Connecting Rod Wear

[mark]

'79 GS 850 G

I've pulled the head, cylinder block and pistons on my '79.

My question has to do with the connecting rod bores that hold the wrist pins. The connecting rods are coated with some sort of bronze finish. What I'm noticing is that in the bore that holds the wrist pins that this bronze coating is pretty worn in a few places in a way similar to the way it might wear on the brake caliper sliding pins for example.

My assumption is that this isn't good, even though the wrist pins don't seem to have too much "play". You can't just put in new connecting rods because the crank is pressed together and the connecting rods are all one piece. So to fix it would be big bucks. I also don't want to dump money into a top end (it needs a cylinder hone and valve grind), if the bottem end is marginal.

Any ideas on how to check these connecting rods to see if they are worth working with? Thanks.

 

[flyingace]

If there is no noticeable play of the wrist pins within the con-rod bores, I would just put it back together and not worry. Mine were all fairly tight and that was good enough for me. The engine has been fine for 2K miles since.

Besides, overly worn con-rod bores is more than I want to deal with.

 

[mark]

Besides, overly worn con-rod bores is more than I want to deal with.

I agree. I just have a bad feeling about this.......

 

[49er]

Mark
I have the top end apart on my 79 850. The con rods are forged and are a brownish/bronze colour. The gudgeon bores on mine are a steely grey.
After 78000kms, the axial wear on the wrist pins and little ends is minimal. I would have honed and re-ringed my engine if I wasn't planning a rebore.
Check the side clearance on your big ends with a feeler gauge. The clearance should be 0.004-0.025in with a service limit of 0.040 (1 mm).
If your engine has less than 100,000 miles on it and has been well maintained, you should go ahead with the planned hone and re-ring.

Ian G

 

[mark]

Well, this makes me feel somewhat better about my situation, but not over confident. My bores are that steely grey color as well and most of the bore even seems to have a kind of cross-hatch to it. They are smooth and the wrist pins tapped out easily........but they do have "wear spots", lighter areas like you'd see if you were sanding paint and beginning to get into the primer.

I think I'll throw the thing in the back of my pickup and take it to a guy who knows what he's doing.........it's worth that I guess. Just when I thought I was about homefree......

I think I'm beginning to understand the wisdom of the phrase, "If it ain't broke, don't fix it".:-k

 

[Guest]

I took a crank to a machinist once because I was unsure. The piston in question had spit out a ring and ate up the bore and the piston. there was a small amount of visible, but not easily felt wear. My mind decided that there was a problem, so I took it to a pro (machinist). After trying to see what I was talking about for a little while, he said,

"If I have to think about it for this long, you're fine."

so if there's no or virtually no play on the pin, you should be kosher.

If I drove 500 miles on a blown ring on a motor with 52,000 miles on it and was fine, I think you should be OK.

PS - I did replace the cylinders and the destroyed piston. It now runs nicely.

 

[rustybronco]

What does the shop manual list for the small end bore diameter and the wrist pin diameter that should be your answer, you can get the size of the the con rod bore with something like this http://www.msdiscount.com/store/col...l+micrometer&compidcookieset&sessioncookieset and measure the results with a micrometer.

 

[Footy.]

Well, this makes me feel somewhat better about my situation, but not over confident. My bores are that steely grey color as well and most of the bore even seems to have a kind of cross-hatch to it. They are smooth and the wrist pins tapped out easily........but they do have "wear spots", lighter areas like you'd see if you were sanding paint and beginning to get into the primer.

I think I'll throw the thing in the back of my pickup and take it to a guy who knows what he's doing.........it's worth that I guess. Just when I thought I was about homefree......

I think I'm beginning to understand the wisdom of the phrase, "If it ain't broke, don't fix it".:-k

If you can still see the cross hatch honing pattern over most of the area, i am sure you will be OK. Any noticable wear should be on the down side of the rod opposite the oil hole.

The info from my manual for my 78 GS750 should be the same for your 79 GS850, as they run the same pin size.

Original spec small end pin/rod clearances are;

0.011-0.014mm or 0.0004-0.0008"

Wear limit is given as:

0.090mm or 0.0035"

I have machined many bronze bush type small end bushes in engine reconditioning, and that three and a half thou wear limit would give a really noticable "rock".

In my experience this kind of flash plated steel rod (bushless design) used on most Jap engines, is very hard wearing and tends to put just as much wear on the pin.

Of course as usual the service history of the bike effects wear rates if neglected.

You could try feeling for any difference in fit with a new pin?

Footy.

 

[Nessism]

When in doubt, measure! Mic the pins (in the area where the rod rides) and borrow/buy the proper tool to get into the small end bore and measure that as well. Compare to the service limit. While you are at it, have the cylinder bore measured as well. The Suzuki service manual give excellent instructions on how all this is done.

Good luck and keep us posted.

 

[DaveDanger]

Footy hit the first thing that came to my mind. Obtain a brand new wrist pin and try it in the small end knuckles of the connecting rods and see how much difference there is between that fit and the current wrist pin's fit. If the new pin reduces the free play to near nothing, replace them all and continue riding with whatever extended life that provides. If the new pin makes a minimal difference, you have to consider that there's no significant wear to worry about.

 

[mark]

Any noticable wear should be on the down side of the rod opposite the oil hole.

This is excatly where the "wear" marks are!

Thanks guys for the very helpful information, wisdom, tools etc. Its going to take a little bit to check it out right, get new pins etc.

This is turning out to be interesting (and I'm totally serious). I like working with motors.
Thanks again.

 

[SkeletonLake]

Had my '79 850 apart several years ago and it was much the same, wrist pins too. Unless there was a bottom end knocking before you pulled it apart, don't think you have anything to worry about. If the cross hatch marks are still visible, I'd just put in new rings, have the head done like you say. button it back up and enjoy for many more thousands of miles. Replace all seals and gaskets too...

 

[mark]

Good luck and keep us posted.

Loaded the bike up a few days ago, drove to a machine shop and had the bores measured and inspected.

They said I'm within spec.....actually that it all looks fine. New wrist pins and good to go like you guys said.

Thanks again for all the responses, I was worried but now its looking good. Hopefully I can get the bike back together by next spring. I'm a friggin' worry wart I guess.