BransdaleBeast Posted July 31, 2013 Share Posted July 31, 2013 Hello all, Sorry for the essay, I'd be interested to hear peoples thoughts and thought it'd be best to include as much as I could. My little DT has developed a knocking sound (started off quiet and gradually got louder over about 50 miles) The bike has done 16500 miles and I've put about 500 miles on her in a month. After reading the Haynes I convinced myself it would be a big-end bearing that had worn. So I took the head and barrel off with relative ease and noticed a few things; Bolts weren't evenly tightened on head and cylinder. There is only one piston ring yet there are two grooves in the piston. The dome of the piston is quite heavily carbonated and seems to have 1 or 2 fairly substantial 'pips?' that have left impressions in the head. The head gasket was just a cheepo piece of thin painted steel (white), and seems almost brand new. In the top of the cylinder head, where the carb warmer pipe meets, there are lots of small black flecks? The bearings on the small and big end of the rod feel quite good to me. There's a bit of side to side play, but up and down there is no movement as far I can tell. I'm wondering if the previous owner has changed the piston and small end bearing, but the cylinder itself has worn to the point of needing a re-bore and upsized piston? With no experience with this kind of thing I can only guess, but the cylinder looks ok to me without any serious scores or scratches. Is it normal to have only one piston ring? Can having too much clearance between piston and bore create a knocking/rattling sound? I don't really know what I'm doing so any help would be greatly appreciated. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted July 31, 2013 Moderator Share Posted July 31, 2013 Hello all, Sorry for the essay, I'd be interested to hear peoples thoughts and thought it'd be best to include as much as I could. My little DT has developed a knocking sound (started off quiet and gradually got louder over about 50 miles) The bike has done 16500 miles and I've put about 500 miles on her in a month. After reading the Haynes I convinced myself it would be a big-end bearing that had worn. So I took the head and barrel off with relative ease and noticed a few things; Bolts weren't evenly tightened on head and cylinder. There is only one piston ring yet there are two grooves in the piston. The dome of the piston is quite heavily carbonated and seems to have 1 or 2 fairly substantial 'pips?' that have left impressions in the head. The head gasket was just a cheepo piece of thin painted steel (white), and seems almost brand new. In the top of the cylinder head, where the carb warmer pipe meets, there are lots of small black flecks? The bearings on the small and big end of the rod feel quite good to me. There's a bit of side to side play, but up and down there is no movement as far I can tell. I'm wondering if the previous owner has changed the piston and small end bearing, but the cylinder itself has worn to the point of needing a re-bore and upsized piston? With no experience with this kind of thing I can only guess, but the cylinder looks ok to me without any serious scores or scratches. Is it normal to have only one piston ring? Can having too much clearance between piston and bore create a knocking/rattling sound? I don't really know what I'm doing so any help would be greatly appreciated. 1: Is it normal to have only one piston ring? 2: Can having too much clearance between piston and bore create a knocking/rattling sound? 3: I don't really know what I'm doing so any help would be greatly appreciated. 1: NO 2: YES, it's called piston slap...google it 3: Get yourself a service manual it will tell you how to check the cylinder bore and piston wear under all that carbon on the piston crown there may be a number engraved into the piston, this may indicate the current rebore size so you can buy a set of piston rings to suit...unless of course it is also badly worn and you opt for a rebore Link to comment Share on other sites More sharing options...
BransdaleBeast Posted July 31, 2013 Author Share Posted July 31, 2013 so you think the rattling is likely just piston slap from a worn bore? and would that also cause the ring to disintegrate? I'll take it all into work tomorrow and give it a proper measure on the CMM. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted July 31, 2013 Moderator Share Posted July 31, 2013 .........................Can having too much clearance between piston and bore create a knocking/rattling sound? .................... so you think the rattling is likely just piston slap from a worn bore? and would that also cause the ring to disintegrate? I'll take it all into work tomorrow and give it a proper measure on the CMM. I didnt say I thought that was the problem, I just answered a question you asked! But while youre in there check out the piston / bore wear, you'll find info in the service manual Link to comment Share on other sites More sharing options...
BransdaleBeast Posted July 31, 2013 Author Share Posted July 31, 2013 haha, sorry just a bit of a wishful thought! yeah will do, thanks. got it all out and cleaned up and sure enough, there's the remains of the ring in the head and top of the piston... think I'll be getting a new head and a re-bore professionally done. now that the piston is off I'm suspecting the top end has worn, as the bearing rocks about quite freely in there! I'll have a mate of mine who knows his stuff coming round tomorrow for a skeg, so hopefully he'll give me a better idea of what's acceptable where. Link to comment Share on other sites More sharing options...
Recommended Posts