Hi everyone, I hope you can help me out with a question I have. I ordered up a cylinder+piston kit off ebay for a buddies 1981 DT125 I am helping him rebuild. It showed up, I cleaned it and went to bolt it on.

1. The first thing I noticed is the cylinder did not have holes through it for the factory cylinder nuts. No big deal, I have plenty of hardware laying around and there is clearance to tighten them from underneath the cooling fins. Done!

2. The next thing I noticed is the reed valve bolt pattern is larger. This is a bigger issue but I was guessing I could find something that would work from a yz/different year/etc. Never looked into it because:

3. At this point I figure I should cycle the bike before I put the head on as things are clearly not standard...and the biggest issue becomes apparent, the piston extends past the cylinder so far that the top piston ring becomes uncompressed.

And idea what this cylinder may be for?

 

Best to load a pic if possible or pic of old & new cylinders. But to save the situation yamaha seem to alter things like conrod length & pin height in the piston by 6mm over the years. So would a 6mm base plate do the job or is it more than that. Or could you find a shorter rod & rebuild the crank. More to consider than that i know but just throwing ideas around at this point. damn frustrating i realise as been there over the years. many times. Base plate sounds best except a bugger to maker one without good gear.
Sorry didn't see pic or i wasn't logged on maybe.

Thanks for the reply, did my photo in the 2nd post not upload? I'm new to this forum layout so may have done it wrong. I will try again in this post. I will measure the height and see, a spacer was my first thought but like you said I dont really have the equipment to build one.

Edit: edge of cylinder to edge of piston appears to be approximately 4.5mm

New cylinder and piston from China? 

Someone said here once they are selling "1982" or later pistons on ebay...  we never got that model in the US but pin height is different. 

Probably best to get the right cylinder if you can...  you could try to see if original piston pin height would work but then you still need new reed valve and manifold and who knows what else... some of the Chinese pistons self destruct quickly too. 

I'd get the original cylinder bored.  Would be cheaper and better in the long run for sure. 

Hello, What you have is a 1982 and up cylinder kit for a South East Asian market bike, they have a different piston rod length and deck height.
The fix is to use a genuine piston from yamaha or pro-x or wiseco or other reputable japanese manufactuerer fit for your year and model.
The chinese ebay piston kits are no good.
The improper alloy aluminum will quickly lose its shape and seize.
It will crack the intake skirt and send pieces into your bottom end.
The wrist pin is not hardened properly it will wear instantly.
The pin clips are not proper spring steel, theyll come loose and your wrist pin will catch a port, potentially causing catastrophic damage and a seizure that could put you in danger on the road.
I like cheap stuff, but i've learned this from experience first hand.
Possible issues with the cylinder itself:
I recently was given one of these chinese 66mm DT175 cylinder kits to me for free to check out.
Piston to wall clearence was wrong right out of the box, in addition to the issues listed above.
Here are the issues I found with the cylinder itself:
Mine was stamped 18L, which is yamaha model code for 1984 and up South East Asian market DT175
My bike is a 1981 US market DT125.
cylinder studs line up ok, but original aluminum bolts cant be used, need to source your own fine thread pitch nuts for the cylinder base.
Biggest issue: transfer duct footprint of the base gasket does NOT match up, and the late model aftermarket cylinder does not properly seal to the crankcase, and would cause a huge airleak. The only way to fix this is to relocate the ducts by welding up the edges of the ducts on the bottom of the cylinder and then do a bunch of porting work on them, and then file the base of the cylinder flat again, to get everything to line up with the original 81 US DT125/275 base gasket.
old cylinder base gasket vs new cylinder base gasket:
 
late model cylinder ducts: too far out and poor flow around the sleeve.
 
late model cylinder:
if the ports are aligned, youre lucky, and they will be poorly aimed, not blended with the sleeve, and obstructed by casting flashing
 

I used epoxy after a carbide burr to rough the surface to correct the boost port
 

I did find that my bore was honed to the correct size, and was nice and round. Hand chamfering of all the ports and a hone was needed.

As for the piston, pin, clips, and bearing i received in the kit: trash, throw it out or melt it down, just dont put it in the bike.
Not only that but its the wrong compression height for early models, so it poked out the top 5mm.

I used a 66mm pro-x piston kit for yamaha blaster, it lines up with the late model bridged intake port and has the correct compression height and is a quality piston. The 175 uses a wider wrist pin bearing too if you do a conversion youll need that. If you have a 125cc kit youll need to find an original piston/bearing or something else made in japan.
I also had to use a reed valve and intake manifold for a yamaha Blaster, as the original US model one was smaller.
the pro-x piston was 65$ on ebay, and the reed valve+manifold kit was 30$. This lets you use the stock carb in stock location.

to sum it all up, make sure your base gasket lines up, get a genuine piston and a Blaster manifold and reed valve 

 
proof that anything is possible
aka 
just because you can doesnt mean you should! 

Wow, there's a lot of useful information. Just bolt a 175 top end on & watch his eyes light up with surprise.

I don't think you can do that with a 78 and later DT125/175 like you can with the earlier bikes.

I see both 125 & 175 in "81 had the same 18G base gasket so i 'think' you'd be safe as with older models. Just my opinion only from looking at partzilla numbers & drawings. 125 & 175 always run the same stroke--the up & down bit., think 50mm. No14 in both links below. I've done that swap with my AT1 using a TY175 cylinder but think i have a baseplate under that & goes like a rocket.
www.partzilla.com/catalog/yamaha/motorcy...linder-head-cylinder
www.partzilla.com/catalog/yamaha/motorcy...linder-head-cylinder