Tappet - flat or domed? that's the question

[MadMarx]

Hello@all,

on the market are only domed tappets available.

All tappets are domed to the center.
Wouldn't a perfect flat tappet work better with our cams?
I assume the domed tappet to prevent the tappet from correct rotation and does rise the surface pressure.

If a perfect flat tappet would be the better choice then I would like to regrind the tappets to flat ones.

I would like to know your suggestions.


Cheers
Chris

 

[mgNOT]

MadMarx,
As I understand it, the tappet is ground with a slight crown and the cam lobe has a slight taper from one side to the other. This causes the tappet to have contact with the cam lobe in a point slightly off center, resulting in rotation of the tappet. By constantly rotating, the wear on the tappet is distributed and this results in longer life. If the tappet did not rotate, the cam lobe would wear a groove in the tappet face and the result would be a shortened tappet and cam life. I believe the tappet bore is slightly off center to the center of the cam lobe also. None of this is necessary in an engine with a roller cam since the tappet has a wheel that rolls along the cam lobe. In fact, the tappet is held in a constant direction in this setup to prevent the tappet from turning and the resulting sliding of the tappet wheel. I hope this helps.
-Dennis

 

[MadMarx]

Hi Dennis,

Unfortunately there is NO taper on the lobe, just a this slight crown on the tappet as you say .

The tappets didn't like to rotate much what I read out of the wear pattern but they don't stuck in their bores, they move very well.

One said to me that these tappets are domed because of they wear most at the center and to prevent them becoming concave they are slightly domed to have a convex or flat surface after while.

But I'm question this because if the cam and tappet fail from this because of a lack of rotation the idea is quite false to dome them.

Cheers
Chris

 

[MadMarx]

I had a talk with a famous German cam manufacturer (Schrick) and they say that tappets should be very slightly domed to handle geometrical imperfections. You can run flat but it is better to have a dome less than 0.002mm (0.000079").
If the cam has a taper grind then you can have a dome of 0.004mm (0.000157")

The tappets on the market seems to have 0.007mm (0.00275") dome which could be too much and a cause to cam failures. Depends on the cam if there is a taper grind on.
So I think the better idea is to grind them flat and give them a slight dome by hand on a lathe with fine sandpaper....

Cheers
Chris

 

[Simon TR4a]

Chris, the idea of doing any work by hand on a surface ground to a precision requiring 6 decimal places sounds pretty scary!
(Even if it is a very hard surface.)
Simon.

 

[MadMarx]

As the manufacturer say flat would work also then a few seconds grinding with fine sandpaper on the outer parts should dome enough I would say.

But anything is better than to run them as they come from the dealer.