Bike Maintenance

Hey Roy sounds like you havent done this before? I thought you did the top end yourself last time. I would suggest you get a copy of the service manual. If noone gives you one pm white rocket he can email you one ( im on holidays). Didnt the spiderman dude on here do a full write up on a 300 rebuild? Search for it. Head uses 2 orings. Base uses one gasket normally. The plastic sleeve they come in show the size. If not use a mic or digi verniers. You choose which size base gasket according to the X dimension (manual). Dont risk guessing or running the same as last time. There is also a technique to measuring X to make sure its right. If it were i i would pull down all the PV parts and oven clean them. This will take some time and you are best to make a guide out of a plastic/wood/metal ruler (or similar) to set Z dimension properly or piston hits PV.
The marks on the bore sound like cold seize marks. You would do well to warm your bike up longer in future. If you cant feel the marks with you fingernail dont worry about them. Get
Scotchbrite pad and run around the bore at the same 45deg angle as the crosshatching. If you cant find SB pads use sandpaper 320 in long strokes. The coating is tough, you wont do damage unless you go nuts in one spot. Wash and blow dry. Always use a new gudgeon pin and roller bearing + clips. I wrote up a tech tip on getting the clips on. Check the carbon reeds while its all apart somone said they chip and break easy. Run a new BR8ES not a 7 KTM are mad. 0.6mm gap from memory. 95 preffered over 98.

Here is the link to Menaces 300 rebuild thread.

Click here

Hey Roy, the pistonwash (clean area) on the crown is very small, but I digress that is not what we are here for.

First thing to check I’d ask you to make is those suspicious looking marks at 5 and 7 o’clock in the bore (intake side less of a worry.. but never can be too careful) can you feel them with the fingernail test and eyes closed?

Sorry forgot to mention that to measure your piston on the bottom of the skirt then measure your bore to get your overall piston clearence. Even the no1 piston you installed last time may have to bigger clearance. This is a good place to start.
Cheers
Robert

Hi Roy
The wear marks are from the small bridges between ports wearing out first as they have less surface area ,put a piston in my old ktm 200,it looked the same,sold it,and it went for a little while,then died.would recomend looking at bore recoat for piece of mind.Try putting a straight edgefrom top to bottom and see if there is any wear on that area.
Think from memory my 200 had rubber orings on the pipe were it fits inside barrel,but still needed hi temp silicon to seal it

Looks like some progress here.
I didnt look at your pics Roy but whatever the marks are dont sweat it. Yes the bridges do cop the most wear but unless you seized it really really bad (you didnt) i would move on. Good point about ring gap. KTM does not specify the min ring gap! See if one of these knowledgable peers here can tell you the min gap fot 2st 300cc ring. I cant remember off the top of my head. Wiseco writes it on the install guide. To file the ring gap you draw the file through the gap from the centre outward NOT up and down. Measure the ring 10mm+ from top lip of bore. Read that manual off Rocket first before we repeat it word for word, then we can help further. Its good stuff. One tip with X is your straight edge/rule can be positioned across the top of bore, use the rear wheel to rock the piston on its TDC. Use less gasket until piston stops ta tap tapping on the rule. My last rebuild required .4 + .2 = 0.6
As for the exh flange i only use copper gasket maker there to form a gasket. Follow its instruction. Supershit Auto had a special on that range last week. Theres black blue and copper. Copper is for highest temp. I also clean out the exp chamber “port” and cure a thin layer of this stuff to help the exp chamber orings seal and so it doesnt flog out so easily.