DIY home piston notching
A race engine was brought to our shop after a failure. Before reassembling the engine I decided to check the piston to valve clearance since this was a hybrid engine using an older head on a newer block.
We placed modeling clay on top of the piston, bolted the head in place and timed the cam. Then we carefully rotated the engine two turns. We took it all apart to measure the thickness of the clay to determine the clearance. However, there was no need to measure as the clay had been cut by the valve indicating there was already some slight contact between the valve and piston.
We carefully examined the old pistons from the failed engine and sure enough when we knew what to look for, there were impact marks on the tops of the pistons. We determined that notches were needed to be cut in the pistons to make the engine runnable.
We came up with the idea to use an old valve coated with abrasive grit to do the cutting. After some internet research, this was found to be an acceptable practice. We did some research to see what had worked and what had not.
I repaired the damaged head enough to use it as a guide for the notching procedure. We used adhesive backed 40 grit sandpaper cut slightly larger than the valve and pressed in place. In order to get the paper to stick, we had to wire brush the valve face, clean it with brake cleaner and then use a piece of duct tape to remove the last bits of dust off the face. The sandpaper held pretty well when this process was followed.
We put the valve in the old head and use a drill to spin the valve stem. We gave up on the idea of using a collar to measure the depth cut and just made the cut in several small steps. We did not bolt the head on just used the dowel pins to locate it on the block. This allowed up to lift the head easily and check the progress of the cut.
To reduce the amount of metal shavings left in the engine, we used masking tape to seal off the top of the piston being cut. When the head was lifted off, we used a shop vacuum to clean up before removing the tape. We simply cut through the tape rather than trying to predict where an opening was needed.
After we had determined the amount to cut by repeating the clay molding process, we moved to the next piston. We found that the best way to keep the clay from sticking was to have the piston top as clean and smooth as possible. Using oil or WD40 just made it stick worse. We used a dremmel to smooth the cuts made by the sandpaper on the valves.
By using a valve to cut the notches we assured ourselves that the notch was in exactly the right place to clear the valve and at the correct angle. Making these cuts on a milling machine would have involved tearing down our fresh short block and collecting data on the center of the desired cat as well as the angle. Our DIY method eliminated al lot of these variables as well eliminating the expense of the machine work. The results looked fantastic. Time will tell how well it performs but I expect it will work well.