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Rocker arm pad radii
https://www.ausmini.com/forums/viewtopic.php?f=5&t=87525		 Page 1 of 1
Author:  Besser [ Tue Oct 21, 2014 10:57 am ]
Post subject:  Rocker arm pad radii
Looking to refinish my rocker arm pads and wondering what the radii on it should be.
Author:  36 [ Tue Oct 21, 2014 11:54 am ]
Post subject:  Re: Rocker arm pad radii
The technically correct answer will depend on the ratio of the rocker arm, the position of the rocker arm in relation to the valve head, the amount of lift the cam lobe has and the shape of the cam lobe.

The practical answer would be to just copy the original radii as it was originally manufactured. That would get you a reasonable result.
Author:  Besser [ Wed Oct 22, 2014 9:08 pm ]
Post subject:  Re: Rocker arm pad radii
Cant understand how cam lobe effects it, wouldn't that just increase swept quadrant.
Author:  GR [ Thu Oct 23, 2014 6:16 pm ]
Post subject:  Re: Rocker arm pad radii
HI Besser
The radius on the mini rocker is a !/2 radius on both 1275 and small bores, has nothing to do with the cam lobe.
Graham Russell
Author:  Besser [ Fri Oct 24, 2014 9:13 pm ]
Post subject:  Re: Rocker arm pad radii
Thanks Graham! you re the first person to put a figure to it Ive found.
Was grappling with the concept of "following the manufacturers curve" the reason I'm regrinding is the manufacturers curve is worn so a follow the surface to clean up the face was always going to increase the radii.

Ok so next question to the AUSMINI....

If I place the radii centre outside the centre line of the valve, do I get more lift and higher acceleration of the valve at the end of the lift?
Author:  drmini in aust [ Fri Oct 24, 2014 9:33 pm ]
Post subject:  Re: Rocker arm pad radii
Besser wrote:
Thanks Graham! you re the first person to put a figure to it Ive found.

Ok so next question to the AUSMINI....

If I place the radii centre outside the centre line of the valve, do I get more lift and higher acceleration of the valve at the end of the lift?

It would be a marginal lift increase.
The usual way to increase lift with stock rockers is to fit offset bushes. These do NOT give you anything near 1.5:1 ratio. If you want that, buy 1.5 rockers.

I used to mod stock 998/1098 pressed steel rockers by fitting my own .044" offset bushes. This put the ratio up from 1.2:1 to 1.31:1.
This was measured with an RE13 cam (.290" lobe lift). Stock rockers are NOT 1.25:1.
The late 1275 oval pad ones gave 1.35:1 with these bushes.
The offset bushes you can buy from UK are less offset than these.

Note if fitting offset bushes the rocker shaft must be pushed back by drilling out the rocker posts.

Here's a pic of my offset bushed rocker-
Image
Author:  Keith C [ Sun Nov 16, 2014 8:28 am ]
Post subject:  Re: Rocker arm pad radii
It is entirely possible to generate more lift from the rocker pad by reprofiling it as opposed to fitting offset bushes and the palava that entails (needing to offset the posts backwards to correct position of the pad on the valve tip). The amount of increased list is entirely dependant on the starting point of the pad shape to it's final posituon at full lift. Grinding down the pad to lower the initial start position, then blending that in to the original pad profile will increase valve lift. A bad by product of this is that it will increase valve side loading. Not too much of an issue on race engines. Bad on high-mileage race engines. The other isue here is that experience and skill are needed to do this well. I made a special jig to allow me to do this on my bench grinder with terrific results.

And a word on 1.5 ratio rockers - they should only be used on cams that were designed to run with them, and heads that flow well enougn to make use of them. Not at all common. They rarely give better performance on small-bore motors without these essential criteria. 1.5 ratio rockers are an affectation of what people demanded after a certain 'expert' proclaimed them as necessary.
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