The pistons are scored, and the bore matches right where the pistons scored the cylinder sleeves. New set of rings, if you could find real low mileage stock pistons, then hone the bores, you;d get better results. You have to check for play on the connecting rods, but that gets a big more difficult. If you have a dial indicator and a base that can be firmly mounted, you may be able to check for play in the direction of movement. If you're into working on engines, why not. You make a nice trike out of that. That engine and drive shaft will hook up to a solid rear car axle really easily. A little bit of tube manufacturing, or a beat rolling chassis, you could make a nice trike.
I have been known to do all kinds of things. Garage is also home to a nice igbt arc welder. Recently changed jobs and I have time to do stuff again (evidence provided 😂). Trike isn't really my style, I had considered building a turbo kit though, not sure how well this engine would hold up. There's a guy on YouTube called doctor motorcycle who does a lot of simple builds on old metrics, worth checking out if you're into that. Gives me plenty of ideas.
I would go for 4 cylinders if adding a turbo. I like the trike Ian Rousell made on his show. It's a Rat Rod trike. I'd like to make this style bike, with thinner tires though and dual disc brakes, maube even an Earl's fork up front. I like this style, no fiberglass hanging all over the place. I don't like the color. He made this trike on his show, the whole thing, tank too, even used a Volkswagen Beetle Carb, and made an intake manifold for the Gold Wing 1100 engine. This bike really moves. All all metal too, no plastic or fiberglass.
I like that you are tying to save this old engine but I agree with Kawasakian's concern about the piston scoring. At a minimum I would measure the piston to cylinder clearances and if out of spec I would consider going to first oversize pistons if you want a lasting repair. This assumes you have a liner that can be bored.
Rebuilding a motor is really a complex job. These days I would only rebuild a motor for a vehicle that had provenance. It had better be really special. This engine, IMHO, doesn't fall into that category. It's low horsepower, and large displacement baffle me. The retail used price of these bikes are low. The whole top end, pistons and all are shot. I would bet that the pounding for the V twin has taken out the rod bearings, and side thrust washers. You'll need all new crank bearings, and if you take a bike engine apart and don't do what needs to be done, you're just throwing your money away. If you have money to throw away, that's great. The top end is so damaged, it points to the bottom end really needing to come apart. In a worse case scenario, you take it apart, you have separated the metal, you'll get a lot more for the scrap value. I scrap metal all the time, Brass, and aluminum are getting a lot of money right now.
You assume it needs crankshaft work which we don't know yet. But if it does, this engine does not use side thrust washers and the plain bearings are only $19 per set. The OP says he has lots of time on his hands. Why not let him decide? After all, he has the benefit of having the engine in front of him and all we have are photos.
If no play can be detected on a vertical pull on the conrod, I would leave the lower end alone. The other option which may be too late if oil has been discarded, is to send the oil out for analysis and if it comes back full of bearing metal, well, you have your answer and it only cost $20 to find out.
It's always let the person decide. That's a given. A plain bearing motor has to be checked, you, as you know, and I mean "you" in genera;. can't just slap plain bearings in. The metal paticles that swirled around that motor no doubt took it toll. I purchased a 1964 230Sl Mercedes Benz 230 SL as the guy would not accept that the botom bottom end needed to come apart. I showed up with $8000 dollars in my pocket. He said a guy was going to come over to try and start it. I told him. "If anyone tries to turn that engine over, my offer goes away, I then pulled out $6000 dollars, and his wife yelled "Take the money, take the money!!". He took the money. I came later with a flat bed and took the $17,000 dollar car home. Only 400 were produced, it's estimated only 300 or less are still around. If this guy had let the Mercedes Benz dealership do the bottom end, he would have had his one owner (he purchased it new) $45K 230SL Benz to this day. The bottom end has to come apart. When he picked up the Mercedes from the dealership, it smoked like a Cumulus cloud coming out the rear end. Yes, I basically stole the car from the guy, but I knew beforehand what was going to have to be done, another $10K put into the car to finish it off, and get around $4 bucks on every dollar I put into it. I know how to rebuild those motors. I have rebuilt many. As I wrote earlier, if you really like working on engines, go for it, but do it right. He has so far. The bottom end has to come apart to clean the inside of the cases properly, and check the bearings properly. I only give advice to get the best results. To me time is money, not just parts. Looking at the condition of the cylinder heads, I wouldn't waste the $20 bucks and time. Taje it apart, clean, measure fix as necessary. He would end up with an engine he would have to break-in again.IMHO
I made a lot of money off of that car. I never finished it, just did some Tig welding on the aluminum doors.
It looked just like this one, same color and removable drop top with convertible top underneath:
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