1975 YZ250b crank ?

[vandy010]

Would it be possible for Jeff to get out of his predicament by fitting a spacer plate under the barrel?

if as suspected Jeff has a 360 crank in a 250 donk then a spacer would help the piston when at the top of its stroke but when the piston is at the bottom of its now longer {360cc} stroke, the piston would go well below the exhaust port hieght with all the port timings being wierd
i think

[firko]

Vandy, I'd like to come back with a dazzling piece of theory to disprove your point but being a Philistine on these things can only come back with..."I've seen it work". Can anyone who's done the conversion fill us in on whatever it is that needs to be done?

[vandy010]

i do remember you talking about it before Firko and i'd be keen to know more about it as well.
i guess this whole rod kit/piston/crank/250/360 thing for these motors just needs a bit of sorting/knowledge before we hand over our cash to avoid the situation Jeff now faces.
but having said that, my engine rebuild was pretty straight forward with no hassles, so here's hoping Jeff gets it sorted with a minimum of fuss.

[firko]

I just spoke to Frank Stanborough and he reckons the port timing will remain the same as the top and bottom of the stroke remains the same POSITION because of the 6mm spacer making up the difference between the 64mm 250 and 70mm 360 strokes. He theorised that the transfer porting may be a bit different but that shouldn't have much effect.

I apologise for the slight thread hijack but I'm merely throwing another possible fix into the equation

[Freakshow]

I must be going mental but im sure the 3rd post down gave you the answer already.

Fit a spacer, thats all that is required. 

5mm is the differance between the stock piston and the wiseco.  add a gasket either side and you will be at a squish of 50 thou, if you want more add the 6mm spacer.  its up to you what compression you want.

BUT THE ANSWER IS ADD A SPACER.  it will cost you $3 or alunium and 20 mins of your time.

[firko]

5mm is the differance between the stock piston and the wiseco.

Heres the part number from the new crank.  275-11422-00-00.

275 prefix is for RT1,which is a 360

Freaky, while I agree with you that a spacer is one way to fix Jeffs problem, I don't quite get your reasoning. The piston doesn't seem to be the problem here, it's the stroke of the crank, possibly having mistakenly fitted a 360 70mm stroke crank instead of the 64mm stroke 250 item. The reason for the needed 6mm spacer is to make up the 6 mm difference in stroke, not because of any piston difference .

[Freakshow]

Reason is regardless of what hes doing in his crank he is still 5mm out at the top and were it passes the ports. 

if he can find a geniune YZ piston he would instantly be 5mm lower , IIRC
or use a Piston port Dt1 piston and drill his own reed valve cut aways in it.
or use a spacer.
 
All cheap and easy weekend fixes to this problem.

all 3 answers will give the same result - he wont top the head out anymore.  ( he never said i have a bad running bike or my crank runs against the cases, your all creating new issue for him to worry about that are not required unless he sights them also as problems, which so far he didnt.

the problem IS in the piston he is currenlty using ( wiseco) to the standard YZ barrell height, are completely differant and will never go together unless you do fork about and stroke it back in 5mm somewhere but why would you go to all that effort when there a 4 easy fixes   

From what i get the guy is looking for a simple and cheap solution.  regardless of what the probelm stems from.  doing 1 of those 3 things will solve all his problems cheaply.

if the crank he has turns fine in that motor then there is no float issue and he is fine,  if  his question was how do i drop the rod/crank  stroke he would just use the 311 rod kit, again problem solved.

Guy just wants to get the bike to run....... lets not make it too hard for the poor fella

[Freakshow]

The 80's YZ rod your referring to vandy is 130mm it wont fix his issue here he would need the 125mm rod kit. 

Also remember that kits runs a 20mm or what ever it is top small end so there are more must buys ( YZ 465 small ned bearing) and kinda extends the must have bits you gotta buy to work.

fit a spacer :O)

[firko]

the problem IS in the piston he is currenlty using ( wiseco) to the standard YZ barrell height, are completely differant and will never go together unless you do fork about and stroke it back in 5mm somewhere but why would you go to all that effort when there a 4 easy fixes

I don't understand your point Freaky? Jeff's already said he's using a 214 and Wiseco piston. What I don't get is you saying that Wiseco pistons have a 5mm higher deck height than the stockers. I've got Yamaha and Wiseco (and Rocky for that matter) pistons for my under construction DT2MX engine and they're exactly the same dimensions. If you read back over the entire thread you'll see that his "mistake" is using a 360 crank, nothing to do with pistons.

Reason is regardless of what hes doing in his crank he is still 5mm out at the top and were it passes the ports.

It's 6mm...the difference between the 64mm 250 crank and 70mm 360 version.

the problem IS in the piston he is currenlty using ( wiseco) to the standard YZ barrell height, are completely differant and will never go together unless you do fork about and stroke it back in 5mm somewhere but why would you go to all that effort when there a 4 easy fixes   

I think you've misunderstood my reply. The only work involved is to make a 6mm spacer plate. No rod change, no piston change no anything change.

If Jeff wants to keep the bike as a 250 instead of the 298cc he gets with the 360 crank he's going to need a new set of 64mm stroke flywheels for his new 214 rod and big end bearing and do it all over again . OR.. I can give you an already dismantled crank for nothing Jeff but I suspect you're in America so it'd be expensive to ship. I'm pretty sure the DT1/DT2 crank is pretty much the same as the YZ-B (except for the steel band so it'd be pretty easy to find a usable one on eBay or Marks Swapmeet for under 50 bucks.

[Freakshow]

yer im just saying he needs a spacer.  but if he uses another piston like the PP dt1 hell also pull back the 5mm, just the piston and rod he has will always add up to the same problem. he can change one of them and also fix the issue.  I Have the same thing 2 gaskets and a spacer, it works fine.  check the ign though to be sure you have the correct set out space and tapered section for whatever your using, im using PVl on all of mine and thay ll fit in, bit im not sure if that left crank has any model changes in it that may effect standard set off of the tiggers.

Firko Are your pistons DT2/3, mx 250/360 and so on your referring too

What im saying is the wiseco is infact taller (as is the mxa and those listed above) and the rocky your talking about, when compared to the other options.  go check the DT1 PP piston  im sure there 5mm shorter as is the pin height is on the genuine YZA pistons (IIRC)

there is a height differant in the yamaha pistons, what im saying is no matter how you skin it that set up of piston and rod he is using is always 5mm higher - or ITS ALL AT THE HIGHEST YOU CAN GO........  IFFFF you use the lower pistons or the other rod you can go under that by 5mm off the bat and add another gasket to pick up your squish.

if you read back over the entire thread you can see it for yourself as shown here by vandy

[evo550]

I have tried the spacer under the barrell trick on a cr480 once, the problem I encountered had to do with non alignment of exhaust and carb to their old position, both the exhaust and inlet port where raised about 10mm, which then meant the center frame down tube fouled the exhaust port....about $500 down the tube. I'm not sure of how much different the new and old heights are on the YZ, but with center exhaust ports it's something to be mindfull of.

[Jeff Allison]

Thanks for all of your help everyone.  I will probably go with the spacer to start and once I can get some extra $$ I will probably put it back stock. 

I really appreciate your help with this.  Once I get it together Ill post some pics.  I really kind of like the idea of a stroker motor though.

If anyone ever needs anything picked up in the St. Louis, MO. area and held for a while let me know,  I am sure the wife wont notice something else stuffed into the garage. 

thanks again
Jeff

[firko]

 Freaky, maybe it's my meds kicking in but I don't seem to be able to get my point across so I'll leave it to someone who can explain it better than me. You talk about keeping it simple but your plan requires a new piston, cutting port windows in it and a 5mm plate. Mine, requires just a plate..simple.

[Lozza]

5mm is the differance between the stock piston and the wiseco.

Heres the part number from the new crank.  275-11422-00-00.

275 prefix is for RT1,which is a 360

Freaky, while I agree with you that a spacer is one way to fix Jeffs problem, I don't quite get your reasoning. The piston doesn't seem to be the problem here, it's the stroke of the crank, possibly having mistakenly fitted a 360 70mm stroke crank instead of the 64mm stroke 250 item. The reason for the needed 6mm spacer is to make up the 6 mm difference in stroke, not because of any piston difference .

Close,but a 6mm increase in stroke is 3mm at the top and 3mm at the bottom of the stroke. Rod length does affect port timing but not by that much. I would find a piston with a lower comp height

[jimg1au]

i bought a rod kit years ago for my cr250m it came with a hot rod kit and a spacer plate and 2 gaskets.the motor has been fully recod and checked out by a race mech.he changed the pin on the piston as the barrell is ported like swiss cheese and machined the head as piston still travelled too high needed a bigger spacer.i have run this it starts ok and runs well.have not raced it yet.the most problem was lining up the exhurst hangers ad the pipe was 4 mm higher than std.
just check the piston travel with the spacer in.the stroke defines were the porting comes in you could have a 10mm bigger rod and spacer it will still work.
stroke and dia change cc nothing else.
jim