Opinion on rebuilding KTM engine

[Mario]

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!

 

[augustiron]

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

 

[Junkie]

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.

 

[OaklandF4i]

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. :thumbup

 

[Butch]

Ah, just buy an Alta. I hear they are having a big liquidation sale.

Sorry. Couldn’t resist.

 

[Mario]

Thanks for the feedback, I'll give it a try to rebuild myself. :thumbup

 

[OaklandF4i]

Thanks for the feedback, I'll give it a try to rebuild myself. :thumbup

Their shop manual is really well written and easy to follow other than calling bolts screws in the translation. :x

The toughest part of the KTM top end job will be installing the piston oil scraper and rings. :laughing 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. :afm199

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 :thumbup

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. :wtf

 

[Mario]

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

 

[Mario]

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 ...

 

[OaklandF4i]

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. :laughing

 

[Mario]

That's some great feedback! :thumbup

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 :ride

 

[augustiron]

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.

 

[afm199]

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.

 

[Mario]

Engine Dynamics it is! Once the rain season starts, the engine comes off and to Petaluma. Thanks all for the feedback! :thumbup

 

[anytwowilldo]

Ah, just buy an Alta. I hear they are having a big liquidation sale.

Sorry. Couldn’t resist.

:rofl:rofl:rofl

 

[Mario]

Decided to pull the trigger on this and began the process this weekend.

Step one: get the engine out... and while at it, strip down to an clean chassis to service the entire bike

5.5hrs of work condensed in 2 min video


youtu.be/p6K-a9sje3A

 

[OaklandF4i]

Decided to pull the trigger on this and began the process this weekend.

Step one: get the engine out... and while at it, strip down to an clean chassis to service the entire bike

5.5hrs of work condensed in 2 min video


youtu.be/p6K-a9sje3A

:thumbup Looking forward to following the progress!

 

[Mario]

Well... there is no going back now! It was actually pretty easy. A bit stressing to get the cylinder out, but the rest was very easy :cool

I did another leak test, this time I measured less than 5%. I'm a bit puzzled about this because I measured 10%+ last year. It's been sitting for months now, so I wonder if sitting made some stuff seal better...

On to my shitty analysis, the cylinder still looks good, cross hatch is still there, a few wear marks front and rear wall of cylinder. When I did the leak test, majority of the air came from the crankcase, so I am assuming worn rings. There was some air escaping the valves, but having no experience on this, don't know if this needs to be air tight or not.

Both cylinder and head are going to the shop for the pros to tell me if I need any work done or not.

This was about 2hrs of work, mostly reading the manual and labeling all parts in ziploc bags.


youtu.be/jIPZtylBK_8

 

[OaklandF4i]

Nice job! Nothing like the confidence a project like this builds. :thumbup

Can you feel those visible marks on the on the front and rear of the cylinder with your finger nails? If not, I'd be tempted to run it again. But I rebuild my top ends frequently with fresh pistons and try to get as many hours as possible out of cylinders. Replating isnt cheap. Measuring the bore and looking for ovalling of the cylinder will tell for sure... and Edco will do that.

If you want to button it up and forget it again for a couple of years.... thats different.

 

[Mario]

Can't feel any of those wear marks. But being the first engine I tear apart, I'll leave the judgement to EDco. Having it apart, I'm guessing piston and rings are mandatory. Don't know if they'll recommend something with the head... I'll take it up there this coming weekend.

Thanks for the feedback btw :thumbup