Here's Cycle X's web page for pistons.
http://www.cyclexchange.net/Engine%20Parts%20Page%20%28Non-Cafe%29.htm Sometimes a piston deal will pop up on Ebay or the Forum's Race and Performance parts for sale and wanted on the high performance section
The biggest performance improvement for most Hondamatics is ring seal. Compression improvements always help.
Seca CV carbs are a bolt on with K manifolds and pod filters. Some main jet tuning is recommended.
As you go for bigger pistons, remember that you need more ignition advance to fire the mixture over all of the piston surface. Ignition using a points eliminator and good coils also helps.
There is a weight limit for bigger pistons with stock rods. You don't break the rods like the clutch versions may do. The small ends of the stock rods will oval with the increased piston weight. Effectively you will stretch the rod a little reducing the valve to piston clearances. I've done that with a race Hondamatic 1,000+ cc motor. 836 pistons near the stock OEM piston weight should last for many years. 10.5 to 1 pistons will run on pump gas without issues. You'll have difficulty running lower compression since most pistons are now 10.5 to 1. Thicker head/base gaskets will allow you to drop the compression ratio if you want to run cheap gas.
As you improve power, your oil's heat will increase. A bolt on oil filter adapter to an external oil cooler from a parts yard or Ebay/Craigslist is insurance for warm climates and or higher rpm extended running.
Bolt on is a simple term that may end up not so simple if you ignore physics. Bigger pistons, more compression, bigger carbs, better ignition, and a good cam will really impress you when you get it done. The challenge is to not wind it too tight for too long without balenced pistons, rods, and good valve springs. That's where Sam spent serious money to allow him to wind it really tight. (10,500 rpm shifts)
If you have very high mileage on your motor, the clutch packs will need replacing. Usually that shows up initially on the shift with an rpm spike. Old motors with top end improvements may still have worn out bearings and clutch packs.
I would look into the 836/850 cc area with overbored stock sleeves. A 'K' clutch stock cam, and at least stock K carbs and manifolds with a good head that has valves that have been freshly touched up will really wake it up. If you put more air into the motor, you need a way to get more air out of the motor. There's lots of exhaust options beyond improving the sound of it. The stocker will work if rust hasn't weakened it too much. See Jim F. for the gasket sets. They're here now, but may not be in the longer run.
Sam's stock Hondamatic ran low 18's in the quarter mile with a lot of rust in the tank. With an 836 kit, good CV carbs, good ignition, the 295 aftermarket cam, aftermarket rods, a Mike R. ported head, and an 4 into 1 exhaust pipe it ran a best of 12.30's in the quarter. That's with streetable 10.5 to 1 pistons. If you hear of Bill or Sam racing his Hondamatic, go check it out someday. You'd be very surprised at how docile the motor is. It revs much better than a stocker, but it will run 5 mph around the pits for hours if you want.
I've raced Hondamatics since 1984. I used to drive it to and from the track for a couple of years. The gearing wasn't so street friendly, so I started hauling it in a pickup. Sam's bike is now up to 915cc and will eventually run 11's in the quarter. My stock '84 GPZ1100 will barely break into the 11's in the quarter. They will surprise you with even mild performance improvements.
Jon Weeks
