Thread: 72 CT90 restomod help

Hi all, I just picked up a little '72 CT90 that's been in the family for over a decade, and been seized for over a decade... I am a machinist and thought I would do a big bore build on it, but I need some pointers... First, how do I tear down the cylinder? I got the little valve cover plate off, but can't figure out where the head/cylinder bolts are? I've never rebuilt something this tiny, more into the big 4 cyl bikes, but figured this little scoot would be a fun resto project. Secondly, who makes a reputable big bore piston/ring set for the 90? Third, how big can I go? I was hoping for 120 or 125. Can i bump compression? Gimme links. Thanks, daveyg.

Ok, so I got the top end all torn down, took a few pics. Went to the local Honda dealer, picked up a set of .050 overbore piston rings... My question now is this, what is the exact size diameter of a .050 overbore cylinder? I'm a machinist and just wanna know if this things been bored before, or if I'll need to get a even bigger set of rings. The current bore diameter (where it's not rusty) is 1.970" So what is the proper bore diameter .050" over stock?


Also, the piston says on the side (near the pin hole) "ART CS 90" but my frame says CT90 Is it possible that this piston is not the original? Some of the phillips screw heads in the cyl head were mildly stripped... I guess the proper bore diameter clues will help me diagnose this but, just another curiousity... If it has already been bored .050 what are my other options? can I get a .100 over set of rings and a piston? What would be my displacement at .050 over? or at .100 over? can the stock cylinder be bored .100 over or will that cause other issues? Thanks for helpin a CT newb out! Daveyg.

Art was an OEM piston supplier. Usually, oversize pistons are marked ".25" etc right on the crown. Make your life a little easier and go metric, like the rest of the civilized world did decades ago. I know we still cling to the imperial system of measure here, but after a while you'll grow weary of breaking out the calculator to do the conversions.

I'm kinda curious as to why you picked up a set of "0.50 over" rings without the matching piston. That's second oversize and you're not dealing with a 50 year-old Briggs & Stratton. At the 8-10,000rpm these little Honda motors can spin, you'll have nothing but grief with mismatched piston, bore & rings.

Incidentally, it's not uncommon to find one of these motors with so much bore taper that it takes going directly to second oversize to restore concentricity. It's also not uncommon to find examples of these engines that have already been taken 0.50 over, years ago. 49cc & 72cc engines can be taken out at least 1.00mm (4th oversize), at least pistons are available. Don't know what's available for the larger engine family to which your 90 belongs. However, generally speaking, you'll have fewer choices but enough to get you going again. Worst case, you can have the jug sleeved for around a C-note. Time to break out the repair manual, micrometer, etc and remove the guesswork.

Ok, so I don't have a repair manual. Also, the cylinder bore below where the rust occurred still shows the crosshatch from a rebuild a mere 20 miles before the bike was parked(according to my dad's memory) And the rings I got, which I was told were .050 overbore (the dumbass technician probably saw .5mm over) slide right in (with a little finesse, as is typical with pison installation) the un-rusty side... So I guess I just need to know what the exact stock bore is. So if someone can help me with that info I would greatly appreciate it. bikez.com says stock bore is 50.0mm which equates to 1.968" and my cylinder is 1.970 which i attribute to the hone of the previos rebuild. So what if i hypothetically bore it .25mm, use my stock piston replacement that has 20 miles use, and my .5mm over rings? Think it'll hold up?

Std bore on CT90s is 50mm (1.9685") Std piston-to-cylinder clearance is 0.01-0.04mm (0.0004-0.0016") Yes, that's 4 to 16 ten-thousandths of an inch, or roughly half a thou to a thou and a half. Compared to most other internal combustion engines they're tight little buggers. If you buy a factory piston they will usually be just slightly under the specified bore size, i.e. a stock 50mm piston for you 90 will probably measure 49.98-49.99mm, so you can usually aim for the exact oversize.

The 1.970" measurement you came up with suggests that it's time for an oversized piston. I prefer to bore a cylinder with the oversized piston in hand, so I can aim for the appropriate finished size. The problem is that getting perfectly centered in the area of greatest cylinder wear is somewhat hit or miss with my ancient boring bar, so I often have to bore first, then order the piston. That usually means I end up honing a little more than I like, but that's the best approach I can come up with in my situation.

HTH,

-kevin

Miniac, thanks for that info! For now though I've just decided to get a set of stock rings, and just gonna hone and hope she doesn't smoke too bad. But I will put a .5mm over piston on order for future use. Tolerances really helped, that's what I was looking for in the first place. Thanks again, daveyg

dratv.com sells a 54mm piston and rings for the ct90. They are only about $40 i think. Then you bore your old cylinder to 54mm and put it together and you have a 104cc. My friend took his pitted ct90 cylinder to a cycle shop and they bored it for $70.

Perfect thanks! I looked that up and only problem I saw was the super high compression ratio... I don't really wanna run C16 race fuel... Ok, nevermind, I found that they make one in 9:1 after a little while, that should work quite nicely, plus it's only $59! Thanks alot for the site reference Merz!