Lets take a brief look at our KTM 300SX TBI cylinder during a top end replacement We discuss some things to look for regarding carbon and build ups, plating, honing and general ring gap tips for your two stroke
I'm not expert engine builder, but i rebuild a handful every year for myself, family & friends. My #1 piece of advice for anyone doing a topend is checking ring end gap. I haven't seen one come out oh the box in spec in a long time, not sure if it manufacturing short comings, or if the suppliers simply want to build the spec to the tight dimension, but I've had to file rings on the last dozen top ends ive had my hands on, both 2 & 4 strokes.
Thank you for your time and very detailed explanations!! You are a great contributor to our sport! Answered several of my questions in this video and also in your replies. You commented on proper break in, could you give us your method? Sad to see the trolls, keep up the good content!
What fuel would you recommend running? Been hearing a lot of people say you need to run race fuel, but the manual recommends pump, heard some run a mix of pump and race. I’ve been running pump 93 and it’s been running great so far. Thanks!
If the ball hone doesn’t do anything then 1: why does it look like it does? 2: why do they make them and why does everybody use them? And 3: if the cylinder looks like the one in the video are you just leaving it?
4:08 I installed a brand new top end and very similar but much less pronounced mark like that appeared. I saw it while checking the top of the piston with an endoscope some time ago. Pulled out the exhaust and reached with my finger and I couldn't feel it so I left it as is. Weird thing is I checked yesterday if it got worse but to my surprise it just completely disappeared. I didn't know it was a common thing though.
I've experienced the same thing. Did a top end, then took it back apart because the powervalve wasn't working, and those marks were there. Put it back together and did a top end 30 hours later and no more marks.
I have 60 hours of motocross on my 300sx. I noticed it looks like I have a polished area half way down the cylinder about the size of a nickel. I think i may need a diamond hone if there is no cross hatch left. What piston to bore clearance for running an A vs B size piston? How much bigger is a B?
Nice video. What's your thought on piston sizing for all the guys doing it at home without a reliable way to measure the cylinder? I know everyone says "after X hours, slap in a size B". X might be 50 or 100 but doesn't really matter. I don't agree with this philosophy. I've run original size A pistons on cylinders with 300+ hours on them. I did put a size B piston on my 23 300xc because it was the only piston I could find last fall. I've just been careful to let it get up to temp gradually. but I wish I could have kept size A in it. I'll probably go back to an A next time...
I mostly run "A" in two stroke stuff even when bore worn some. If it's a slow speed trail guy then now - but hard mx or hard use the extra clearance is nice.
With your time with the tbi, have you experienced the random bog some get? I suffered through 3 months of trying to get rid of it on my 2023 300xc. Usually when it was hot after running it hard, just sometimes you twist the throttle and nothing, then it goes away. Ultimately it bogged/cut out on a small jump and injured me bad. It’s gone and now I’m on a yz250 2024. There are some others with my issue and some injuries too on the big ktm facebook groups. My opinion is the mapping gets messed up after its hot and constant on/off throttle use like on a track. Some kind of glitch maybe..but very dangerous for those that jump the tbi’s.
Thanks for this. Picking up my bike friiday. Was doubting with all the negative talk about the piston shattering. Great info. Got a question tho. Should I stick with the recommended motorex 2t @ 60/1 or is there something better. I'm an ex.motocross racer mostly riding track in northern ontario canada.
depends on the gray spot - scotch brite doesnt do anything to the plating if used by hand - so you can try it. But if you have worn through the plating to the casting - it wont help you
@@HPRaceDevelopment Ok. I saw that gray spot when I was changing the piston and decided to just do nothing about it. Now I have ridden like 25 hours on it and its working good so I was just wondering if I should do something about it when I change my piston next time. I was able to see some cross hatching there but that gray spot took most of it away on that one spot.
I put Apex head on yz250x at 21 hours, piston top had 0 darkness, 0 carbon. Looked like new piston and felt oily. Same with inside of head, no darkness, no carbon.
What about the 2024 KTM65 my nephew got one of the first ones in and it only ran for two hours then the engine failed. It has been repaired but it took two tries to correct the problem with certified mechanics doing the work? So could you tell us anything you know about the problems with the 65 motor? Those light transfer marks in the cylinder are gone with some scotch brite and then a ball hone. You have to really wash the cylinder of any motor after doing that and there will be no residue left to ruin your motor and the rings will seal just fine. This works on all two stroke motors whether it be motorcycles or chainsaws and the motors run great!
My feelings and years of trying various methods and dyno testing remain - ball hone is so you feel good about yourself when you do work. its functionality is basically worthless. As to the 65 im told they have some cyl casting issues or powervalve fitment issues or both.
@@HPRaceDevelopment Yeh that’s exactly what was wrong with the bike both of those areas. The bike was supposed to go to the #1 plate rider but he’s not confident and going to use his ‘23 Husqvarna instead! They wee supposed to have taken care of those from the ‘23 model but I guess not? 🤔
Looks like it - but it's just a discoloration - the bottoms have some sort of coating from the OEM - not sure what it is, looks like an anti corrossion coating
Every since I've started using AMS oil in my 2 stroke, I've never had the coating rub off like that. Just something that I've noticed. It could be due to something else.
I actually read on wiscos website that there rings are supposed be used at tighter spec than oem, (unless i read wrong). The rings for my bike were tighter than oem and still put the piston on and has been running for about 15 hours now?
@@adriangalvez5104 Jake said we should listen to what the manufacturer says my reply was to his comment - but for everyone. Doing so would assume wiseco actually tests their product…which they dont. We do…and come up with specs that actually work for what we see.
We think people chasing this miss the boat. It does a tiny bit but not much - and without ecu tuning it's hard to move it far enough to make a difference you can really like. I've yet to see a situation where tightening it up adds power - people keep telling me this is what it does but I keep testing it otherwise.
how long do most motors last? I want to know when to do engine rebuilds and its really hard to tell on the internet. for two strokes with mx riding, wot, high revs, not insane but quite hard running usually, how long will a top end go before the piston is either too worn or the material has fatigued to the point of danger? also for the bottom i guess a good general rule of thumb is that the bottom end (including conrod) should last twice as long as the top so you generally do your bottom end every second engine service right? is it good practice to replace the rod too in case it is suffering from fatigue and lost a lot of its structure integrity? since you are a performance shop i was hoping that you guys could give me a straight answer unlike the rest of the internet which probably doesn't even know what a piston looks like. if possible do you guys also know the hours for 4 strokes and good engine rebuild practice for them too, like how often to replace the piston and rod and stuff like that? and how do these service intervals change for moderate riding such as trail and calm riding were you don't push the motor hard much? sorry for the spam of questions.
With 2-strokes it depends of the engine size, how much oil in the premix and what oil is used, how hard you ride it etc. With 4-strokes it also depends of the engine size, how hard you ride it and what oil you use and how often it will be changed. Also if you are using a forged piston or cast piston top end service hours can change. I´m sorry but there isn´t easy answers for these questions. @HPRaceDevelopment can continue if needed....
@@HPRaceDevelopment strange. I thought the service manuals were extremely low hours compared t reality for liability reasons. I have plenty of friends of people I know at the shop who have up to 140 hours on a stock ktm 2 stroke for hard racing. I think that is overboard and i would do 80 ish hours for hard racing but its hard to tell what the best service interval should be.
@@garbageman3992 when we work with top level amatuers or pros, the service manual is usually not often enough. if you do the engineering math on high cycle fatigue you will find it lines up with the oem manuals.
@@HPRaceDevelopment yeah the pistons especially need to be replaced frequently as they are aluminum and thus they fatigue even under low loads, they fatigue quickly too. but what are your thoughts on conrods? since rods are steel and have a fatigue limit where they will theoretically last a lifetime if they are never pushed beyond it. do rods get pushed beyond the limit even at low rpm's or can a motor that is never pushed hard theoretically live on one conrod for like 1k hours? if pushed hard like mx racing does the rod fatigue at a similar rate to the piston, should it be replaced as often as the piston?
I'd never buy an injected bike unless I also found a way to make the jetting richer. They come from the factory jetted lean to pass emissions tests. Look at how the top of this piston has no clean wash. That means it got too hot due to not enough fuel droplets contacting it to cool it down, leaving areas near the transfers and boost port clean of burned oil. Also look at how much oil burned onto the sides of it. Not good. It needs a more heat resistant oil that is made to work with injectors. But at least it's not too too lean which is known by burnt oil under the piston crown.
This must be a race bike, ive pulled pistons out of 250's and 300's with double the hours and it didn't look anywhere near as bad as that, but they were medium speed single track riders.
What wear? The coating coming off in 90 percent of this piston is just the coating coming off as I described. It appears to have gone through one or 2 too short of warm ups followed by hard track action where we see minor polishing on both sides. Trail bikes dont see as many cycles per hour as mx bikes. Hours a shitty measurement of usage
@@HPRaceDevelopment I didn't say anything about wear. The burned oil at the ports and on the crown is more than I see on these mellow ridden trail bikes.
I have a Brand new 23 TE 300 He ‘ haven’t ran it yet ‘ it’s only been started once. I want to change the Oem Piston out for Wiseco or Wossner . Wonder which you’d Recommend?
The wiseco doesn't have a pin height difference? Really? They gave that up? Rule of thumb measure everything. We were making 65-72hp on gas 20 years ago from 250cc trx and cr..what is that 300 making?
@@bigrich7026 In race applications single ring is good. This is a vet rider who wants 50 hour changes - dual ring is fine. On my dyno id pay good money to see a 65 hp 250cc on gas. This 300 as it sits on our dyno is 50-52
KTM as well as other new generation 2 strokes have to be running lean compared to old school types to get past CARB and euro standards. I raced off road/desert in open class for 20+ yrs. We never thought 2 strokes would make a come back. Are you using a forged piston?
These bikes don't pass any emissions standards in sx or xc form - and I can garuntee you they don't pass any standard on the TBI. Premixed fuel, and two strokes will NEVER pass. Additionally the concept that "lean" is what makes an engine pass emissions is flat wrong. If you dig into what they are testing for and the actual standards - you will see that going leaner isn't the answer to passing emissions, as leaner burning mixtures produe NOx. two strokes by their nature have a large amount of unburnt fuel that migrates through the engine into the exhaust and out - which dramatically increases their "hydrocarbon" ouput in testing. There is no way around it with the scavenging system a two stroke has - and unless you direct inject it after the exhaust port closes - it's a significant amount. To put it flatly - a two stroke will never pass without some exceptions to the rules. The good news is - they currently dont' have to pass - and the usa/carb standards are incredibly lax - but only applied to non race models which ktm is smartly re naming line ups to get around. @@Profabdesigns
40 hours...that piston seems clapped out... I thought the bike could go 80 to 90 hours before top end.. But maybe if you do don't clean the air filter...? I guess I am old dog and don't abuse
@@fredmitchel1236 Ill never understand how people assume an hour number. The math is simple. Whats the applied force, whats the applied cycles, whats the material choice, whats its high cycle fatigue life - convert hours to cycles and you have a change interval This piston looks fine, is dimensionally same as new and shows no “signs” of wear. You change pistons in race use because its the stress crack fatigue you cant see that causes exploded parts, not the visual wear you can see
The elephant in the room here is there was zero measurable wear on that piston. Don't change it if it an't broke. Maybe put rings in it but I bet there would be zero compression increase once the new piston and rings are broken in. 300's can do 200-300 hours on the factory piston. Most of the wear in happening in the barrel. If you want that fresh factory feel - replace the piston and barrel. Otherwise leave well alone - spend time getting jetting perfectly right for the day and you will get more out of the bike.
You might want to do a little research into high cycle fatigue and how it relates to aluminum. Visible wear is NOT the primary concern on parts changing. Its the wear you cant see that results in shattered pistons, snapped connecting rods, etc. Do yourself a favor and google high cycle fatigue - then dig deeper into cast aluminums, then into cycle counts of a piston on a 2t at 6000 and 8000 rpm averages per hour - you might find you never want to push a piston too many hours ever again
If you want to go that route, get yourself a good crack detection spray kit to use every time you open engine and change the rings around old cast pistons. And then post the results. There are a lot of people interested. :)
