Crank Bearings?

So I have a XJ750X engine that's apart. The crank bearings have gone... missing. I know what the calculation calls for be to order but I have a full set of new blue bearings in hand. Could these be installed or do I have to drop big money buying the other colors? Also I remember seeing a chart once that gave the actual thickness of each color and I can't for the life of me find it again.

 

The colors correspond to specific measurements that set the clearance between the bearing shell and the crankshaft journals. If that clearance os incorrect the oil wedge that the crankshaft runs in will not develop. That leads to some very bad things happening.
You absolutely must use the correct color (size) bearings in each location.

In your situation I'd be inclined to think that this is a sign to clean and re-organize the garage.

 

isn't something etched in the lower case to figure the bearing size, this is beyond my pay grade

 

Scratched/scribbled/etched into the upper crankcase is the main journal saddle "sizes" (and, you need to view them from the front of the engine for them to be spatially correct). Stamped into the left crank journal counterweight is the crankshaft journal sizes. You subtract one from the other, that gives you the bearing "size" you need, which is then translated to a bearing "color"...........

http://www.xj4ever.com/catalog/frame-engine-id.pdf

 

And then you take measurements with plastiguage to make sure the original bearings still fit correctly.

 

On the subject of crankshaft bearings do the journals need to be checked with a vernier caliper to make sure they are the same diameter?

 

Just went around in circles with this same question earlier this week.
The shell is the subtraction of the case measurement (scribed in case) and the punched measurements on the crank (all in chacal's explanation in his pricelist on how to do it properly, or the service manual...which has the colour chart).
You can't measure the shells accurately with anything, square (vernier, micrometer) and you can't identify the shells by the number's stamped into the back of them and the colour is just not on them anymore so...
The trick is to definitely keep them in order and neat for refitting and if you read through the posts on crank shell bearing replacement, throughout the forum, you will find that it is done rarely on these bikes and if you read chacal's pricelist... you will also see why... mucho expensive!
So... I decided my shell bearings actually look pretty good even though they look quite blotchy and a bit weird here and there.
Good oil is the key I'm thinking.

 

Not really. You only really do that if the journals are damaged and need to be reground. They just don't wear unless something went horribly wrong and bits of metal went flying around inside the engine, or it sat with water in the crankcase for a long time. And then you want to use a micrometer for the measurement (or rather the machine shop doing the work will).

The plastigauge measurement of the bearing clearance is the important thing. That will tell you if the stock bearings are still in spec, or if you need to go to the next color.


As for the shell color, it can be hard to see, but there is usually a bit of color left. Use a strong light. The color marks are on the edge of each shell, at the bottom of the dip.

 

This is so very true.

And even more so.

There must be a translation somewhere in the archives of Yamaha Engineering that unlocks all the secrets, but I've never seen it. Some service manuals give a bearing measurement by color (for the rod bearings) while other, different service manuals give the measurements for the crank main bearings (but, no service manual gives these measurements for both types of bearings). And the tolerances between the different "colors" is incredibly tight. For example, the blue crank bearings are 1.5002 - 1.5006mm, while the black bearings are 1.4998 - 1.5002mm, and so on.

I've never heard of motorcycle cranks being "ground" (like in automobiles/trucks, i.e. Chevy/Ford/etc. V8's) since the range of bearing sizes available is not wide enough to accommodate any true grinding, and there isn't any standard "undersize" or "oversize" bearings available. And the main bearing saddle size (machined into the case) is also a factor n these engines, unlike most automotive engines, which don't seem to take that factor into account, at all. Of course, V8 engines aren't expected to live very long turning at 10K+ rpm's on a periodic or extended basis, either, like these engines do. But maybe Formula 1 engines are done in this same manner.......

But, whatever and however they did it, it seems to work.....I can't remember ever hearing about a crank or rod bearing "failure" (wear or damage, yes, but never a failure) in these engines, I recall seeing some Utube videos where people purposefully try to blow up the engines (drain all of the oil and run the engine at high speed) and even in such a situation it took a LONG time before seizure occurred (and I'll theorize that the seizure probably occurred in the piston/rings rather than the bearings).

Pretty remarkable engineering, with the drawback that we're sort of left in the dark about the "numbers". No crank or rod journal specs (diameters) are given, either.

The plastigage measurement is key, but, I don't think it's intended as a method of going to the "next color", for the reasons specified above. If the clearance is too big, you replace the bearing with the same "color" (size) as what the case/crank numbers indicate, and if a new bearing still doesn't reduce the clearance to specified, then Yamaha says to replace the crankshaft.

 

There are other ways though. So long as the clearance is correct it doesn't matter how you get there. Though you are correct that reinstalling a new bearing of the originally specified size is the first (and usually only) step. What I left out is the need to regrind the crankshaft journals to allow for the use of oversized bearings, or the use of metal redeposition and grinding to return the crank journals to their original specification. All of that is beyond what the home mechanic can do (and is not commonly needed), which is why I ommited that.

 

Metal redeposition - not normally used for any automotive krankshafts, let alone tight tollerance japanese howling machines, I wouldn't recommend it.
On the subject of measuring shell bearings - it is possible, and quite easy, you use a micrometer and shiny new ball bearing on the inside of the shell.
Once you've measured the shell bearing I'm not sure what you'd do with the information, other than to aid assembly/clearances setting. The Rover K series in the uk is similar, I allways measure top and bottom shells, sometimes even use used ones (if I can't get new ones to the right size) to get the right the clearance right.

 

Interesting replies I did not realise the cranksahft journals rarely need ground on the bikes. My father was a carpet loom mechanic and he taught me a lot about cars too when I was a kid and I knew car crankshafts got reground. Great forum this is the knowledge you have is great and Yamaha made a great series of engines when they designed the XJ range. I wonder who has the highest mileage XJ here? I watched a video recently of an XJ 900f rebuild on Youtube and the builder wrote 'the engine did not need any work it's an XJ.

 

I just used fancy talk for welding the crank journals. Common practice on all sorts of engines. I had no idea that the term referred to another specific process.

 

Aaah good one, bit like using a bore gauge. Excellent.

 

Just don't forget to measure the ball bearing first, and subtract after.
I've done that brain fart, and it costs a bit.

 

Good info. As stated I was aware on how to get the correct color sorted for the case but being in Canada by the time I pay exchange rate and shipping it's pretty crazy cost wise. Another site stated that the tolerances weren't that large between the bearings and that the Blue's were the tightest. That site suggested that a full set of Blue's could be used in an engine with some miles/wear. Also since I have a full set of NOS blue shells that I got in a parts lot it would have been nice to save the $$. I guess I'll just have to save the pennies and get the proper colors from Chacal or get another bottom end/crank.

 

Whoever wrote that advice about using the blue shells has no clue about how crank bearing tolernaces work, and that crank journals don't wear unless there is a lot of grit in the engine. If just using blue shells were a good idea, that's what would have been done at the factory. It would have saved Yamaha a lot of money.

 

That's true, the maximum size difference between any of the "colors" is 0.020mm, and it could be 0.012mm, depending on the individual shell that you have, since each bearing has a size (thickness) tolerance of 0.004mm.

That might be correct, or exactly opposite. The BLUE size bearings are specified to be 1.5002 - 1.5006mm, and I presume (but am not sure) that this is the overall thickness. The YELLOW size bearings are on the other end of the size spectrum, and they are 1.4860 - 1.4900mm thickness (again, if I am understanding their specifications properly, which is hard to do considering the radio-silence from the Imperial Yamaha Navy regarding these matters.....). A thicker bearing shell results in LESS clearance between the shell and the crank journal surface. So the Blue shells may very well be the tightest ones.


The journal oil clearance are 0.020 - 0.044mm (650 thru 900 engines). If you use shells which are too "tight", you'll wipe the babbit off in a big hurry. If the shells are too loose, you'll lose lots of oil pressure to the rest of the engine (in a best-case scenario).

 

Oil pressure is due to resistance against pump pressure in other words as I understand it if bearing clearances are too large there will be less resistance from pump pressure, is this what would reduce the oil wedge?

 

No I don't believe you need pressure at the bearing to form a wedge. However, if you loose all your oil down the nearest bearing, the rest will get starved. Result - scrap engine.