Boring

After getting word from Harold that the cylinder would need to be bored .50mm over (or the 2nd over) I started sourcing a piston and a ring/rings.

Yamaha switched to a reed valve induction system in 72 which made more power and was more efficient presumably (they've been using reeds ever since). Pistons for those engines are easy to find…like order-them-in-a-catalog easy.

DT1 pistons…not hard to find, but they’re the order-them--on-ebay kind of not hard.

Found a decent deal after little to no searching; a DT1MX Piston. When I bought it, I knew there is a different piston ring configuration than the DT1E (what I have)

I learned after getting the piston and looking at it that the skirt is also much shorter (this will affect port timing) and that the piston is a good few ounces heavier than the old DT1 piston.

Though thinking on it now, the old piston was gouged, had another ring grove, and lived a hard life in its latter years. It’ll find its way to a book shelf or a desk to lead a less stressful life…

There’s something really reassuring about going to a place of business that’s named after a man and actually talking to that man. This was the case at Harold’s Machine Shop.  It was a 2 man operation in Richmond, Tx as best I could tell. I think is comrade was named Donnie. I gave him the piston and ring so he could do the finish work and I picked up the cylinder the next day.

 Steel liner needs to be filed to smooth the transition between ports/cylinder wall. 

Steel liner needs to be filed to smooth the transition between ports/cylinder wall. 

I’m a little sad that it didn’t come back hot tanked or cleaned, but for $64 I have no room for complaint.

I was unable to find what the proper ring gap should be. I feel like the gap might be large, but can’t say for certain without the data. We’ll see how it runs…

Somewhere in the time before, after, or while the cylinder was in the shop, I slowly collected my pieces from USPS. A tachometer drive gear, a shift shaft washer, a tiny oil pressure spring and a 5/64ths diameter ball bearing. A rubber “oil pipe holder” and a grommet for stator wires.

I’m happy to say that it was a day ago when I put the clutch side engine cover on with black oxide allen screws and plan to never open it again. (But I’m a realist. I know it’ll be open again…)

 Gear on bottom left is Kickstarter. Didn't look right. Should have known...

Gear on bottom left is Kickstarter. Didn't look right. Should have known...

 How we fix torn gaskets. 

How we fix torn gaskets. 

The gasket kits for these engines, or maybe just the ones I’ve gotten (because this happened with the DT2) are some odd sort of deformed. Not enough to matter from a functional standpoint, but just enough to put weird stress on the gasket at certain bends.

I put the kickstarter shaft in (thinking that it was all too easy) and buttoned the whole case up. After kicking it a few times, I noticed the kick starter shaft was installed wrong. Goody. So off the side over comes, I re-set the kick starter shaft (not rocket science, but a return spring has to catch on a certain knob) and upon trying to put the cover back on…tore the gasket. I’m thinking it’s nothing Motoseal can’t fix.

I was going to just stick the autolube system on (oil pump) and let it ride….but it kept me awake at night not knowing if it was full of sludge or solidified castor oil or what. So I took it apart….

Last time I took one apart (DT2) I ruined it.

 2 Stroke oil pump. No need to premix as long as you have faith...

2 Stroke oil pump. No need to premix as long as you have faith...

These bikes (all bike, really) have road maps..exploded part diagrams for every little system showing what part goes where, what part number it is and how it interacts with other things around it. It’s like a mechanical, 3d paint-by-number.

 Part 23 does not exist on earth. Also note #12- a screw. Behind those 4 screws is the insanity.

Part 23 does not exist on earth. Also note #12- a screw. Behind those 4 screws is the insanity.

The oil pump is an exception. There’s no diagram showing what’s in the pump. It’s some gears, microscopic (honestly, damn near…) springs, some seals, and a crazy array of different sized washers of different thickness that are of critical importance.

I ended up being glad I took it apart because there was some crap in there…but it was a very painstaking time. SO much so that I took 0 pictures. As a plus, though…I lost the little spring and the directional gear that ratchets on them. BUT I found the springs. And managed to get the gear back on. Doesn’t sound hard…but it was an amazing feat. I’d read from a few people “If the gear comes off (off the spring loaded pins) throw the pump away and find another one.”

Feel pretty good it’ll pump oil…I modified the lower banjo bolt since they don’t make and it’s impossible to find a spring that goes in it. Drilled the hollow bolt out a tiny bit and was able to get a later model spring (from a 76 DT250 oil pump) to fit. No idea if it’ll operate correctly so careful testing will be done before firing.

Next steps- Black letter the “YAMAHA” on both sides

Port and polish the cylinder, polish the head.  (tools are in the mail. Doing it by hand got old fast.)

Powder coat the cylinder and head

Bolt it together.

The end? Don’t know what to do after that…

 Sealing it up. 

Sealing it up. 


I do feel pretty good about this engine though. As far as tolerances, parts, function, and the fact that literally no stone (or bearing) has gone unturned.

And in the end, it’ll amount to a loss of money because no one will buy it, I don’t have anywhere to ride it, the “bill of sale” is some scribbles in red ink from a 14 year old, and I don’t really know what to do with it…

But it sure is a lot of fun building it.

Should probably take up fly fishing instead. 

Best since Day 1.