Hello & welcome

Not a clue......but I am sure someone else will have

 

thanks for the welcome

 

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.

 

couldn't you just bolt in a 400 or 450 lump? I think the mounts are the same

 

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.

 

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.

 

A

 

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.

 

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 ?

 

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.

 

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.

 

I think a 400 barrel would be ok as it's 85mm ish high so plenty to play with.

 

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.

 

Thanks again. Wonder if its skirt length, think I read somewhere the pistons are different lengths. Does give me more options for finding a barrel but I will try get a 450 first as I would be using the 450 piston which Kevin posted the part number for. All good fun.

 

This has been bugging me today so I looked up the 450 barrel on a KTM fiche and it's actually the same part number as the 400 59530005000. If you google the part number it only comes up as a 400 I guess this is because the 400 predates the 450 in EXC guise by a couple of years.

 

I did the same and bought a 400 barrel today. How did the guy remove material from the base of the cylinder, was it in a lathe or a milling process ?

 

He definitely took some off the top and and bottom to match the genuine 350 barrel I have.
The picture below confirms this and gives you a good idea of the spigot length, you can also see the machining line at about 4mm. You can also see roughly how much has come off the top, I don't think it's critical other than making sure the crank/conrod misses the spigot and the total height is correct.
Looking at the machining marks I'd say it was all milled but it has since been etched by the nicosil process so hard to tell. I'll see if I can't find my calipers and give you more accurate dimensions.
You do need to make sure the dowels are machined to match the cases and head.

 

He definitely took some off the top and and bottom to match the genuine 350 barrel I have.
The picture below confirms this and gives you a good idea of the spigot length, you can also see the machining line at about 4mm. You can also see roughly how much has come off the top, I don't think it's critical other than making sure the crank/conrod misses the spigot and the total height is correct.
Looking at the machining marks I'd say it was all milled but it has since been etched by the nicosil process so hard to tell. I'll see if I can't find my calipers and give you more accurate dimensions.
You do need to make sure the dowels are machined to match the cases and head.

View attachment 34512 View attachment 34511

very useful pictures thanks, gives me a good idea of what I need to do. there's a few precision engineers about so as you say I can take both in and use the 250 as a template. They all seem to advertise CNC machining. I can see this barrel been worth more than the bike Ha Ha.

 

You need to copy the 250/350 barrel otherwise the camchain tensioner will be in the wrong place.

 

Good point. When the 400 barrel turns up I can do a bit of measuring and head scratching. As they say measure twice cut once, what could possibly go wrong.

 

Slightly more accurate measurements but battery had failed, I'm sure nothing is taken off the spigot so I make that 3mm off each end to make a height of 76mm and put the chain tensioner in the correct place.

 

thanks for the pics again. I have been measuring the 400 against the 250 and the cam chain tensioner position measures within a nats hair the same on both.my plan is to just take 6mm off the bottom. just need to work out if I need anything off the spigot. depends on crankcase depth and the rod clearing.

 

Seems reasonable, mine measures 69mm

 

KTM must have a bit of tolerance on the cam chain tensioner position when machining.

 

And then a kit turns up.
350 kit

 

Thanks for the heads up, I had seen it on ebay, looks like just the box to me with the removed 75mm 250 bits in it. I have sourced most of the parts now, gaskets, jets, needle and a cam chain link. The OBEVP needle was a bit scarce. Couple of engineering places near me advertise CNC machining so plan is to take the barrel in next week and then get the piston and have the cylinder re-plated. Using the bike every other weekend at the moment so no rush really, it's a bit steady between lanes but right enough for now.

 

Yes, I think you're right, I'd not looked at the pictures.