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Discussion Starter #1 (Edited)
Thought I'd make mention of my dealing with my top end rebuild to help others.
Basically, I bought my 2011 350SXF with 154hours on the factory piston. The first owner raced it, the second owner trail rode it and I added a few extra hours 160 before deciding to replace the piston.
I did this job with the engine in the frame however I've been told its better to remove the engine! Being a mechanic by trade (I don't like removing something unless its a must do!) Firstly the tank needs removing, the rear sub frame can be pulled backwards and other items removed. Even getting the HT plug lead out is tight... The spark plug is thin wall 14mm tube socket. The head is tricky to split from the barrel as the dowels had rusted slightly sticking the two together (take care trying to split the two apart you don't want to damage the gasket surface between the two). When the head is removed the barrel comes off with plenty of room. The head is tricky to get back on with limited room and while trying to place the cam chain in position. Points to help, work with a clean bike, brush everything down off the frame before removing the rocker cover. Then check the valve clearance (this can be done with the head on a work bench and cams fitted) My valve clearance was just in spec with the range advised by KTM. I did get shims and get the clearance right in the middle of the range One inlet was tight on min the exhaust valves both at max gap.
The piston I removed was a factory KTM piston and in pretty good condition, the Teflon pads had worn down and it was due for a piston... Really 3-4 times overdue by KTM standards but I'd say it could have possibly done another 30-40 hours at a gamble of course.... the barrel was also really tidy, slight top ring mark from carbon build up. After researching the controversial topic of preparing 4 stroke barrels, Diamond hone must be used on Nikisil vs sand paper or normal hone with a good wash out (I decided to go with what mates had done & the local KTM workshop incl the local Honda dealer also, they use 400 grit sand paper and form a light 45 degree cross hatch pattern just to remove the glaze & carbon then clean and wash the cylinder x 3 times with warm soapy water) As the KTM 350 piston is $550 NZD I opted for an after market one still forged (Meteor) at $220 NZD and gaskets after market also $80 NZD. TAKE CARE! THE 350SXF HAS 2 SIZES OF PISTONS TO PICK FROM. I won't discuss doing the entire job but more what I found interesting or challenging? The timing up the cams/crank etc is easy if the KTM method is used. The chain tensioner is easy to remove and install.
After completing the piston change I changed the oil and started the bike. I rode it down the road. I noticed it blew a little smoke out the exhaust when I gave it a rev (it never used to smoke at all). I did some research and found that it was advised to not use synthetic oils when running in the top end. I drained the oil, changed the filter and cleaned out the steel factory gauze filter (the steel filter had a few very small specs of steel) This confirms the importance of changing the oil filter after a rebuild! I should also highlight (DON'T GRIND TITANIUM VALVES) my mate learnt this the hard way when the engine shop lapped his valves in... It didn't last long at all! I used good car engine oil as mentioned by a tuning specialist. I went for a 10min ride with some load on the engine but it still blew a little smoke under load. Finally, I took the bike for a trail ride and when it had warmed up I gave it a good blast through the gears up and down a slight incline... not at full throttle, but I certainly gave it a good blast. After this trail ride the bike has stopped smoking and is going very well. This smoke was a little scary as I didn't want to do the job twice and I'd never had a car smoke after a rebuild. I will be switching back to synthetic oil soon. I know this link would have been used before on this forum, I guess this is conformation on the attached method working for my bike, I couldn't say if using synthetic oil was my problem with getting the rings to bed in or because it hadn't been given a good enough blast! however, after changing the oil to mineral and going on a trail ride, This smoking problem has gone. (http://www.mototuneusa.com/break_in_secrets.htm)
(ADDED SECTION)
Update on the bike smoking... After my bike had been taken for a good ride it stopped smoking right. Sadly, the next time I started the bike from cold it smoked again under the same conditions (bike warmed up at idle, blip throttle and smoke appears). Remember the bike never smoked before this job. I thought about all possible reasons behind this smoking problem and came to this conclusion... Leaving the head and valve stem seals alone, after replacing a piston is probably not the best idea! The compression has been raised and therefore the vacuum on the inlet side increased, given the valve guides had some wear (I found this wear after removing the head again and removing the valves) slight wear between the guide and valve. The engine had now created enough vacuum with the new piston to draw oil down past the old valve stem seals and into the engine. When the engine had become hot the smoke stopped! as this clearance had closed up. I will update the overall outcome soon, I expect it will be sorted. It appeared ok after a ride around the block straight after completing the job.

Note: I still believe the points made by Moto tune USA (regarding top end run in) valid, I have seen more evidence regarding using mineral oil and not idling the bike for prolonged periods straight after rebuild. Apart from the annoyance of doing this top end job twice and having to replace the head and barrel gasket in order to replace the valve stem seals it gave me a chance to inspect the new piston and barrel after 2 hours of riding. Everything looked great.
 

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Great write up.馃憤


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Discussion Starter #4
Update on the bike smoking... After my bike had been taken for a good ride it stopped smoking right. Sadly, The next time I started the bike from cold it smoked again under the same conditions (bike warmed up at idle, blip throttle and smoke appears). Remember the bike never smoked before this job. I thought about all possible reasons behind this smoking problem and came to this conclusion... Leaving the head and valve stem seals alone, after replacing a piston is probably not the best idea! The compression has been raised and therefore the vacuum on the inlet side increased, given the valve guides had some wear (I found this wear after removing the head again and removing the valves) slight wear between the guide and valve. The engine had now created enough vacuum with the new piston to draw oil down past the old valve stem seals and into the engine. When the engine had gotten hot after a few laps the smoke stopped as this clearance had closed up. I will update the overall outcome soon, I expect it will be fine now as it appeared ok after a ride around the block straight after completing the job.
 

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I've seen the same results after a piston ring
replacement on a VW diesel.
I guess a single cylinder bike would even have more
vacuum on the intake stroke.


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Discussion Starter #6
Thought I'd better update the post.

The bike started smoking more after a 2 hour trail ride, I started the bike the following day and it smoked even worse.
Take 3. I removed the head and barrel again, this time the HT plug connector snapped on removal. $100 NZD through KTM.

Turns out the barrel was past the KTM recommended wear limit of 1.9 thou, my barrel around 2 thou. so after sending the barrel to be diamond tip honed, they can't do it.

I also wanted to replace the inlet valve guides as the inlet valves felt pretty loose in the guide with the spring removed.(possible cause of smoke also).

The short and least painful solution given I had already bought a new piston and 2 gasket sets is a cylinder re-coat of my barrel.

This is a breakdown of costs.

New valve guides 2x $58 NZD each guide all 4 guides are the same. I'm just doing inlets. (KTM has a special tool to insert into guides to check wear limits, if the tool fits, replace guides). Given you can wriggle the valve we are replacing them!

Re-coat the barrel $395 NZD
Install guides and cut valve seats inlets only $160 NZD plus guide cost.
Another set of rings $50 NZD
Another gasket set $70 NZD
Freighting parts $50 NZD
NGK blue Spark plug connector $100
Keep in mind I have already spent $400 just to replace the piston and then trying to fix a smoking bike.

All in all this will have cost me around $1300 while doing all assembly and dissemble myself.

Big bore 365cc kit option. I was quoted $1400 NZD however I would then be wasting a $250 piston already purchased.

Without any bitter tone, Buying a 350sxf at 160hrs might not be the best idea, I had already spent $1200 making this bike the way I like it. What I could have bought with what this owes me now... an almost new bike perhaps. That said I really like the 350sxf now.

My bike rode like a dream until I decided to do maintenance such as replacing a piston. I have since found other people that have had the some type of problem after a piston change and have just put up with the smoke, that said smoke is telling you something and any potential buyer won't buy a bike that smokes.

Buyer beware, used bikes can start to cost you substantially more than you intended to spend especially high performance 4 strokes.
 

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This is a very nice write up.
Thanks for sharing.

I will be going through the same process myself in the next coming weeks.
I plan to pull the head and barrel before ordering my piston and hopefully the barrel will be within spec to use either the A or B piston. If not then off to the re-coaters and likely back to an new replacement Vertex "A" piston.
While the head is off I will send it to an expert engine builder let them change the valve seals and re-cut if necessary (I'm no mechanic and disassembly/reassembly and piston/rings install will be more than enough work for me lol). Also main reason I wanted the head cleaned up is exactly for what you described. I am worried putting a new piston/rings with all that compression that the bike will gain that it's not had for quite some time will/might vacuum push oil through the valves (take away my oil burning through the oil ring problem, and just replace with oil burning problem through the valves ironically).

Glad to hear it runs well now.
Mine luckily doesn't smoke at idle, rather only on the hard throttle pulls pointing towards the oil ring; but if I put the new piston and don't freshen the head maybe will have some valve leaking at idle after the rebuild.

Will post more as my project progresses.
Cheers!
 
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