Engine bearing help

[blue giant]

Tod,

I have the Clymer manual and I'm going by figure 100 on page 86. That is how I have measured for my replacement bearings. I also have a Yamaha service manual for my XJ and it is the same almost word for word. It would appear from both that the number you subtract from is the first number or the number on the upper half of the con rod.

I hope I'm explaining that right.

[trbig]

You're explaining it fine, I'm just wondering if I'm not?? It makes me wonder if they kind of split the diffference for some reason and used a green bearing on half and a brown on the other half? I've always only seen a single number stamped there, so that's where the confusion is coming from.


Tod

[79XS11F]

Those numbers don't look right??? Any chance that the rods are original and the caps are from another engine? Caps are interchangeable and if these are not the original caps I would use the numbers that are on the rods.
I really can't see them "splitting" the difference from the factory.
Rob

[trbig]

The rods and caps are a set. The "H" between the 4 and 5 is stamped onto the joint between the numbers, and they all line up and continue from the cap to the rod.

I'll probably leave these rod bearings alone and just replace the crank ones. I'm glad to see that it doesn't make sense to someone else.. lol.


Tod

[79XS11F]

If your crank bearings are at the limit it is very likely that the con bearings are also ready to be changed out as well. It would be a shame to change out the crank bearings and leave the cons in there while the engine is opened. Is there anything about it in Yamaha's tech releases? Perhaps the thing to do is go with which ever numbers give the tighter fit. and check with the plastic gauges.
Rob

[trbig]

I don't know about the tech releases, but it's not a bad idea about installing and checking with a plastigauge... I just don't have a whole set and would have to order what I need. As high as they are, there's no way to justify ordering two of them.

I have a couple calls in to local Yammy mechanics and am waiting to hear something back from them. I will also take a couple rods off tonight and see if by chance I can see any color on the bearings. I'll plastigauge them also to see where they stand.


Tod

[mro]

Customer Relations (800) 962-7926

Yamaha...
I'd call and ask.
Or they may be able to tell you who to call.

I've seen 3H5 on the top of pistons
but don't remmeber what your showing on the rods.


mro

[trbig]

Yup, the 3H5 is for the later model pistons. You'll see that on the cams too. I will call that number and see if they can tell me anything. No one has called back from the other two places. Thanks for the number.


Tod

[trbig]

No answers through customer relations... they revert you back to the dealers for questions like this. Thanks though.


Tod

[AlanB]

Hi Guys,

we recently beat this question to death on the UK site, and had the different colours of shells properly measured.

The basic conclusion was that the shells are different by such small amounts that once the motor has turned over, the colour system is meaningless.

If the shells are worn enough to need replaced, just fit the heaviest (least clearance!) shells you can find........ they WILL NOT be too tight (although plastigauge won't hurt!)

AlanB

[trbig]

Thanks Alan.

Just thinking about it, it would seem that you are more or less using your crank for a grindstone to cut the bearing to proper thickness if you were to go to the thickest one.

Each bearing size is .0010" (.25mm) larger. This doesn't seem like a lot, but the correct clearances are to be .0017" to .0025". I am on the high end.. somewhere on the .0022-.0025 range I believe. If I were to put on a bearing 2 sizes larger, it would effectively close my gap completely and leave no room for oil, or the cap bolts couldn't be torqued properly. You'd be smashing bearing together instead of rod and caps. It would seem that after the bearing wore to where it needed to be in this case, the bolts wouldn't be tight enough any more?

That's the way it would seem to me anyway.... Feel free to point and laugh at my reasoning!


I just plastigauged the conrods. Something has FINALLY gone my way with this thing. They look new and measured almost too tight! Close to .0015". So they are staying and the crank bearings are going.


Tod

[AlanB]

Hi Trbig,

Here is a chart of bearing sizes borrowed from the Yam triples site;


That gives the shell sizes as 1.5mm, plus or minus a tolerance. Those sizes are quoted in thousandths of a MILLIMETRE..... not thousandths of an inch!

The differences between shell sizes are around four thousandths of a mm........ One mm is around 42 thousandths", so that makes the differences between shell sizes less than a fifth of one thousandth of an inch......... Like I said; too small to matter!

AlanB

[trbig]

I don't think that's right. I may be really wrong, but every bit of info and on every parts list I have seen to this point says that the bearings are graduated in .25mm increments. That comes to just under .001 inches. That's why if you were getting out of spec at .0025", the next size bearing would close it up .001 of an inch and get you back in spec... actually just a touch tight.

It seems to me that someone's math is wrong. With an accepted rate of .0017-.0025 inches for spec in the book, you could never make up for bearing or crank wear according to that chart. It just doesn't make any sense?

Anyone have any new bearings they could mic to verify this?


Tod

[Ken Talbot]

Originally posted by trbig
Each bearing size is .0010" (.25mm) larger.

Quick metric correction - 0.0010" = .0254mm

[trbig]

Hmmmm... which would make about 10 times too much, huh? lol.

I don't know then. Thanks for pointing out my math error Ken.

So according to that... everyone should just throw yellow bearings in everything and be done? No measuring involved? It just seems to me that those little guys in white coats thought all this through better than that. (No, not the ones that are coming to take me away)


Tod