DT1 250 pre 70 legal BArrel

[Nathan S]

Remembering that all three of my motors are stamped DT1F:

This is my untouched DT1F (reed valve) motor for my resto bike. It has a 311 rod. Its on the original bore, so I assuming that its the original rod.


This is my race bike motor (and no, I'm not going to pull it apart to take photos). Note the small lump on the side of the barrel just above the base gasket - this is the top of the pad that used to have "246cc" cast into it, before I took to it with the lathe.


This is my 'spare' motor. Note that its missing material from the bottom of the barrel, but less than my bike's barrel. Nifty's is like this too.

214 rod in the spare motor:


Yamaha piston from spare motor. This one hasn't had anything removed from the bottom of the skirt, which makes me think that there are two or three different piston-port pistons.


Finally, the spare motor with piston installed, shown that the deck height is well and truly in the ball-park (if not ideal):

[JC]

Freaky,

I'm just going on what 2 early Wiseco manuals say. They list # 234 for "YZC,D,E,F, DT2,3, MX,IT" only.

# 230 & 398 are listed for  74-75 YZ. I suspect they're 5mm shorter deck-ht like DT1. And that fits w my YZb engine using #234 piston, not the proper #230 or 398 & needing 5mm spacer under barrell. The YZa/b was said to be developed from the DT1 engine (ie not DT2/3)

AS mentioned previously, I've measured DT1 piston-port piston (several times) & its 5mm lower deck-ht than DT2/3, but barrels are same ht (110mm)

Nathan, Yeh seems to me there's 2 diff cases too. My DT1F (DT2/3) engine has 311 rod also. I'm fairly sure the YZ had 214, but its obviously the same length cos yr engines have both & both need decked barrell.

[Freakshow]

im lost .  nathans other motor shows a 214 rod ? so are we now agreeing its the pistons that change in hieght not the motors ? make life easier if thats the story.

the Yz was alway a reed was it not ? so the piston should be the same all the way.  I have a 234 and the 230 piston at home so i might pull them out and measure them up, but i just assumed they were the same i have never had them side by side as they were both i thought reed window pistons.

i got 2 Dt1 motors  here somewhere after next week i might pull of the heads and suss the rods, be inerested to see how many are 214 or the 3 series.

 i have a Dt2mx barrel on a yz bottom end, and thats fine, so too if i fit a dt1 barrell - they all line up fine.  i dont have a PP barrell to sit on it to see if there is a differace to agree with ya finding there, but ill have a hunt up through the Dt1 c, f or mx Bottom ends i got and see if there is some kind of a pattern in there.

interesting thread fellas :O)  thanks for all the pics Nat makes it easy to see what were all talking about here.

Neal told me he only fitted 1/4 spacer ( which later became a GYT kit) and had an angled intake inlet set up to a mag carby yam gave him with a pancake air cleaner, when i sussed him out about his go fast in the day set up, there was no mention of rod changes, but if i speak to him later ill quiz him on that too, because he ran the PP and then into the reeds before he retired in 74, they dont call him proffessor for nothing.

[vandy010]

most interesting indeed fella's!
Freaky, i just measured my PP barrel and it's 110mm from base gasket to head gasket and so is obviously "undecked" and STD.
i have no DT motors at my disposal so i can't measure anything else and only have the one RT motor. prefer the big bike.

[JC]

It is a bit of a conundrum.

The pistons are definitely diff hts. PP pistons (& YZa/b as far as I can tell) are definitely 5mm lower from gudgeon to top than all other reedvalve pistons. But the barrells are all 110mm high & the rods all seem to be same length whether 311 or 214.

So if the rods & barrels are same length/ht & pistons 5mm shorter something else must be 5mm shorter also. The only thing it can be (it seems to me) is the cases are different, w the vertical distance between base-gasket surface & centre of crankshaft 5mm less on the PP DT1 (&YZa/b) cases than on all later reedvalve engine cases.

I measured that distance on a DT2/3 (DT1F) engine & later DT engine. Its 80mm (or v close to it) on both, but I don't have a PP DT1 (or YZa/b) engine .

Another way around decking the bottom of the barrell 5mm when putting DT1 barrell on DT2/3 cases would be to use a 5mm taller piston. I'm fairly sure the TM/TS250 pistons were about that, cos when DirtBike mag built a project DT1 in early 70's they used a Suz piston w a spacer under the barrell. I'll have to check what thickness.

[Freakshow]

when putting DT1 barrell on DT2/3 cases

Hang on i thought we were talking about a PP barrel on a DT1F reed rebottom end ?  thats what Nathan has up in pic's.  IM wanting to put a Dt1F bottom end onto a PP barrel,  thats what i though we were nutting out.  can we be sure we are talking about that specific set up when we conclude this delema.

[facthunt]

Does this help?
214 rod is 130mm between centers, 311 rod is 125mm between centers.

[vandy010]

and so it is, the jigsaw pieces are coming together

[Nathan S]

Brilliant post, thanks Kurt.

That explains half the mystery - but I'm not sure why Nifty's or my spare motor have needed material removed from the bottom of the barrel.

[Freakshow]

if you used a 311 rod with a Dt1 piston you get that short stroke so the only way to pick up that squish is to drop the barrel or head, maybe the barrell was easy for them to do b8ut why you needed to re cut the ports to the specs you did .

Sounds to me like a 214 with a Dt1 PP piston is correct hieght

[JC]

Yes, that solves it. Rods are diff lengths, so Zedder listing of same rod in all those bikes must be wrong.

214 rod w DT1 PP piston should be right Freaky (w'out moodification). By the way, DT1F engine # is DT2/3 engine.

Suz TS250 piston is 6mm taller from gudgeaon to top than DT1 PP piston, so that could be used w 311 rod & a couple of extra base gaskets as an alternative to decking the barrell when using 311 rod.

[firko]

I have a DT2MX crank, presumably with 214 rod in original DT1 early number cases and a twin ring piston port piston. Am I reading correctly that this setup shouldn't work? I'm not about to pull the engine apart to check but I'm 99.9% certain that's what I've got and the engine runs fine and has for twenty years. The slight doubt I have is that Gary Treadwell modified the barrel and Peter Reynolds the head and I can't remember the details.

[vandy010]

the way i read your situaton mark is that you have the "long" 214 rod with a "short" DT1 piston.
as opposed to a "short" 311 rod "long" DT{reed}piston.
it's early but i think i need beer!

[Freakshow]

yes to all that, its obviously the combination of rod and reed that can make a problem.  but 214 and PP as you say JC seems like thats how it was.

[JC]

Yes Mark, I think yr combination should be right too.