top end help

[4x4 AG]

OK, so i thought the compression was a little low on the new quad. I am not sure that i checked it completely right, but after kicking it over for a while the best i got to, cummulative, was 150 psi. So i pulled the motor to look inside. attached is a picture with the dome off and piston at TDC. I am not sure if these numbers mean anything to anyone, or if you can even read them. But here is the help i am looking for.

I have never built or re-built a two stroke. only 4-stocks and pretty much the shops did them. So where do i start? I tried to roughly measure the bore with a MM ruler and it was ~69 mm which lead me to believe the someone did the 270 BB on this engine. But how does the sleeve look to yall? i am not sure how thick it should be. How do i decide what piston to use, and if i need a re-bore? Do i need to buy some micrometers and other measuring tools?

Thanks guys.

[Honda 250r 001]

you could prolly just throw a thinner base gasket on there and the compression would be up to normal. Im betting the compression was always at 150 because of that huge squish gap.
To take the top end of that 250r you need to drain the coolant, remove coolant hoese, remove tank, remove head stay, reomve pipe, remove intake, 4 nuts on the base of the cylinder and its ready to pop off.

[4x4 AG]

yah i got the engine out already. Much easier the the 4 strokes i have done.

I am not remembering a gasket when I pulled the dome off. but should i try some different gaskets to get the compression up? Thinner base gasket and gasket between the spacer and cyl?

Also, how do i know when it needs to be rebuilt? and how thick should the sleeve be?

Man, I am such a noob. :D

[sixer3]

Do not use a thinner base gasket (gasket between the cylinder and the crank case) use a thinner head gasket. gasket between the top of the cylinder and the head or dome i think you are referring to it as. Be careful with this though. if you raise it too much you will need to look into using different fuels.

as for the cylinder, it's impossible to see in that pic whether or not its still good, i wouldn't worry about the thickness of the sleeve so much as the condition of the surface inside the bore. Though the thickness of the sleeve may come into play if/when you need a re-bore

[4x4 AG]

ok. So maybe now i have some new questions. on the piston it says 3.50MM. So i am guessing that is 3.50MM over standard, which is 66mm. So that would make it a 69.50mm piston. That is the largest that Wiseco goes. Does that mean that this is the last bore for that Cylinder?

How is a 270cc big bore made? This is still the stock 246cc jug/cyl, as it is stamped on the side. What did they do to make it a 270cc?

[sixer3]

they remove the stock sleeve, bore the cylinder and insert a larger sleeve to accept a bigger bore, so if that is an aftermarket bigger sleeve which it must be, to replace it, you would need to re-sleeve with a "big bore" sleeve

You can use a Yamaha YZ 250 70MM piston from 92-98. With a 70Mm and 72MM (stock) stroke, you will be at 277CC…a 69.50MM piston would put you at 273CC’s.

One caution with the YZ pistons is you need to make sure the ring ends are trapped within your cylinder walls on the intake side. This is not an issue with Honda OEM cylinders that have not been ported. However; it can be an issue with ported cylinders that have been aggressively worked on…

I would also have the cylinder base decked to allow the piston to come up to the top of the sleeve or be no more less than .010” from TDC…in your picture; TDC appears to be +1MM or more down the bore. This would bring your compression up to 170PSI-185PSI depending on your head.

Carlos.

[JohnBeton]

Originally posted by 4x4 AG
...Thinner base gasket and gasket between the spacer and cyl?...

You're talking about a spacer, so you have a spacer underneath the cilinder ? If so, can you check the squish without the spacer installed ?

[wilkin250r]

Do a little research on "deck height". It's the distance from the top of the cylinder wall down to the edge of the piston at TDC. Yours looks to be a little big, which means your base gasket is too thick, or your spacer plate is too thick, or some other goofy reason.

There might be a REASON for this, so I don't know if I'd go changing it without knowing the rest of the story, but let's start working out the details. We'll start with deck height.

[4x4 AG]

ok, so i am learning some now. I think that it has a spacer plate under it, as it is an 86 motor. So aren't those the long rod motors that run the spacer plates?

Also, can some one post a picture of what my deck height should look like. I am not necessairly looking for race gas compression, but i think 180 psi is what the manual calls out.

thanks for th replies, keep them coming.

I will measure the deck height tonight, and the spacer.

also, what tools do yall use for measuring these things? calipers? rulers? how percise do i need to be?

[wilkin250r]

Calipers at least, maybe a ruler if it is high quality and precise enough. You want to be accurate down to half a millimeter (0.020 inch) at the absolute very least, and that's STILL too big when you get to the building stage, but half a millimeter is fine for getting a general idea of what we're looking at.

Calipers are best. Ideally, we want to be accurate down to 0.001 inch when we get to the building stage and planning your squish band.

[4x4 AG]

ok. I got alot to learn. Squish band???

I will try to find some calipers.

[4x4 AG]

new picture of TDC. THis look better?

[4x4 AG]

doesn't look like there is a spacer plate on it after all? should there be on an 86? OR maybe it isn't an 86 motor?

[4x4 AG]

Intake....does it look like work has been done? Like i said, i am new to this and don't know much.

[4x4 AG]

another shot of the Intake.

[4x4 AG]

and lastly, a shot of the Exhaust side. Look like anything has been done?

[wilkin250r]

Originally posted by 4x4 AG
ok. I got alot to learn. Squish band???

I will try to find some calipers.

Look at the underside of your cylinder head. See the dome shape? That's called the combustion chamber. But the dome isn't as wide as your entire piston, right?

That outer edge, that ring that goes from the combustion chamber out to the edge of your cylinder walls, that is called your squish band, and it's really critical to proper engine building.

http://www.aircooled-rd.com/images/rdheat1.jpg


This picture of from THIS (http://www.aircooled-rd.com/default.asp?txtPage=squish1.htm) article. It's originally written about a 2-stroke streetbike engine (the same engine that later became the Yamaha Banshee), but the theory applies to the 250r as well.

[wilkin250r]

Let's not get ahead of ourselves, you don't need to worry TOO much about that yet. But as you can easily see, your deck height and squish band height are directly related.

So get us a measurement on your deck height, and we'll go from there. One thing at a time.

[4x4 AG]

yah, i don't have anything to measure that distance with right now. all i have is rulers and tape measures. So that won't get me what i need. I posted that one other picture of the piston at TDC to show that my other picture was a little off. There was a little left in the stroke. I guess i will have to see if i can find a depth gauge or something to measure that with.

The other things i posted was general questions. I am trying to determine if i would be better sending it to a shop for rebuild and porting, or if what i have just needs freshing up that i can learn to do.

[dober250R]

it looks like someone only cleaned up the ports on that cylinder.... What is the tolerance for the compression? I would recommend just slapping a new set of rings on the piston, and put it back together. Then re-check the compression. A set of rings on these 2-strokes cost next to nothing. The top of the piston looks relatively clean. I wish I wasn't rusty on 250R's... Back in the day, I had a head that was milled, and ran a custom waterjet copper head gasket, and that thing ran like a raped ape. I'm getting the itch to get another 250R....

[dober250R]

Make sure you get a metal head gasket, not a crappy fabric Cometic gasket...

Local pawn Shop might have some good deals on used machinist tools. Try to find a “blade depth mike”, these are normally older tools made by Starrett or Brown & Sharp.

The “blade” is thin and rotates 360 degrees in relation to the mike head allowing you to measure right up to the very edge of the bore & piston. Depth mikes have a 2”-4” wide base, providing the most stability and extremely accurate & consistent
measurements…great tool for the non-machinist types; hard to get a wrong dimension with this tool.

Carlos.

[wilkin250r]

I you were going to build high end engines for a living, then yes, get professional tools, as Carlos suggested.

But for just a general build, a set of calipers is fine.

[4x4 AG]

yah i am planning on picking up a standard caliper and a set of feeler gauges. Probably do that this weekend and see what i have.

I am planning on tearing the trans side case off as well and looking to see how things look in there. i knoe i need to replace the kickstart gear, so i might as well inspect everything and order all the parts at once.