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Discussion Starter #1
Hi, probably sounds a mad thing to do on an old 04 250 rfs but I like a project. I have a tidy 250 rfs and a 2T 250 exc. I used the 2T for timecard and H&H but as that's on the decline and the Ryedale was called off this year so I took to green laning again. To get a bit more go out of the rfs I was thinking of making it 350 with an 89mm piston.
Can the RFS cylinder be over bored to except a dry liner or does it remove the aluminium into the water jacket and become a wet liner ?
Any recommendations of an engineering firm in Yorkshire welcome, had stuff done at Drakes in the past.
Cheers Si.
 

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Buy a good used 450exc cylinder and machine it down to the appropriate deck height. It’s what ktm did when they made a 350 kit. It may be worth asking one of the more established dealers if a 350 kit is still available, it’s unlikely but worth asking.
 

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Discussion Starter #5
Buy a good used 450exc cylinder and machine it down to the appropriate deck height. It’s what ktm did when they made a 350 kit. It may be worth asking one of the more established dealers if a 350 kit is still available, it’s unlikely but worth asking.
I wondered how KTM did it, tried finding picture of the 350 cylinder which is like hen's teeth. I have a spare 75mm 250 cylinder to play about with so if I could find the liner specs it might work out a cheaper option to bore and sleeve the 250.
cheers.
 

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Some info here from our 350 kitted bikes. 350 kit info
I ended up modifying a 450 barrel but you need to make sure you machine the oilways and locator lug sockets to match the 250 barrel. I probably have the details here somewhere if you go that way, if I recall it's some off the top and some off the bottom of the barrel not just one end.
The 350 kit is worth the hassle as it makes for a lighter feeling bike than the 400/450/525 with closer gearing although that closer gearing does restrict your road speed a lot.
 

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Fit an iron wet liner. That's the way the larger bikes go out to 540, 560, 613 etc without using the large bore KTM parts. The piston you need is a standard 450 EXC piston
 

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Discussion Starter #9
Some info here from our 350 kitted bikes. 350 kit info
I ended up modifying a 450 barrel but you need to make sure you machine the oilways and locator lug sockets to match the 250 barrel. I probably have the details here somewhere if you go that way, if I recall it's some off the top and some off the bottom of the barrel not just one end.
The 350 kit is worth the hassle as it makes for a lighter feeling bike than the 400/450/525 with closer gearing although that closer gearing does restrict your road speed a lot.
Thanks for the good info. As I have a 250 barrel to mess with I will go that route for noe as it might be cheaper option.
 

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Discussion Starter #10
Fit an iron wet liner. That's the way the larger bikes go out to 540, 560, 613 etc without using the large bore KTM parts. The piston you need is a standard 450 EXC piston
Seen a couple of places offing wet liners which looks more machining for O rings but a better job. To get a dry liner in looked tight. I was looking at this liner as a possible dry liner fit but there would be not much of it left. Ill get a couple of quotes. Might need to get a piston to send off with it rather than just boring to 89mm. Thanks for the advice.
 

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Best off boring it and seeing if it stays dry.
 

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Discussion Starter #13
Best off boring it and seeing if it stays dry.
Had an email back from HT Howard a place down south that will give it a go but if it goes into the water jacket its toast. If they can get a liner in then I will buy a piston and bore the liner to fit. Dont want to shell out £160 on the piston just yet. Do you know it anyone has tried an 80mm piston from a excf bore kit ? Depending on pin size and if it clears the valves might be an option but only a small gain.
 

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I'm fairly sure the water jacket in the 250 barrel is smaller than the 350 piston so you'll definitely go in to it.
This is the detail of my modified 250 barrel, the 450 barrel has smaller water jacket.
"89.4 dia x 99mm liner in what appears to be a 250 barrel with large water ways.
Custom JE piston no 522417 flat top"

Both times we've had steel liners in the 350 kits the bikes suffered from overheating when the going got hard/slow, whereas the nicasil lines ones didn't. I do wonder if you need a slightly bigger bore/clearance to use the 450 piston in a steel bore.

I paid £90 to have a 450 barrel modified plus re-nicasil cost. Second hand 450 barrels are cheap enough.
 

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Discussion Starter #15
I'm fairly sure the water jacket in the 250 barrel is smaller than the 350 piston so you'll definitely go in to it.
This is the detail of my modified 250 barrel, the 450 barrel has smaller water jacket.
"89.4 dia x 99mm liner in what appears to be a 250 barrel with large water ways.
Custom JE piston no 522417 flat top"

Both times we've had steel liners in the 350 kits the bikes suffered from overheating when the going got hard/slow, whereas the nicasil lines ones didn't. I do wonder if you need a slightly bigger bore/clearance to use the 450 piston in a steel bore.

I paid £90 to have a 450 barrel modified plus re-nicasil cost. Second hand 450 barrels are cheap enough.
thanks for the advice, your right about the 250 barrel, it's a non starter really. the 450 is sounding the way to go. The 250 barrel is 78mm in height, I read the 450 is 8mm taller, did you take about 4mm of each end of the 450 barrel ? Who did the work engineering for you ?
34485
 

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34487
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Pictures are a standard 250 barrel and a modified 450 barrel, I had the machine shop copy a genuine KTM 350 kit barrel but I guess you could get then to copy the 250 dimensions. YOU DO NEED TO GET THE DOWEL LOCATORS MACHINED TO DEPTH AS WELL.
I'll ask the person who did my machining if they will consider doing more but it was a bit of a favour.
 

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Discussion Starter #18
PS I'm in south Devon as is the machinist.
appreciated the pictures. its a nice job. I will definitely start sourcing a 450 barrel. if the machinist is willing i could always post it down from yorkshire once i get hold of one. if not no worries, the fact its possible is a start. does it need to be a 450 exc or would a 400 exc do. both say 89mm but must be different length cylinder.
34494
 

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Stolen from one of KevinYorks past posts.
Deck heights
"450SX/SMR are 69mm
540SXS and 560SMR are 76mm
250 EXC is 78mm
400/450/520/525 EXC 400/520/525SX and 525 SMR are all 84mm"

That would suggest the 400 is the same barrel as a 450 but it's a different part number.
 
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