Ian Howell

Member
Jul 9, 2002
1
0
Hi Eric
I'm a experienced machinist in SOUTH AFRICA and would like to know the specs[head,ports,crankshaft] for converting the 2001 YZ250 to a 300.
 

EricGorr

Super Power AssClown
Aug 24, 2000
708
1
The best combination of bore and stroke that I've found that comes the closest to your goal is a 69.5mm bore and a 76mm stroke for 288cc. The main problem with the YZ and RM is the cylinder head stud location and relatively small diameter of the inner o-ring. Relocating the o-ring is possible by welding the top of the cylinder and re-cutting the groove to a larger diameter in order to get it farther from the bore, but thats very difficult.
I use the Wiseco 69.5mm from a TRX 4-wheeler part # 526MO6950 then weld up the outer edges of the rear transfers to accomodate the ring centering pins. Then I have Crank Works off-set the crank pin holes by 2mm and press in a Hot Rod kit. The crankcases will also have to be machined to accomodate the connecting rod and a spacer plate 1mm with 2 standard gaskets must be used on the cylinder base. This set-up produces and engine with a strong pulling mid-range and good top end. I change the porting to suit the application and that may include a thinner spacer plate for more low end. This set-up is the most reliable that I've found. The 74mm piston that I designed when I worked at Maax Power can be adapted but its not very reliable. Some companies are using that piston with stroking to get it out to 327cc but theres only so much you can do with the original design. The last thing you want is a 250 that you have to rebuild once a month.
Good luck on your project, Eric
 

bclapham

LIFETIME SPONSOR
Nov 5, 2001
4,340
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found anotherr good old thread

any ideas on the cost of this stroking process and cases machining work?

how does the 273cc engine work if you use the 69.5mm piston with the standard stroke?

thanks
 

EricGorr

Super Power AssClown
Aug 24, 2000
708
1
The 69.5mm bore uses a Wiseco 526MO6950 and welding and narrowing of the rear transfers needs to be done. That limits the top end and targets the low end.
The 2002 cylinder casting is thinner around the cylinder walls and I've seen 2 of the 69.5s develop water leaks into the bore from casting porosities. So I've stopped offering the 275 kit on the YZ.
Same thing goes for the RM 250 cylinder, the manufacturers are making their castings lean and its tough to do big bores, between the narrow margin between the bore and the inner O-ring groove to the problems with the thin castings.
I think we'll see the use of aluminum sleeves in the future for big bores. LA Sleeve is making cast sleeves out of hyper-eutectic materials and Advanced Sleeve turns sleeves out of forgings from Wiseco. Of course these sleeves must be plated to be used in an engine.
 

bclapham

LIFETIME SPONSOR
Nov 5, 2001
4,340
0
it sounds like the 68.5mm piston is a good way of getting more life and power from an old yz250 cylinder and still keeping the power characteristics of a great MX engine without making it into a woods bike. i think what we are seeing is that we rapidly approach the limits to how big we can take 2-strokes before the costs goes up exponentially! i suppose $1000 spent on a stroker would be better spent as a deposit on a CRF! its hard to justify that sort of money when you can buy a wiseco crank and rod ready to go for less than $200

thanks again for your input eric.
 


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