OzVMX Forum
Clubroom => Tech Talk => Topic started by: melx on April 27, 2015, 01:57:31 pm
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Hi all, I have a 75 YZ250B race bike on a standard bore that needs new piston kit, these don't seem to exist anymore, I have heard some bikes have a 5mm spacer fitted below the cylinder fitted and then run a Yamaha TY250 piston? I can not find any prior comments here on this. Can anyone give me a link or comment if this is a good solution? Is it as simple as adding the spacer or are other issues then requiring adjustments for? At least those pistons 74-75??) seem to be available for easy purchase.
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I would mesure the piston and see if there is anything that fits(you may need a spacer below the cylinder as you have mentioned). Here you find some piston dimensions http://www.mitaka.co.uk/CATALOGUE/PISTON_DIMS_ALL.htm Wiseco,Wossner etc catalogue should also have piston dimensions. I think that you need a dealers catalogue. Not sure why I can not find the catalogue online. Good luck.
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Mel - both Speed and Sport and NWVCP list various pistons - or alternatives - for the 250. Speed and Sport listed more choices.
But be prepared to pay as they appear expensive.
Hope that helps.
regards
Rossco
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Heres one: US eBay auction ending tomorrow.
http://www.ebay.com/itm/NOS-Yamaha-Piston-STD-1974-1975-YZ250-YZ360-432-11631-01-98-/121628690611?hash=item1c51a288b3&vxp=mtr
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here in nz they were called mx250b in "75 & yz250c in "76 [just making sure we're talking the same thing [not earlier chrome bore DT1 shape motor yz monoshock], 509 pistons are still on fleabay 200981048071 or 380978576775 for a couple of starting points. Don't think you said what oversize [or i missed it] but anyway--hope this helps. The original yamy piston was a good strong one. Cheers, OTH.
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couple more. not much use without rings though.
ebay 181098736414
http://www.cmsnl.com/mx250b-1975_model8540/piston-std-3rd-os_5091163701/#.VT29jcZcP5M
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check the part numbers first as the 360 and 250 pistons are different and not just stroke variance.
MX 250b is a different bike but some of the pistons maybe compatible - they are between the 360's however have different piston cutaways/windows. I think this is what North West will be listing. Drop them an email - they are good to work with.
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Thanks, I will try those references, I seen some advertise std pistons that list both 1975 250 and 360 YZ but obvious they are different bores. Seems on Yamaha parts they share the same part number which confuses. The ebay ones I got answers on were actually 360 pistons.
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The trouble is with the numbers used the parts numbers are the same except the first 3 digits which are the model . Parts ziller lists start on the 4th mumber .
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except - I have just noticed after Mels comment - NWVCP list 431 and 432 under both 250 and 360 - makes life interesting.
But as mentioned - write to them - they have been good to me in the past.
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Thanks for that tip, it all makes sense now to me on part numbers now, I found https://www.motorcyclespareparts.eu/yamaha-parts/yamaha-model-prefix-motorcycles-1970-1994.aspx which lists the first part numbers to give the model. It shows :
431 is YZ250A
432 is YZ360A
483 is YZ250A
483 is YZ250B
484 is YZ360A
484 is YZ360B
using this reference the piston advertised now on ebay as std 250B piston has a part that starts with a 432 number so is in fact a 360 piston.
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Wossner list the same piston for early YZ250 MX250 DT250 TY250 and IT250 they go in .5mm sizes up to 3 mm over. The one you need is 8179D050. PM if you need anymore info.
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One question is it still a chrome bore . unless the bore has been re sleaved with a cast one you are in for a new cylinder or re chrome .
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Hi Melx,
I am not 100% sure but the YZ250B chrome bore barrels used the same piston as an A model 431-11631, I think the later in production B's model came with a bore able liner.
I think those piston were 431-11635 to 431-11638.
Be aware of the piston pin centre to piston crown height using pistons from 76 up era. I think the A model has a different piston crown height from the piston pin centre than the later B model 1975 MX250 & YZ250C model.
I converted my YZ250B to take the easier to find 509 piston MX250B, using a different rod than the YZ250B stock rod, the rod kit was from a later model YZ250 that pressed straight in No barrel spacers required.
I think the std YZ250B rod is 130mm centre to centre, using a later model piston you will have to spacer the barrel up from memory.
There is two types of head gasket for both the 250 & 360, a circle ring type and the other one with four 8mm stud holes and four head nut holes, not sure were in the production run these changes were phased in.
The 432 coded piston are 80mm Up for the 360cc motor, the Yamaha parts books of the era last something in translation, lots of errors in the books.
Regards YZB 500
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pic of the bike would be good please just to be sure of the correct model. just google imaging yz250b 1975 it shows the DT1 shape motor [trick yz] i rode one for a year back then & has a chrome bore. Then googles mx250b 1975 & shows the next generation heavier [solid & reliable] different shape next generation motor which i had one of 'new'. This [cast bore] tales the 509 piston--boreable cylinder. Trick DT1 shape "75 has a chrome bore [originaly] so have to be careful with ring type [fkn stupid 'me' finished of a pretty stuffed DT1GYT kit cylinder by using wrong ring--wasn't happy lol. If your chrome bore has--had a liner fitted then maybe the 509 piston kit would work [think it would]. Thought the pin to top measurements were the same on all the reed valve 70mm yamy pistons but maybe not. Also be careful on ebay as they often say things like [fits 250 & 360] which it obviously doesn't [70mm & 80mm]. My rant for the day. Cheers.
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just closing the google images of the yz250b & saw a pic of the Harley mx bike with the front forks on the back--'what 'were' they thinking [plough a paddock lol].
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The Wossners that I mentioned earlier have a pin height of 33mm, that is from the centre of the pin to the top outside edge of the piston. The DT1 piston had a 28mm pin height which it seems the YZ A&B share. So without a rod change or 5mm spacer you are stuck with genuine os pistons assuming your bore is cast iron. The LA sleeve catalouge list a sleeve for YZ250 1973-75 and it has them as originaly chrome bores.
If you still have a chrome bore and go for a taller piston you will have to use cast iron rings or the chrome will be destroyed.
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i know there's a difference in height between piston port DT1 & reed valve from DT2--on [& YZ] but thought all reed ones stayed the same. Think i've dumped all my old pistons bugger it--don't ya just hate that. Cheers.
ps, got a pair of 250a/b crank wheels on trademe plus a [not too healthy] right side crankcase--think it's still up there!!.
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have you got a link for the yz stuff on tradme or the wording
cheers
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Hi, trademe auction number. Thanks. 879709939
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http://www.trademe.co.nz/motors/motorbikes/parts-for-sale/engine-components/auction-879709939.htm