rd200 piston question

[frogdink]

so i know that i need new pistons, but im not sure if they are standard size or not. on the top of the piston it is stamped with .514 or something like that with an arrow pointing up. i am told that the arrow is just to show direction of piston. but the numbers have me wondering what pistons to buy....any ideas?

[Crazcnuk]

Uh, Measure it?

[Miti-Babe]

Frog

Pistons usually go up in 0.25 mm increments...

1st O/S = +0.25 mm )
2nd O/S = +0.50 mm )- bigger than a stock piston diameter
3rd O/S = +0.75 mm )

It's not normal to get bigger than 3rd O/S for most bikes... Most manufacturers don't make them bigger because the weight of such a big piston causes vibration which isn't balanced out by the crankshaft flywheels - they're still the correct size to balance out a stock piston...

This is also why big-bore kits have to be specialist parts... They make bigger pistons that weigh the same as the stock parts to stop the vibration happening...

I suspect you have 2nd O/S pistons fitted, but measuring them is always the best bet anyway... You'll need to check the bore too... Look on the bright side... You may only need piston rings..

HTH

Miti

[frogdink]

...

thats an idea crazcnuk, what would i do without experts such as yourself to guide me through this.

Babe, what threw me off was that it was not just .500, it is .514, but i will look into it more there are plenty of second oversized pistons on ebay if that is the case. thanks for the help.

[Crazcnuk]

Well, your question was what does the number mean.

I assume the piston is bigger than 1/2 inch in diameter?

If it is then the .514 obviously does not mean .514 of an inch. I thought that would be fairly obvious.

Besides that, you NEVER go by numbers stamped on a piston. You measure the bore, and you measure the piston and find out what the clearance is, then decide if you need to bore or not. If you do need to, by this time the piston size needed will be fairly obvious.

Sorry if I was a bit crypitc.

[frogdink]

my boss let me borrow some snap gauges, i will measre them this week. the problem is the engine wont turn over, one of the pistons was stuck, but i had to move the head up to free it rather than the piston down. and it still wont turn. i tookk the engine out and started to take apart and everything looks really clean (not all rusty). as a begginer as far as engine work goes. do you think this is something i should tackle and try and be talked through, or should i bite the bullet and drop it off at a shop and tell them to do what they need to do to get it operational and fork over a bunch of cash....im asking because i have no idea what i could be in for...

[Crazcnuk]

I assume by 'head' your talking about taking the cylinder barrel off?

The usual sticking points are the rings, which, if you have the barrel(s) off, bypasses that, bearings, or the transmission is in gear.


A lot of the older bikes, and 2 strokes especially, used ball or roller bearings on the crank which may be gummed up or rusty.

Also you may want to check the clutch. Even if the tranny is in nuetral, the clutch turns the input shaft in the transmission. If something is stuck in there, it can keep the motor from turning over. Make sure the clutch is free when you pull the cable.

What some people do is fill the transmission and crankcase full enough to be above any bearings with ATF, diesel or crankcase cleaner and let it sit for a couple of days. THis can often free up anything that is stuck.

[frogdink]

yes, i am talking abnout the cylinder barrel. please excuse my non-knoledge of the names of this stuff. like i said its my first time doing anything this in depth. since i am replacing the pistons anyway, would it hurt anything to bang the hell out of the piston to see if the will break free......i have a feeling i know the answer to that.

[Crazcnuk]

The only thing that really sticks between the piston and the bore are the rings.

If the barrel is still on, just put a mixture of ATF and diesel on top of the piston and let it sit for a day or two. It should find it';s way all the way down past the rings into the crankcase. Once that happens a few light taps with the wooden handle of a hammer, or a rubber mallet should break it free.

[cloerts]

I had the same problem with the same bike, the problem was that part of the pistons broke off and lodged in the crank case stopping the rotation of the engine. I found a new tranny/case on ebay for about $35.00 and a new set of pistons for $50.00 $80.00 and problem solved.....Now if I could just find time to put it all back tgether....