dogodell Posted October 7, 2009 Share Posted October 7, 2009 hey, let me start by saying that I have the skills to rebuild a car engine but have never been inside a bike engine before... with that in mind I am still working on my 75 mx400... found that it had very little compression even after I fixed the "decompression" valve cracked it open and discovered that sometime in its life it decided to eat what I think is part of one of the rings.... needless to say not a pretty sight with metal everywhere top of cylinder and piston both have more craters and pits than the moon... I pressed out the piston pin (luckily I found the little clips before I tried it) and the needle bearings in the rod just slid out and can be wiggled around easily??? Is this normal??? Second issue is that while the bearings in the cases for the crank appear to be tight (no play in the crank in any direction as far as I could tell) the rod bearing appear to be very loose... I can easily take the rod and move it around??? Once again is this normal and how much "play" should this have??? last question is... would it be possible to replace the rod bearings without breaking the cases and removing the crank??? well I lied one more question... how bad would it be to do a quick top end job and leave the rod bearings as is for a couple months of riding??? (its almost winter and I planned on a complete rebuild then anyway) thanks in advance Link to comment Share on other sites More sharing options...
dogodell Posted October 7, 2009 Author Share Posted October 7, 2009 oh yeah and if this is the wrong section for this sort of question than someone please tell me so I can put it in the right one thanks again Link to comment Share on other sites More sharing options...
Moderator Cynic Posted October 7, 2009 Moderator Share Posted October 7, 2009 hey, let me start by saying that I have the skills to rebuild a car engine but have never been inside a bike engine before... with that in mind I am still working on my 75 mx400... found that it had very little compression even after I fixed the "decompression" valve cracked it open and discovered that sometime in its life it decided to eat what I think is part of one of the rings.... needless to say not a pretty sight with metal everywhere top of cylinder and piston both have more craters and pits than the moon... I pressed out the piston pin (luckily I found the little clips before I tried it) and the needle bearings in the rod just slid out and can be wiggled around easily??? Is this normal??? Second issue is that while the bearings in the cases for the crank appear to be tight (no play in the crank in any direction as far as I could tell) the rod bearing appear to be very loose... I can easily take the rod and move it around??? Once again is this normal and how much "play" should this have??? last question is... would it be possible to replace the rod bearings without breaking the cases and removing the crank??? well I lied one more question... how bad would it be to do a quick top end job and leave the rod bearings as is for a couple months of riding??? (its almost winter and I planned on a complete rebuild then anyway) thanks in advance Hmmm, car engine and bike engine's are different but its only nuts and bolts. The little end will rattle like a kids toy on the bench the important bit is if there is any vertical movement when its in place. For the cost of em if your not sure change it. Same thing goes for the big end, some side to side is ok around 1-2mm is not unusual but there should be NO vertical play whatsoever. As for the not quite last question, no the crank and big end bearings need the cases splitting. Last question. Its a 2stroke, the high wear rate of a 2 stroke engine means that they are designed with top end rebuilds in mind. Ridden hard you wont see much more than 7-8000miles maybe more if you are gentle from a piston set. If you have been kind to the old girl with good oil and let he warm up properly and such you may even get away with just a set of rings and a hone. As to which section, here/ workshop/off road ,dual purpose sort of fits all of em but dual purpose or workshop will prob get you quicker answers. Link to comment Share on other sites More sharing options...
dogodell Posted October 7, 2009 Author Share Posted October 7, 2009 that explains a lot... instead of 1-2 mm play I have more like 1/4-1/2 inch (sorry dont know the equivalent and Im a carpenter in the US so its easier to estimate in what I know so I obviously need a connecting rod bearing which means splitting the case... ??? do I need to remove anything on the clutch side of things??? (pulled the cover off and noticed that there is no bolts from that side... or can I just remove everything from the flywheel side??? also is there anything that I should watch out for transmission wise when I break the cases open i.e. is anything gonna fly out at me that I am going to have to put back later or will it all stay in place so I can at least get a look at it and know how the thing works and fits together??? thanks again Link to comment Share on other sites More sharing options...
Moderator Cynic Posted October 8, 2009 Moderator Share Posted October 8, 2009 that explains a lot... instead of 1-2 mm play I have more like 1/4-1/2 inch (sorry dont know the equivalent and Im a carpenter in the US so its easier to estimate in what I know so I obviously need a connecting rod bearing which means splitting the case... ??? do I need to remove anything on the clutch side of things??? (pulled the cover off and noticed that there is no bolts from that side... or can I just remove everything from the flywheel side??? also is there anything that I should watch out for transmission wise when I break the cases open i.e. is anything gonna fly out at me that I am going to have to put back later or will it all stay in place so I can at least get a look at it and know how the thing works and fits together??? thanks again 1/2 to 1/4 inch, oh dear. The primary instruction for this type of problem....... Beg, borrow or steal a manual. There is no way round it a crank that badly buggerd will have wreaked havoc with seals, bearings etc. Read and digest then strip and inspect. There is nothing complicated inside aside from the crank will need to be rebuilt profesionally. But if you supply the parts thats not expensive. You dont really need any special tools bar a flywheel puller so long as you have a decent bench and thats not vital and a reasonable tool kit you will be fine. One thing is for certain though, it aint gonna run as it is. Link to comment Share on other sites More sharing options...
dogodell Posted October 8, 2009 Author Share Posted October 8, 2009 lol alright that's about what I was figuring thanks for the help Link to comment Share on other sites More sharing options...
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