1975 YZ250b crank ?

[Jeff Allison]

Hello, 

I have a 1975 YZ250b that I am working on. It was a barn find and is 99% complete (even has the silencer).  The bike was left outside and it rusted the crank (could have been reused probably).

Sad I know, whats really sad is that it had been in a basement until this guy needed the space

Heres where my problem is The local Yamaha shop had half of the crank (I found the other half on ebay).  Had new rod main bearings and crank installed.

Put motor together and it is hitting the top of the head.   Could this of been a YZ360 crank they sold me ? if so is there any solutions to fix it, wihout tearing it down and using the old crank ?

Thanks
Jeff

[oldyzman]

Justa thought- Measure the stroke, this may give you a clue....
Brett

[Freakshow]

the problem you describe there is the piston hitting the head - has nothing to do with the crank size,

the gen YZ pistons are 5mm lower, you are obviously using a MX250 or later yZ piston or wiseco, therefor you need to make up a 5mm spacer to stick undert he barrel.  thats what they all did after the chrome bore YZ piston ran out.

[Davey Crocket]

Are you still running the chrome barrell? or has someone put a sleeve in it in it's past life?...is it a geniune 248 rod?

[Jeff Allison]

The rod that was in it was a 214, and it was bored 060 over with a wiseco piston

Jeff

Looking at the cranks side by side the rod in the new crank is level with the crank and the old crank looks to be just below it on the big end.

when the jug is put on the piston comes up to the 1st ring almost half way.

So I should get a 5 mm spacer and this will solve my problem ? 

thanks
Jeff

[farmpro]

It sounds like the new crank halves are from a RT or YZ 360 if the rod is a 214.Especially if you look at the crank halves from the end and the pin is further out to the edge on the new crank.

Bill Davis            Texas Vintage Racing Club

[74yza]

74 and 75 yz250 cranks have a 214 rod and are fly cut then banded w/ a steel band (band is spot welded around edge)  They are very easy to recognize  Is yours that way ?     

[Jeff Allison]

The old crank is banded.  The new one is not.

[Jeff Allison]

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

Will it work with the spacer as mentioned in an earlier post ? 

Whats my best option ?

thanks
Jeff

[farmpro]

275 prefix is for RT1,which is a 360 and won't work at all.I wouldn't try to use a spacer on it.

[Jeff Allison]

Looks like I may have to save up some $$ and have it tore back apart.     Like a little kid at Christmas anticipating the toy he wanted and opening the gift to find something that was totally useless to him.   Ahhhhhhh.

So the spacer will not work ?

thanks
Jeff

[vandy010]

Jeff,
this has been posted before and it's not really about 250 v's 360 cranks but more about the earlier 214 rods v's the later DT series rods and thier differences.
just another thing to look out for.
we did discuss a lot of this some time back in the Yamaha section, so a search through there may reveal a link to it.
my RT1 yammy uses a rod kit from a {82~ish} YZ250 as it's the same as a 214.
my local Yamaha shop/guru sorted it for me using a pro-x kit.

[Rossvickicampbell]

I think it is something like 80-89 YZ250 rods fit the 360 - have one in mine.

[JC]

Jeff,

While there is a problem w diff length rods & diff height pistons, as stated above, which was thoroughly discussed & resolved in a previous thread, it seems to me from the evidence given that you have a 360 crank w the longer stroke, as you initially wondered.

As also stated above,  275 is a 360 prefix, & more importantly, the crank part # for RT-3  360 listed on yamahaparts.com is the part # you gave. It is also listed as an optional 75 YZ360 part #.

250 cranks are 64mm stroke, whereas 360 cranks are 70mm stroke

The simple thing to do to be conclusive is measure the stroke of the piston in the barrell, remembering to account for the distance the piston goes above the top of the cylinder at TDC

[firko]

Would it be possible for Jeff to get out of his predicament by fitting a spacer plate under the barrel? A couple of folks I know did the 360/250 stroker conversion back in the 90's, the result being a very nippy open classer of around 320cc from memory.