My first silicon contra seal was made from a piece of 12mm ID x 18mm OD high temp (500 deg F) silicone hose or tube. I cut a 4mm slice off of it and grooved the contra to fit the silicone with a very tight sqeeze into the contra bore. I had to make a special tapered tool with a lot of lube to press the contra into the bore. I think this was my downfall on the first contra. I think with the 3mm groove in the contra and the tight silicone fit, the silicone wanted to go somewhere and was pressing up and down on the flanges of the contra, and it broke.

On the second contra, I made a special tool to hold the outside diameter of the silicone ring perfectly round and had a hole bored in the tool that I put another round cutting tool through to slice the silicone to a 1/16" or about 1.5mm wall thickness instead of the 3mm it origionally was. I then made a new contra with a 3mm wide x 1.5mm deep groove to fit the silicone ring. This groove is 1 mm in width smaller and 1.5mm in depth shallower, which should make the contra stronger. I machined the groove until I got a snug fit with the band into the contra bore but not near as tight as the first one.

I don't know what the benifits of the band vs orings would be but I do like the idea that any swelling of the ring will only help in the sealing capabilities of the ring.

I followed the diagram of the MVVS .49 plan very loosly in only really taking the concept from the plan. I decided that instead of streching a 8mm OD band over a 12mm contra, it would be better to have a band that was more of the correct size. My thoughts were that if the band got a nick or slight cut in it, it could tear and if the streched band contracted, it would begin to leak. With the band not having to be streched, the chances of tearing will be reduced. My contra bore started out at 0.710" diameter ( sorry, don't have any metric tools), and now is 0.736". I had to dress up the bore a coulple times between contras. I found a way to make a ultra fine feed on my lathe using a cordless drill as the cross slide drive and after boring, I use chrome polish to polish the bore to a mirror finish. With the fine feed, it only takes a minute to shine the bore. I have about 4mm of flange below the ring and about 3mm above the ring. This gives me considerably more meat on the contra to reduce the risk of the lower flange breaking again.

I did find that it likes about 3/4 throttle max. I was looking at a few older carbs I have and might put an older magnum 53 carb on it. It is a little less than 1mm smaller in bore diameter.