Kt100guy85,
There is lots of good information in this thread posted by many experienced racers. I thought I would aggregate a bunch of stuff that I want to add, so I’ll stuff this all in one message instead of responding to other messages.
First off, regarding the sleeves that go onto a crankshaft where the main bearing sits, those usually work best if installed on a brand new crankshaft. In other words, in my experience they don't fit very well if installed on a crank that has worn main bearing journals. That said, I've rebuilt Yamaha's that had a fairly worn crankshaft, with new main bearings, and they've run very well. That's not to say that worn crankshafts are faster, because they're not. However, it's not as big a problem as some people think it is. Obviously if the crank is worn a few thousandths or more, then it's definitely an issue. If it’s just light or moderate wear, I wouldn’t worry about it at this point.
Note that the main bearing required for a standard crank is a 6304, and the bearing for a sleeved crank is a 6205. The OD is 52mm on both bearings, and the width is 15mm on both bearings. The 6304 has a 20mm bore, and the 6205 has a 25mm bore.
Regarding the connecting rod and where it should be located, I personally prefer bottom located connecting rods. The rods are the same though as far as “widths”, and can be located either top or bottom depending on the bearing and washer set up. As others have said do not locate them both on the top AND the bottom, it's one or the other, but the rod MUST be located on one end. If you're setting up the connecting rod to be located on the bottom I personally shoot for about .012”-.015” side clearance (measured with shims between one side of the rod and the thrust washer next to it). Be aware that how you set up the crank (i.e. the width) will determine the end play that the crank has in the case when assembled. My target on end play in the case has typically been .010”. I will tolerate a touch more, but that is the target. As you can imagine, the “correct” way to build the engine is to build the crank first, then measure over the shoulders on the crank (that the main bearings sit against) to obtain that dimension. It might be something along the lines of 1.805” or so (only an example, but probably will be close to that if you set the rod side clearance at the number mentioned above). If you were to target .010” end play for the crank in the case, then you would need a case that measures 1.815” between the main bearings on this example.
There are a number of ways to get this dimension: If the case measures *narrower* then that, you can simply run a gasket of the correct thickness to obtain your target dimension. If the case measures *wider* then that, there are two approaches: one is to run a shim *behind* one or both of the main bearings. These may be available from some of the online sources, and could be available in various thicknesses. Ideally you want to space both bearings approximately the same amount — for example if the case was .010” too wide, you’d put a .005” shim behind each bearing. (only an example). The other way (more involved) is the narrow the case by machining the sealing faces of the case halves. Note that this needs to be done very precisely, as if it’s not done very accurately, the bearing alignment of the case halves can be messed up. I’d say this last method would probably be the “last choice” as it’s the most involved, and if you don’t have ready access to a machine shop, I’d go with shims behind the bearing(s).
Once you have either spaced the case halves with the correct gasket, or spaced the bearings inward with shim(s), don’t be surprised if the end play of the crank measures just a touch more than you calculated. (maybe a couple thousandths more). This is because bearings have “internal clearance” which allows an inner race to move slightly “sideways” within a bearing. There’s nothing wrong — that’s why I typically shoot for maybe .008”-.010” when setting up a case, but it will end up .010”-.012”.
Back to the con rod for a sec… if you’re having someone rebuild the crank with your existing rod, it’s a good idea to check both ends for how round they are, and the size. I personally set big end clearance at .0013” on new parts, and am willing to live with .0015” on used parts. They can be run tighter (and are set up tighter from Yamaha when new), but for best performance, the numbers I gave are my choice. Much over that, and it starts to be time for a new rod. Note that if the rod is significantly out of round, it should be honed to bring it back round — this could cause it to be oversize too far, which would mean you’d be better off starting with a new con-rod. The small end is not as picky since the bearing only goes “back and forth” and does not spin. I personally target about .0008”-0009” on the small/upper end of the rod. Once again, it’s probably OK if it’s tighter, but I’ve found that number to work well. Note that if bottom locating the rod, the upper end bearing has a wider cage that takes up most of the space between the wrist pin bosses inside the piston.
Bottom line: for a "top located" setup, it requires a narrower upper end bearing with 2 washers that have counterbores in them, and the bottom end cage is a bit wider than the rod --- for a bottom-located setup, it's a wider cage on top, and a bit narrower one on the bottom which should include the two space washers that go on each side of the big end of the rod.
The other thing to check on the rod is how straight it is… both in parallelism of the two bores, and in twist. If someone is in the engine building business, they should have all the equipment to measure, size and straighten con rods.
One last thing on rods: there are a couple of configurations: generally people refer to them as “light” or “heavy” rods, or “old style” or “new style”. The current generation of rods being sold is what I refer to as the “heavy” rod. I personally prefer the “light” rod, but the caveat is that the best ones were made back in the late 70’s and the 80’s. After that, I believe Yamaha cut some corners on material quality, and the “light” rods started getting “egg-shaped” ends if they were run at very high revs for a decent amount of time. With what you’re doing, I won’t worry about it.
The above should get you going on the lower end.
When it comes to setting up the upper end, it all depends how “deep” you feel like getting into this. You may simply want to install a new piston (and ring) that has the correct clearance in your existing bore. Make sure the ring is correct for the bore size, and make sure it has "at least* .010” end gap (contrary to what many believe, within reason... more end gap does not reduce performance). Just make sure it has NO LESS than .010”.
There is much more to it in a full-attack racing environment… how the barrel is honed, setting port heights to the rules, checking/machining combustion chamber volume, etc. Whether you go this route, or just get this thing back together so you can run it … fire away if you have questions.
PM
