During our time racing we have had a few DNF’s due to a cam follower adjusting itself by allowing the bolt to be wound in, therefore we needed a remedy to stop this from happening again a costing us more race finished.
The solution has been to install shim washers between the lock nut and the bolt head. This will stop the bolt from screwing in during running and ensuring that the valve clearances stay as they are intended.
The existing gaps between the lock nut and bolt head are measured to see how much shimming is needed. The gaps in our followers range from 2.5mm to 3mm.
We bought a selection of 2mm, 0.5mm and 0.1mm shim washers with 10mm hole and 16mm OD which fit perfectly on the follower and will allow us plenty if refinement to get the valve clearances just right.
The benefit of using shims is that not only will the bolt not wind in and open up the valve clearance, but the adjustment is simpler. The old method required test fitting a follower in the head and measuring the valve clearance then removing the follower and measuring the overall follower, the locknut was then loosened and the bolt adjusted to the new required length then the locknut tightened and the length rechecked then installed in the head and valve clearance checked again.
With the shims once the valve clearance has been tested once then the correct number of shims can be installed or removed with consistency that the amount of adjustment is precise.
Each follower, in turn, had its locknut to bolt gap measured as well as its overall length before being stripped down and a selection of shims adding to the bolt to match the measurement. The follower is then reassembled and the overall length checked to make sure it is still the same as before.
Once the follower had been reassembled, it was installed back into the head and the rocker arm fitted. This then allows the valve clearance to be measured. If the clearance is to large a shim or shims of the correct thickness is installed and reassembled and tested.
If the required shim thickness is not achievable using the available shims then a shim can be sanded down to get the perfect valve clearance.
The sims will add a little extra weight to the valve train but this is a sacrifice that is worth taking to ensure that the followers are reliable and we finish the races.