Rebuilding 1980 CR80R Engine for Mad Swap...

I say "mad" because I am apparently slightly mad for considering it, but also because I want to swap this engine into a Sachs MadAss. For those not familiar is a small street legal cycle with an interesting industrial design aesthetic powered by a Chinese clone of the venerable Honda CRF50 engine. To spare repeating myself, I recently started this thread in the Honda 2-Stroke forum:

Okay, what I have in an engine from a 1980 CR80R pulled from a junkyard parts bike. I got it complete for $50. After taking the top end apart it seems the piston skirt shattered scoring the cylinder wall and generally banging up the combustion chamber. The connecting rod big-end bearing feels very sloppy and needs to be replaced. Before I put any money into this engine I need to know if I can afford to fix this. From reading around here it seems that standard operating procedure in such a case would be to replace the crankshaft. To my surprise they seem to be available from the Part Store of this very website, however for the extreme price of $430. That would just blow my budget right out of the water.

Looking at the parts fiches, apparently the crank pin is a separate piece and is only $15. Why not simply replace the pin, bearings, seals, etc. and call it good? Forgive me if this is a stupid question, I'm new to the world of two-stroke engines. I've always wanted one but two-stroke street bikes are extremely rare now a days so I have set out to build one of my own. I'm very comfortable with a wrench though I lack the special pullers and such required. I know my way around 4-stroke engines but these 2-stroker seems to have a few tricks to them.

Thanks for any help!

you can do what you just said. make sure the rod is good and new bearings and crank pin (can not use the old pin) new piston and bore job--maybe another head.-----good luck

Very good to hear.

About the head, it is banged up a little. I was thinking of removing the dings with a grinder and polishing the surface a bit. The dings aren't huge, perhaps 0.5 mm deep. Then head might need to be milled down to get the compression ratio back up. That raises the question of what compression ratio to shoot for.

Or perhaps it would be simpler to just find another head for it.

head doesn't sound bad, just clean it up--shouldn't make much diff in comp.

The 0.5mm was just a wild ass guess, but you are probably right. I might still measure the head volume etc. just to be sure. I feel like every little bit helps with an engine this small.

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