10 replies to this topic

[Anthony30]

Anyone tried using these rocker shims from Mini Sport? The Mini Spares 1.3MM shims are just not thick enough. Ideally I want 3MM shims. I would prefer not to use two shims on top of each other, but might need to.

 

https://www.minispor...abs_description

 

Any other companies who make good quality shims?

[Spider]

Make some from some 3 mm sheet steel.

[hhhh]

Are you sure you don't need shorter pushrods?

[carbon]

As above, it would be very unusual to need 3mm shims under the rocker pedestals. Unless you're using valves with 3mm longer stems or have skimmed a lot off the top face of the cylinder head (or base of the pillars) for some reason?

 

3mm shims will give a big change to the rocker geometry, most likely not for the better. The chances are the contact point will be right on the edge of the rocker tips at full valve lift.

 

Have you checked with a dummy build and turned over slowly by hand to check the rocker geometry?

[Anthony30]

Not even torqued the head down yet. The 295 as been skimmed, and the inlets sit slightly higher than the exhaust valves. I might just buy a set of Mini Sport ones, and see. It will probably be enough. I'm not sure.

Edited by Anthony30, 04 April 2017 - 07:21 PM.

[minimans]

OK very timely question. Just how do you determine the correct geometry and therefore rocker shaft height? is there a stated figure for rocker shaft height?

[hhhh]

Standard practice is half lift at 90 degrees between rocker arm and valve stem, but there are arguments to deviate in either direction. With the rocker shaft slightly high, you get faster low lift acceleration (but less high lift acceleration, and less total lift) which is generally beneficial for torque spread. There can't be a standard prescribed figure due to all the variables. Cam profile could already be optimised for the best valve acceleration where needed, or not.

[limby2000]

Hi Anthony, had 80 thou taken of my 295, my tech friend calculated 1.2 mm, i made some from the right gauge plate. I too had the longer inlets (without the clips).

Edited by limby2000, 04 April 2017 - 08:53 PM.

[minimans]

I found this which does a good job of explaining how and why.........

http://www.minimania...h_rods_and_more

 

[hhhh]

If the geometry was correct before skimming the chamber side of the head, shimming the rocker pillars is not the way to make it correct when you find that you can't adjust your valves to obtain clearance. The correct solution is to shorten the pushrods. If your inlets are a different length than your exhausts, at least one of them will have sub-optimal geometry regardless of final pillar height.

[tiger99]

As suggested above, you will NEVER correct the geometry change caused by a skimmed head by shimming the pillars. You will make it worse, ever single time. Only if you actually run out of adjustment do you need shortened pushrods.