I need a piston lesson (NEW ? VALVES!)11/14/12

[MRSOUND]

So I'm taking night classes for small gas engine repair and have taken an atc 90 in as my 1st 4 stroke project. Motor was free but no spark so I've haven't had it running yet. Anyway part of our "lesson" is to measure critical specs as we disassemble he motor. The specs for a 90 cylinder per the service manual is as follows:
STD -1.9685 - 1.9688
.25 - 1.9784 - 1.9788
.50 - 1.9882 - 1.9886
.75 - 1.9981 - 1.9985
1.0 - 2.0079 - 2.0118
My bore Measures 2.124! The cylinder is stamped 89cc. Which blew away my thoughts of it being a 110 cylinder.
I know i can't figure out the true cc's without the stroke dim. which I don't have yet.
Can anyone give me an educated guess?
Is this getting outside the "safe" limits of the cylinder?
Didn't get the valves cleaned up yet to take measurements yet but would they have had to change from stock as well?
Piston measured 2.123 X 2.117 - Instructor said this is to allow for heat expansion. It is stamped on top with 4.00.
Does this mean it is 4mm over std? Is this piston anything 'special" or just oversize?
Lots of carbon build up on the head, valves, and piston.
Here's a couple of pics. Talk to me fellow trikers!

[hippyplz]

Yep, that a 54mm hc piston. Cool find.

[MRSOUND]

So Obviously I wont be able to use the stock head gasket that's coming in the gasket kit I ordered?

[hippyplz]

Yeah, i have a +5 in mine and the head gasket fit fine.

[hippyplz]

Well let me refraze that. A stock 90 head gasket will work, but if you ordered a kit for a 110 i dont know of it would be the same or not.

[Dirtcrasher]

Mike at VMS knows the sizes and requirements. Nice guy to talk to

[atc007]

There is only one way you worry about measuring a piston. Skirt to skirt. No other measurement means anything. Generally the stamping on top of the piston is SAE,rather than metric. Like I told you,I'm no 90 expert. Mike,Shortline,,will see this,he can tell you the day and factory that piston was made lol and exactly what you can and can't do gasket wise. Very cool little piece for sure. I'm guessing a std 90 head gasket will work. But that is hogged out !

[blue27]

lesson #1 you never know what your are going to find until you open it up,lol High compression big bore 90,cool!!! Big overbore piston may be a little harder to find and pricey...If you need a stock cylinder to bore to a more common size piston kit I know we should have some around and I do know a shop local that can bore a cylinder at a reasonable price. LMK if I can help.

[MRSOUND]

Waiting for a shortline response Bill. Curious if it just a big bore piston or maybe something "special". I doubt it but hey, that would be cool.
Bruce- Thanks! I think both the cylinder and the piston are going to clean up fine. Wondering if there were other mods that typically would go along with a bore this big? What else if anything should I look for?

[shortline10]

Head gaskets are the same for the 90/110 after 1974 and I can see that your motor was made after 74 by the head configuration at the head gasket and yes its ok to use a stock head gasket with a big bore kit IMO . Your piston is 4mm over stock so that puts you at 55mm and 118 cc , if you have a stock atc90 crank . Piston looks like a Big Al's to me .

[MRSOUND]

Thanks for the info shortline! I'll get to the crank hopefully next Thursday. I'll let ya know if it is stock or not.

[oscarmayer]

do me a favor, take a pick of the bike and send it to oscar@mayer.net please.
Years ago I build a 90cc engine and did a 110cc bigbore setup with 110cc carb and a few other goodies tweaked cam and stuff. I get a nagging feeling on this particular bike you have that it may be that old 90 I had build up back in the early 90's. That piston loosk EXACTLY like the one I used.

following specs that I can remember were as follows:
Rockeymountain 12.5:1 big bore 110cc piston kit
stock jug bored to 110cc
cam was a web 110cc cam (back then the 110 cam had slightly bigger lobes and better overlap)
we had 3 angle valve job and stock valves with 110cc springs
125cc intake (was slightly bigger than 90 or 110)
110cc carb (re-jetted)
perforamce i think bassoni or supertrap exhaust i cannot remember which one
aftermarket exhaust studs. they were stronger than stocker that always snapped.
open air filter no airbox (uni filter if i recall)

i know there was a few other things. if it is that bike, it started out life in Southern Cali where I found it on autotrader for $350 smoking like a train. I was good friends with an old shop named "Sam and Ed's motorcycle and ATVs" in Azuza CA near West Covina and not too far north off arrow highway. (still trying to fine where they went as one of the borthers a few years after i left was killed in a bike accident (RIP my friend).
if you have photos of the bike before teardown, I can tell you for certain (if some of the specs I stated matched, which somethign tells me yours is fairly close)

[3wheelerfan1991]

do me a favor, take a pick of the bike and send it to oscar@mayer.net please.
Years ago I build a 90cc engine and did a 110cc bigbore setup with 110cc carb and a few other goodies tweaked cam and stuff. I get a nagging feeling on this particular bike you have that it may be that old 90 I had build up back in the early 90's. That piston loosk EXACTLY like the one I used.

following specs that I can remember were as follows:
Rockeymountain 12.5:1 big bore 110cc piston kit
stock jug bored to 110cc
cam was a web 110cc cam (back then the 110 cam had slightly bigger lobes and better overlap)
we had 3 angle valve job and stock valves with 110cc springs
125cc intake (was slightly bigger than 90 or 110)
110cc carb (re-jetted)
perforamce i think bassoni or supertrap exhaust i cannot remember which one
aftermarket exhaust studs. they were stronger than stocker that always snapped.
open air filter no airbox (uni filter if i recall)

i know there was a few other things. if it is that bike, it started out life in Southern Cali where I found it on autotrader for $350 smoking like a train. I was good friends with an old shop named "Sam and Ed's motorcycle and ATVs" in Azuza CA near West Covina and not too far north off arrow highway. (still trying to fine where they went as one of the borthers a few years after i left was killed in a bike accident (RIP my friend).
if you have photos of the bike before teardown, I can tell you for certain (if some of the specs I stated matched, which somethign tells me yours is fairly close)

Cool story, keep us informed.

[MRSOUND]

Emails sent oscarmayer... Don't sound like its the same trike though . This has the air box and stock exhaust unless the motor was plucked as a donor. (I could only hope!) How do I tell if it's a 90 or 110 carb? (remember I'm learning here) Here's pics for the rest of you in case your interested. Warning: this was really neglected and will need alot of tlc (and a new frame...cracked) Pics are "R" rated! (rusty)

[shortline10]

Poor thing !!! Carb is def stock atc90 16mm