1975 YZ250b crank ?

[firko]

Rod length does affect port timing but not by that much

The rod hasn't been changed it's still the same 214 rod used in both the 250 and 360.

[Freakshow]

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.

all im trying to tell you is add a spacer or use the lower piston.  You just do one which ever is easier for him. not both your getting confused or not reading properly.

And yes for your info a DT1 PP or a gen YZA is 5-6mm shorter so its an easy fix changing that than making a spacer if he doenst have the tools or know how.

 Forget what your talking about piston wise the wiseco he has is higher im telling you its the same as the yam piston you have there and they ARE NOT THE SAME as the others.

the other ideas i threw out were just that, so he didnt have to do your other suggestion and change the entire crank......

I wish you'd pass the meds onto me cause im not sure why you dont get what i have said, even when i say it 4 differant ways.

[Davey Crocket]

I need a mogadon after reading that Freaky.........

[JC]

There's a bit of tunnel vision going on here.

If Jeff's problem is just piston height, then Freaky's obviously correct - all he neads is a 5mm spacer under the barrell.

But if he's got a 360 crank, its more involved than that. And potentially catasrophic if he tries to run it w the 5mm spacer.  (see below)


Mark, yes its quite possible to run the longer stroke crank in the 250 engine (as you know), but you have to get the port timing right & the correct sized spacer in the right place, otherwise it'll be a dog.

(The RT1/2/3 & YZ360A/B cranks are the same dia & width as the DT1/2/3 & YZ250A/B cranks. They fit straight in, its just a diff offset of the crank pin which still has clearance on the 110mm dia flywheels. The 360 bigend won't hit the crankcases as someone suggested above)

But its not a 6mm spacer thats required for the 6mm longer stroke. The thickness required is usually half the extra stroke because, as Lozza says the extra 6mm stroke goes 3mm deeper into the cases & 3mm above the top of the barrell. 

But where does the spacer go?  If you put it under the barrell so the top of the piston is at the top of the barrell at TDC you'll usually get very radical timings for yr exhaust & transfers & very mild timing on yr inlet (on a piston port engine  - a reed valve compensates the inlet to some degree, depending on how its set up).  Not a good engine.

Yr rings at BDC will also be 6mm lower in the barrell which often means at least the bottom one will be exposed into the top of the inlet port, & if thats where yr ring ends are (& they usually are on yams of the era), yr asking for instant catastrophe via munched rings etc


As it turns out, if you leave the barrell where it is (not raised) the port timing is often in the right ball-park.  What's then required is that an extra 3mm thick head gasket/spacer be used & that can only be done if the top of the piston is 3mm above the top ring. That spacer needs to be very carefully located on the barrell (perhaps pinned) so it doesn't move off centre & interfere w the piston as it approaches TDC. One still needs to check that at BDC the bottom ring (if there is one) hasn't opened into the top of the inlet port as it's still going 3mm further down into the barrell.

One would also need to check squish clearance & C.R. etc., & that at TDC the bottom of the piston skirt still covers the bottom of the exhaust port w about 2mm overlap. (ie piston skirt will need to be about 70mm on exh side)

(More later, as wifey needs the computer)

[JC]

Just checked a few things in the shed & Wiseco catalogues.

Wiseco did list a piston exclusive to the YZ250A/B  (#389, preceded by #224) so Jeff may well have the right height piston.

But if its Wiseco #234, which is listed for the DT250s, MX250A/B, YZ250s after the 'C', & IT's etc, it is 5mm taller.

My YZb engine came w a wiseco piston & 5mm spacer & I'm fairly sure it was #234 (can't find it at present to check)

The correct deck ht for the YZa/b is 19mm (from top of gudgeon to top edge of piston) if using the early rod (#214)
The wrong piston for YZa/b is 24mm deck ht (unless the shorter later rod is also used)
Easy enough to check. That's the 1st thing I'd be doing.

[vmx42]

As it turns out, if you leave the barrell where it is (not raised) the port timing is often in the right ball-park.  What's then required is that an extra 3mm thick head gasket/spacer be used & that can only be done if the top of the piston is 3mm above the top ring. That spacer needs to be very carefully located on the barrell (perhaps pinned) so it doesn't move off centre & interfere w the piston as it approaches TDC. One still needs to check that at BDC the bottom ring (if there is one) hasn't opened into the top of the inlet port as it's still going 3mm further down into the barrell.

This solution would be fraught with peril on so many levels it is almost beyond description. Truly scary… IMHO 

[Jeff Allison]

The piston is marked 230 with the W on the other side

[JC]

Jeff (Allison),

#230 is listed simply as "YZ250 forged" but its also listed as preceding #389 (& the latter is listed exclusively for 74-75 YZ250), but I wouldn't just trust the catalogue listing alone.

I'd be measuring the deck ht of the piston (top of gudgeon to top edge of piston) to double check. 
If its 19mm, the piston is NOT yr problem. Simple.

Next step for me would simply be to measure the stroke.

If its 70mm, w an 060 piston yr capacity would be 281cc, which is sort of no-mans-land - ineligible for 250 class & somewhat uncompetitive for open class

As it turns out, if you leave the barrell where it is (not raised) the port timing is often in the right ball-park.  What's then required is that an extra 3mm thick head gasket/spacer be used & that can only be done if the top of the piston is 3mm above the top ring. That spacer needs to be very carefully located on the barrell (perhaps pinned) so it doesn't move off centre & interfere w the piston as it approaches TDC. One still needs to check that at BDC the bottom ring (if there is one) hasn't opened into the top of the inlet port as it's still going 3mm further down into the barrell.

This solution would be fraught with peril on so many levels it is almost beyond description. Truly scary… IMHO 

Its also kinda counter-intuitive, but lateral thinking aint such a bad thing sometimes. IMHO either way is fraught w difficulty & potential peril. (Be much better w new liner to suit) But it can be done, as Mark attests. Nothing much is insurmountable.

All I'm pointing out is this way the numbers work out, whereas the other way they don't.

Case in point:

Measuring down from the top of the barrell, my YZ250b barrel is:

Ex 35.5mm  (ie opens ~ 90deg ATDC on 64mm stroke crank)
Tr 52mm      (ie opens ~ 122deg ATDC on 64mm stroke crank)
In 102mm    (opening timing depends on location of holes in piston skirt for reed valve engine)

Raise the barrell 3mm using 70mm stroke crank, port hts are still the same (top of barrel still coincides w TDC) but piston goes 6mm lower at BDC so Ex & Trans open earlier & Inlet opens later

Exh now opens about 80deg ATDC
& transfers about 107deg ATDC.

Thats well into road race territory for ex (TR750 Suz are 80deg, TD/TZ250 yams are 79deg).
And beyond for transfers (TR750  112deg, TZ250  111deg)

Leave the barrell where it is & use 3mm spacer on top of barrell, & effective port hts become

EX  38.5mm
Tr   55mm
In  105mm

Which on a 70mm stroke crank means
Ex opens  ~ 88deg ATDC
Tr opens  ~ 117deg ATDC

Which is very close to RM250B specs, which by comparison are:

EX  38.5mm  (opens 88deg ATDC)
Tr   56mm     (opens 119 ATDC)
In 102mm     (opens  74deg BTDC, but they also have the extra reed direct into the crankcases)


Of course, there are other spacer combinations one could try, eg 1.5mm below the barrell & 1.5mm above (tho that would still be too radical on the exh & trans IMHO).

 

[Davey Crocket]

Those A and B motors where very powerfull and have a big top end hit, I think there was a fare bit of Yamaha road race technology in there, you would be lucky to find an engine that would out drag one in a straight line, 36 and 38mm carbs make em go even faster, when I spoke to Gary Jones at last years Nat's, he said they tamed them down with higher gearing, I have at home an intake manifold and reed cage (NOS) for a 73 OW250 (Torsten Hallman), the dia is quiet small and stuffed very much like a V-force....made a lot more torque.....they knew there shit way back then....just made us wait a lot longer for it to flow on down.

[Rossvickicampbell]

apparently a bit of extra flywheel helps tame them also.  Most amazed - I have a 360 B which goes very well - my mate has a 360B which also goes well but whereas mine just pulls from down low and keeps going (hard) all the time his is more like a 125 with a real hard hitting power band when it does come on the pipe - mine seems linear whereas his gets to a point and then rockets!!!!