Cam degree on ohc with mechanical rockers

Hi y’all,

been doing the cam degree course and had some questions.

I wanted to check if the same principals of degreeing a push rod motor apply to ohc motors with mechanical rocker style assembly.

like old bmw m20 and m30 motors and older Porsche motors. Just as an example.

I assume the rocker ratio needs to be calculated and the lash clearance allowed for if using cam lift numbers at the valve? Or would the manufacturer simply give valve lift numbers as I don’t see that there’s anywhere else you can measure it?

is there a practical example of this style motor to come or is there anyone that can offer some insight in the correct method… I’m dying to build my motor.

thanks,

leigh.

As I think you realise, the rocker ratio may change during the operation of the valve, with some being quite significant.

It really comes down to the specific method the cam' supplier, or vehicle manufacturer, specifies.it's usually either at the camshaft for flat tappets/followers, or at the valve with a specified clearance. Some manufacturers will give a valve lift value at TDC, again with a specified valve clearance.

If it's a simple split you should be able to use, say, 5 thou'" from opening and closing, measured at the valve.

Thanks Gord.

what I’m assuming will happen is the cam card will come with valve lift measurements?

then all I need to do is make sure I can do basic math to correct for any lash. Or could you? For argument sake. Zero the lash mechanically then reset lash after the degree work? Not too dissimilar to putting fixed lifters in place of hydraulic lifters.

I’m also assuming it may work this way cos the sr20 technically runs rockers which will have a ratio. But it seems to be ignored and all measurements are done at the valve from what I’m seeing. Does this mean the manufacturer has already worked out the more complicated issues and is giving us the simplest solution?

sorry for the wayward questions. Just trying to understand as much as possible.

Usually the camshafts, especially the "mild" ones, are made to be "close enough" installed as per factory timing marks on the gears/sprockets, but it's good practice to check.

Unfortunately, there isn't a lot of standarisation on the method the manufacturers use, so it pays to look carefully for the exact proceedure - some will have 0 clearance (I would use 1 thou' to make sure it isn't pre-loaded), some use the actual valve clearance, others somewhere in between - because of the clearance/take-up ramps, a couple of thou' can make a difference, but if it's a simple split timing, it should make little or no difference. Some use lift at TDC - which also means checking the timing clearances spec'd, because this CAN make a difference.

If you have a camshaft set in mind, the manufacturers will oftern have a link from the camshaft's spec's, which you can check. If you google "camshaft timing card", it will bring up many examples of the methods used.