Last year I had a stock high mileage cb750A needing a top end overhaul. We've all seen the smoke on start-up. Not so bad when it gets up to temperature. Oil weeping around several gaskets. Looking at 836cc kits that may exhaust your budget? The head has to be redone/valve guides checked/valve seals replaced/valves checked and ground. Gaskets for the head and base. I was looking for a lower cost alternative....
Yes, there are cast piston 836 kits out there. I wanted a little stronger piston, so I read on Axel's old site about using CBX stock pistons for a lower cost hop-up. "Minimal machining necessary" were the words that I read. Went to Ebay and bought 6 used pistons for about $100. Rings? After a lot of searching I bought 4 OEM Honda ring sets for just under $25 each. Gaskets came from one of the SOHC specialist suppliers on the web. Thought that I'd save at least $100 against the Wiseco alternative.
On paper, it worked well. Got a spare block bored for the pistons. Got the gaskets in the mail. Set aside a weekend to do the overhaul. First problem - the pistons won't fit onto the stock 750 SOHC rods. The CBX rods must have had a taper on the small end of the rods... So, my first trip to the machine shop to get the pistons milled to fit the stock rods.
Another weekend, second attempt. Everything bolted together OK, but it was a little hard to turn over. Used a fiber optic scope, looked into the spark plug holes, and found polished marks where the exhaust valves lightly hit the pistons. Stock cam lashed at stock clearances. Pulled the motor back apart and found a couple of piston domes just brushed the edge of the combustion chamber when spun over... So, my second trip to the machine shop for exhaust valve pockets and a 1mm narrowing of the dome to fit the stock combustion chambers. Yes, the gaskets were stock thicknesses.
The moral to my tale is that you can't believe everything that you read. After two visits to my machinist, I would have been better to have found a deal on a 836 Wiseco kit. Now I have one of the stronger CBX pistoned Hondamatics around. Didn't even try the aftermarket cam I had planned for it. I had that strong feeling that I'd be going back to my machinist for the third time....
Jon Weeks