OzVMX Forum
Clubroom => Tech Talk => Topic started by: Jeff Allison on August 24, 2011, 05:09:30 pm
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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
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Justa thought- Measure the stroke, this may give you a clue....
Brett
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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.
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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?
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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
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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
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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 ?
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The old crank is banded. The new one is not.
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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
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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.
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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
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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.
(http://i1035.photobucket.com/albums/a438/vandy010/ozvmx%20stuff/rodspistons.jpg)
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I think it is something like 80-89 YZ250 rods fit the 360 - have one in mine.
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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
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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.
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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 ???
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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?
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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.
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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 ;)
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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. ;)
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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 ;).
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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
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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)
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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.
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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 (http://i1035.photobucket.com/albums/a438/vandy010/ozvmx%20stuff/rodspistons.jpg)
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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.
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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
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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. ;)
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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
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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
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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.
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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.
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I need a mogadon after reading that Freaky......... ;D
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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)
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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.
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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 :o
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The piston is marked 230 with the W on the other side
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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 :o
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).
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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.
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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!!!!