GPR Camp: Replica Racers

When I tested for squish I got nothing. I mean no squish—the solder came out untouched. I couldn't figure it out. Then I realized. This top end is for a 43mm crank! It wasn't going to work on this engine at all. So I started digging around through all the old stuff. I found a number of options, but none of them were viable. Then I found the original ProRace top end that I used to create the PMPR top end. I had seized it while leading some liter bikes on a ride. Just pushed it too hard. I took a look at it and it seemed to be just transfer. It was the same piston size (A) as the 43 top end so it would work with piston already on the connecting rod. I also use the same head since I had already indexed the spark plug.

I set to cleaning the seized top end with sand paper, and swimming pool acid. I actually got is much cleaner than these photos illustrate, but I didn't take photos. I was just way too pressed for time, and it was cold too. My hands were totally numb as I wrenched on the bike in my feeble attempt to get it all accomplished in time.


Then I installed the top end, tested for squish (it was good), and installed the Top intake on the engine. Put some oil in the transmission. Installed the engine, and started hooking things up. It's almost there. I could probably have gotten by with just putting on the pipe and adding some coolant, but I really didn't want to ride it without some minor testing first. It was time to punt. It's okay. I had a backup plan, and this one should be running sometime this week.

Before I could get started I had to finish fashioning the test kit. This mean adapting the freeze plug with an air valve. Previous plugs employed a large enough bolt that the air valve fit right in. The shorter valve was too short so I had to use a longer one, and I had to ream out the plug until the valve fit properly.






It can be difficult to get a proper seal at the exhaust port. I probably should have tested this with the exhaust manifold in place.


In addition to a leak at the base gasket there was a leak at the spark plug. This happens frequently with ProRace heads and CHT sensors. The configuration of the head doesn't provide proper access for the CHT sensor. Which translates into air leaks.


Having a base gasket leak meant pulling the top end (again). The slathering the base gasket with some sealer. This time I use copper gasket.


The new H2O tank in place. The previous owner cut off the mounting tab. So for now I had to employ zip ties. Eventually I'll bond, or plastic weld a tab on to the tank and mount it properly.


Some progress. Next up is to install the exhaust, add some coolant, and test it out.