe taper patterns. Yes, Chevy V8s all taper in the same direction, but several other V8s don't. Older inline six cylinders can have front three cyls tapered toward the rear, back three toward the front. Some alternate front/rear. That's why a cam regrinder always consults the card or the book. There are so many variations, no one's memory is good enough to remember which lobe tapers which way.

jack vines

 

[kradicke]

Specific to the Triumph 4 cylinders and 6 cylinders, the tappets are dead flat (no crown) and the OE cams are ground with no taper. It is the offset in centerlines that provides the rotation, and as you noted the offset is not consistent in either direction or dimension.

Your camshaft grinder is not the manufacturer of the cam blank. As far as we know, there are three blanks available on the market only one of which is new manufacturer (there are still some old stock Stanpart blanks being used by one camshaft grinder). Two of the new blanks come from the same casting source, the difference is the shape of the raw chill cast lobe. So your cam grinder buys in the blank and then grinds it with the profile you selected for you. The factory Stanpart blanks have a very low chill depth, and thus are not suitable for performance grinds especially when you consider they don't have enough material in the nose region of the blank. Both of the other aftermarket blanks have considerably deeper chill depth, and depending on which blank is used, you can use cam profiles that put the lobe nose right at the same height as the OD of the cam journal (as large as you can go).

The two major things that go wrong with the Triumph tappets and camshaft is 1) mis-matched interface geometry between the cam and tappet and 2) poor quality tappets (material/material hardness) . There are a number camshaft grinders that do not know that the TR6 cam was ground without taper, thus they produce their units with taper. Then there are the shops that do it as original, and grind the camshafts without taper. Now you have to source a tappet that matches, there are a wide variety of tappets available with face radii anywhere from 30 to 90 inches and you also have the few vendors who sell the proper dead flat (no radius/no crown) tappets. Another concern is the usable face diameter of the tappet. Many of the tappets on the market today have a massive edge chamfer, which reduces the effective face diameter by nearly 0.050" (instead of near 0.800" usable diameter, the chamber effectively gives you a 0.750" diameter lifter face). Depending on what your camshaft lobe profile was designed around, 0.750" may be too small and the you'll have some issues with the lobe running off the edge of the lifter face. This is compounded by camshaft grinders use use the same profiles for the BMC A/B series (0.812" lifter) as they do the TR6.

Then you have material and chill hardening issues with the tappets. We recently went through a batch of 65 tappets that were sourced from AE (Federal Mogul). Over 15% of these had insufficient hardness (below 55HRC). That means for a set of 12, you were almost assured to have at least one lifter that was going to fail you. The soft lifters were anywhere from 38HRC to 54HRC, with most being 48-54HRC range... I believe we only had two that were in the 38-40HRC range.

Just as a final note, another issue that contributes to the difference in centerline offsets is the thickness of the shim gasket that is installed behind the front engine plate. The original Stanpart specification gasket is 0.016" thick, and is very high density material so that it doesn't distort much under compression. All aftermarket gaskets (but my own) are nearly double as thick and have a very low density and thus distort quite a bit under compression, especially around the fastening holes in the front plate. So what results is that you move the offsets all toward the front of the block somewhat from the thicker gaskets currently being sold.

Cheers,

Kai