cagimaha Posted September 16, 2009 Report Share Posted September 16, 2009 Right the saga continues... After recovering ever so slightly from the financial impact of a bent crank the joys of two stroke ownership have come back to bite me in the arse once more. The first time it went it did both pots, it had done 150 miles since the rebuild and had been fine up to that point. During the rebuild I followed the instructions in the yamaha workshop manual with reference to torquing the head down. I also re-torqued it after 100 miles. I got it back home and stripped it down then started investigating, the gasket looked like this: After a conversation with Taymar racing I purchased another head gasket (pattern again) and rebuilt it again. Taking the advice I used some high temperature silicone sealant on the gasket and torqued it down to 25ft.lbs which is higher than the yamaha reccomendation. I left it overnight for the silicone to cure. Started the bike and let it get up to 60 degrees, after it had cooled down again I re-torqued it. Then I went for a short ride (<10 miles) without going into the powerband, let it cool down and re-torqued again. Then not a fortnight after the previous failure it did it again and the breakdown man got phoned again. Also to my surprise the woman on the other end of the phone seemed adamant that her trained engineer would be able to fix this problem at the roadside even though I told her the head gasket had gone. Needless to say when the recovery van arrived he didnt attempt to fix it. After tearing it down again when I got home the gasket looked like this: I've checked that the head isn't warped, I've checked that the deck height of both cylinders is the same. I'm at a bit of a loss as to what it could be. Im going to get the NDT gear out and check theres no cracks in the head and barrels which is my last hope. If theres no cracks then I'm not sure what to look at next. The only other thing I can think might be causing it (but I'm not sure how) is that I have 2 base gaskets under each cylinder to set the squish properly. The barrels I'm using have been skimmed at some point as has the head. Also the head has some nasty detonation marks on one chamber (they were there before I got it) but I can't see how they would affect it either. If anyone can shed any light on this it'd be much appreciated. Forgot to mention the engine in question is an RD350 (4L0) motor which was unknown to me and put together with parts from various sources. It's a 250 (4L1) bottom end with the tuned 350 top end fitted. Everything else seems to be fine and when the head gasket isn't disintegrating it goes like stink. Cheers, Ben. Link to comment Share on other sites More sharing options...
Coose Posted September 16, 2009 Report Share Posted September 16, 2009 Two thoughts - are the faces of the barrels flat? I would recommend taking both the barrels and the head to someone reputable and have them skimmed again so that you know all is well. Whilst there, have the cylinder heights measured accurately before they're skimmed, just to see if they're slightly out. Just out of interest how are you measuring the deck heights? My other thought is what piston oversize are you on? If you're approaching 2mm, is there any evidence of the pistons hitting the gasket sealing ring? Again, if the pistons are on the large size it may be that the sealing ring on the gasket isn't making enough of a contact between the head and the cylinders. Assuming that both head gaskets have failed above the exhaust ports, pushing out into the waterways I wouldn't have thought that either of your cylinders are cracked, but the failure is due to one of the above. What squish clearance are you running? You don't really want to be below 0.9mm for a road bike on Unleaded, and preferably Super at that. Link to comment Share on other sites More sharing options...
si.likes.guiness Posted September 16, 2009 Report Share Posted September 16, 2009 i was thinking the same about getting them skimmed as that wear on the gaskets is not normal mate Link to comment Share on other sites More sharing options...
dougb Posted September 16, 2009 Report Share Posted September 16, 2009 i've found on my ypvs 350 and my tdr's that pattern gaskets just don't work,try an o.e one (if you can get one.....),either that or get a copper one.... Link to comment Share on other sites More sharing options...
porter_jamie Posted September 16, 2009 Report Share Posted September 16, 2009 I purchased another head gasket (pattern again) and rebuilt it again. Link to comment Share on other sites More sharing options...
cagimaha Posted September 16, 2009 Author Report Share Posted September 16, 2009 Thanks for the responses guys. Im going to check out the surfaces of the barrels in a little while with a surface plate. Ive been measuring the height of the barrels with a set of mitutyo (or however you spell it) verniers and as far as I can tell theyre both spot on. I can see about borrowing a big micrometer though if you think thats necessary. I'm on 1.5mm o/s with the pistons, there was a slight mark on one of them but that was because as the gasket had distorted it had pulled the sides in as it turned from a circle to an oval. The marks were not on the very edge of the piston crown and they were only at 2 points sort of in line with the gudgeon pin. They havent marked the metal of the piston just removed the carbon, I've wiped the crowns with a rag and you cant see it any more. The squish is set at 1.2mm, the piston does protrude from the top of the barrel at TDC though... The gasket has failed on the inlet side not the exhaust side but I don't think that makes too much difference. I think I'll have a go at making a gasket from some copper using one of the old ones as a pattern and make sure that the piston isn't going to catch the gasket. I'll also check the barrel/head mating surfaces are true. The only reason I thought to check for cracks was that if coolant was somehow getting above the piston it would increase the compression pretty dramatically and possibly blow the gasket. Ill keep you posted! Progress is gonna be a bit stunted though because I'm no longer insured on it and I'm only home for a couple of days before heading back to university. Thanks again, Ben. Link to comment Share on other sites More sharing options...
Coose Posted October 7, 2009 Report Share Posted October 7, 2009 Did you get to the bottom of this? Link to comment Share on other sites More sharing options...
cagimaha Posted October 7, 2009 Author Report Share Posted October 7, 2009 Im back at university now so progress is on hold till I go home for xmas. Ive managed to source another head to try though and I'm looking into getting some copper head gaskets made. Out of interest do any of the PB'ers out there know how thick an LC head gasket is after it has been torqued down? Or how thick to get a copper one made so as not to alter the squish/compression? Cheers, Ben. Link to comment Share on other sites More sharing options...
crash Posted January 26, 2010 Report Share Posted January 26, 2010 i had this problem with my rd lc 250,what i found out to solve it was to lightly coat new head gasket with grease or vaseline on fitting,when bike warms up it burns the gasket to head ,works every time, no leaks, i tryed every thing to fix this problem this works. Link to comment Share on other sites More sharing options...
nzknut Posted January 26, 2010 Report Share Posted January 26, 2010 can you pop "O" rings in? Link to comment Share on other sites More sharing options...
will#224 Posted January 26, 2010 Report Share Posted January 26, 2010 I'm guessing the torque pattern starts from the inner 4 bolts then moves to the outer 4 bolts? I used to build high pressure pumps and we had similar problems, they were cured by using a different torque pattern and applying full torque to the inner bolts before moving to the outer therefore spreading the gasket out. Link to comment Share on other sites More sharing options...
cagimaha Posted January 27, 2010 Author Report Share Posted January 27, 2010 Sorry I'd completely forgotten to update this one! I got a copper head gasket from this guy http://www.coppergaskets.us/ and it seems to have cured the problem. Can definitely reccomend them, arrived quickly and I think I ended up paying about £20 for two gaskets in different thicknesses including delivery from the states. As soon as I explained the problem to him he knew exactly what engine I was on about and seemed perfectly happy to offer me a money back no quibbles garuntee if it didn't work. I managed to get some ancient magic goo called wellseal which is also apparently the best stuff for sealing head gaskets and havent had a problem since. If I get a tune that I'm really happy with then I will probably go down the route of cutting o-ring grooves in the barrels but until then I'll stick with the copper gaskets as long as they hold up. As ever thanks for the help, Ben. Link to comment Share on other sites More sharing options...
stuntmonkee Posted January 27, 2010 Report Share Posted January 27, 2010 Got any pics of your Mito350 Cagimaha?? Link to comment Share on other sites More sharing options...
cagimaha Posted January 27, 2010 Author Report Share Posted January 27, 2010 Yeah there is a thread buried somewhere deep in the depths of the forum. If you do a search you should be able to dig something up. Alternatively there is a load of pics on photobucket http://s455.photobucket.com/albums/qq278/cagimaha/?start=0 I'm at uni in London for most of the year and the bike is back in Cumbria so progress is pretty slow. I need to get the wiring re-done at some point and there is still a couple of things I want to sort out with the fuelling. Link to comment Share on other sites More sharing options...
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