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Old 10-19-2018, 09:42 AM   #1
Mario
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Opinion on rebuilding KTM engine

My KTM 500 sumo is due for a rebuild, running it at the track has not been nice to the engine and I'm getting 10% leak and hard to start. ~120hrs 2,200mi

I am considering two options:

1) tear down and rebuild myself: i would send the head/valves and cylinder out to a shop, I'd just slap everything together with a new OEM piston, rings, bearings, etc
2) send complete engine to a local shop and ride dirty bikes while I wait

I am good at wrenching and reading manuals but never done an engine rebuild. On the other hand, I do not know of any local shop that I can send my engine to.

Can anyone comment on 1)? If you send the head and cylinder out, do you still need special skills/tools to disassemble and assemble the engine?

Regarding 2), anyone know of a really good and reliable shop that can quote an engine rebuild? I don't need race stuff. I don't think I''ll be taking the sumo to the big tracks ever again.

Thanks!

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Old 10-19-2018, 11:57 AM   #2
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A top end is not hard to do by yourself, if you have the time, tools, patience, wrenching experience and a service manual.
Engine Dynamics in petaluma can get the cylinder and head ready to drop back in.

1) no "special" tools needed, at least not readily available ones. Just a good metric toolset.

3) locally or ship to?
Thumper racing, off the top of my head

Last edited by augustiron; 10-19-2018 at 12:00 PM..
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Old 10-19-2018, 12:45 PM   #3
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I'd look into how long the bottom ends on those tend to last.

If you think the crank is probably in good shape, it isn't hard to do a top end (especially if you have a shop cut the head for the new valves).

I'd expect that to have a nikasil cylinder and therefore you shouldn't need to send the jug out, but I could be wrong.
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Old 10-19-2018, 09:38 PM   #4
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A top end is not hard to do by yourself, if you have the time, tools, patience, wrenching experience and a service manual.
Engine Dynamics in petaluma can get the cylinder and head ready to drop back in.

1) no "special" tools needed, at least not readily available ones. Just a good metric toolset.

3) locally or ship to?
Thumper racing, off the top of my head
Pretty much this. I think Engines Only in San Jose specializes in thumper dirt bike engine builds. Never used them or anyone as I do them myself. If they dont do KTMs, I am sure he could refer you to someone if you dont want to use the dealership.

Other than the plethora of torx bolts on the engine, the KTMs are pretty straight forward. Some aspects almost easier to work on then other brands. Split and reassembled a 350 recently.

Until you open it up, it will be tough to say what you need to replace or even give and accurate quote. But your leakdown test can give you some clues. Attach it again, but pull off your valve cover, your airbox boot and filter as well as your oil fill plug on the engine. Pressurize again, but this time listen for where air is escaping. Coming out the end of your exhaust (exhaust valves) out your airbox (intake valves) out your oil filler hole (rings). Not definitive, but get you started.
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Old 10-19-2018, 10:42 PM   #5
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Ah, just buy an Alta. I hear they are having a big liquidation sale.

Sorry. Couldn’t resist.
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Old 10-21-2018, 04:22 PM   #6
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Thanks for the feedback, I'll give it a try to rebuild myself.
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Old 10-21-2018, 06:59 PM   #7
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Thanks for the feedback, I'll give it a try to rebuild myself.
Their shop manual is really well written and easy to follow other than calling bolts screws in the translation.

The toughest part of the KTM top end job will be installing the piston oil scraper and rings. Only KTM specific advise other than the manual is is use/buy a ring compressor (cheap Tusk brand one from Rockymtn is sufficient) and make sure to have some quality torx and allen sockets on hand so you dont strip them with a cheap loose fitting ones.

You will also find the KTM OEM pistons and rings are ridiculously priced compared to other bikes. Use the OEM gaskets and then get yourself a aftermarket Vertex piston and rings.

The KTMs are pretty easy to work on. Good luck

edit: and pay attention to piston orientation. The manual on the 350 we just did says the mark "A" piston top goes forward, where the on the Vertex piston it was just the opposite. Pretty easy to see if you pay attention to the valve sizes and the corresponding cut outs on the piston.
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I'm looking for 1970 or older project like a CB350 or Triumph 650. Cheap and complete. PM me if you have something - will provide it a good home.

If you think me being naked is offensive, dont look!

"You find the biggest meanest bull, chop off his balls, dangle them in front of him, then hop on his back. That should give you some reference point. Either that, or shove a shuttle rocket up your ass. Take your pick." Colin Edwards

'Cycles is a mean toy lady" Big Halsy

Last edited by OaklandF4i; 10-21-2018 at 07:04 PM..
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Old 10-21-2018, 07:23 PM   #8
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Cool, thanks!

This might be a stupid question, haven't googled this yet... what's the process to pick a piston size? have it measured, machined/re-honed to a certain diameter that fits the closest piston within tolerance?

Edit: time to educate myself, hope these two are good

Last edited by Mario; 10-21-2018 at 07:36 PM..
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Old 10-21-2018, 07:24 PM   #9
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Ah, just buy an Alta. I hear they are having a big liquidation sale.

Sorry. Couldn’t resist.
I'll keep an eye on them... those electric machines I understand more than the piston up and down thing ...
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Old 10-21-2018, 07:41 PM   #10
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Cool, thanks!

This might be a stupid question, haven't googled this yet... what's the process to pick a piston size? have it measured, machined/re-honed to a certain diameter that fits the closest piston within tolerance?
Your cylinder is plated like all modern dirtbikes, so they are never bored/machined like old steel lined cylinders. When the cylinder is damaged or worn beyond spec, it is replated to match the proper bore of the standard piston size.

You will inspect and measure the cylinder or take to a machine shop to do it for you. Some cylinders wear (ie enlarge slightly) and are still usable. Then you order one of the slightly larger pistons available, size is based on your cylinder and the specified piston to cylinder gap in the manual.

That said, I almost always use the standard piston size on my bikes and just replate when worn. If you keep track on the time/wear/condition of your motor, you should be able to get multiple top ends off the original plated cylinder. Wait too long, and you risk wearing them out like anything else.

Some brands, like Yamaha two strokes have oem matched pistons to cylinders based on the stamping like A, B, C etc on the cylinder's outside wall. Even then, I just use the standard aftermarket piston. I'm not a pro or looking for the last .001% of HP.

I try to use OEM when affordable and almost always keep to stock compression for reliability, longevity, and fuel (higher compression tends to be more finicky with knock, heat, fuels, etc.)

I'm not a professional wrench or rider, but thats what has worked for me for years. Will always defer to a Pro. Others experiences may vary.
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I'm looking for 1970 or older project like a CB350 or Triumph 650. Cheap and complete. PM me if you have something - will provide it a good home.

If you think me being naked is offensive, dont look!

"You find the biggest meanest bull, chop off his balls, dangle them in front of him, then hop on his back. That should give you some reference point. Either that, or shove a shuttle rocket up your ass. Take your pick." Colin Edwards

'Cycles is a mean toy lady" Big Halsy
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Old 10-21-2018, 08:00 PM   #11
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That's some great feedback!

I'm now leaning towards doing it myself (minus head and measuring of cylinder). The more I learn about these machines, the better I get at keeping up with them
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Old 10-21-2018, 09:30 PM   #12
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Take the head, cylinder and piston to engine Dynamics in Petaluma. They will redo the head in its entirety and check the piston and cylinder for you and advise if you need to overbore, replate, etc. Odds are a hone and rings will be all you need unless there is scoring.
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Old 10-23-2018, 06:54 AM   #13
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Take the head, cylinder and piston to engine Dynamics in Petaluma. They will redo the head in its entirety and check the piston and cylinder for you and advise if you need to overbore, replate, etc. Odds are a hone and rings will be all you need unless there is scoring.
This. And EDCO in Petaluma is a great company.
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Old 10-23-2018, 08:27 AM   #14
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Engine Dynamics it is! Once the rain season starts, the engine comes off and to Petaluma. Thanks all for the feedback!
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Old 10-23-2018, 04:27 PM   #15
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Ah, just buy an Alta. I hear they are having a big liquidation sale.

Sorry. Couldn’t resist.
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