Splitting the cases

I’m posting this in hopes to get some guidance on the matter at hand. I’ve disassembled the whole top end and am slowly getting to everything (boring and installing oversize pistons, checking head and valves for spec/lapping, new cam chain etc etc). The whole shebang.

Anyways, while I have it all apart the machinist I go to for top end work made a comment about the connecting rods and making sure they were in spec. I went home and checked them against my Clymer manual.

Slide clearance on the big end is a tight 0.55mm on each one and the book calls for 0.3 - 0.6mm. For play at the small end of the con rods I’m seeing about 1.5mm of movement on each when the manual calls for 1mm - 2mm.

What are other peoples recommendations for this? I know the con rods are within spec, but they seem to be on the worn side. I’ve never split the cases before on a motorcycle and I would really love not to, but I know since I’m already in this far maybe I should. It’s like a can of worms though and one day I think I’m going to stop here, but then find something else and keep going further. Where’s a good spot to call it and do the work I need to and rebuild? I’m not trying to make the worlds fastest or best engine, but I do want something I can rely on for the foreseeable future.

Any advice, knowledge or past experience will be greatly appreciated. Thanks you.

Given the specs you presented, you seem to have built a good case for not rebuilding the bottom end, particularly because you aren't looking to build the "best engine". I think it will run fine as is.


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Your side-play and "wobble" on the big end are in spec - no worries. The more important question, and the more likely point of wear is the "small end" - unfortunately there are damn few specs for guidance. So a visual inspection of the small end con rod surface and wrist pin is in order and also zero play between the two (vertical, they will wobble a bit).

IMO & IME, if all the above is correct, it's not a "should do" to do a "bottom-end" on a otherwise good XS without provocation.

I'll just add that inside the little ends there's a copper plating the wrist pin rides in. If that copper is mostly gone and you're looking at steel, the rod is pretty much shot. If you're not sure, give us some pics inside the little ends.

On both rods most of the copper is gone except for the back and front sides. I can fit a 0.06mm feeler gauge between the stock wrist pin and the rod. Will this make or break the build? I rebuilt a cj360 in the past and didn’t replace the rods and it’s still goin I suppose. Would this be a machine shop type job or is this doable at home?

Toro said:

On both rods most of the copper is gone except for the back and front sides.

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You're worried about the copper inside the the rod... where the wrist pin goes in... that hole.

Yeah, I'm not sure we're on the same page re the small end ID and wrist pin.
First the wrist pin: there should be no step wear at all across its length and 20mm OD.
Second the con rod bore ID(per book): Small end ID 20mm (.787") with a tolerance of .0011" to .0006". So, .7876" to .7881"

So again, basically zero vertical play between the two. I can't imagine an engine living very long with that clearance allowing a .002" feeler gauge inserted (that's way > than .002")
Pics?

If a rebuild is required it requires hyd. press, jigs, dial gauges, etc. or a very specialized machine shop. I'd venture to say most auto shops can't do it to spec

Jim said:

You're worried about the copper inside the the rod... where the wrist pin goes in... that hole.

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Oh I’m sorry, that’s what I meant. I wasn’t specific enough. Most of the copper is worn off where the wrist pin sits.

There's a small "grey area" between usable and trash. Some good pics will tell us where you are in that "area."

I will send some pics soon or in the morning

The one with the blue rubber bands is the right and the other is the left. I could take a video of the play each has too if that would help.

If it was mine I'd be replacing them.

Yeah, those are a tad ugly. Don't know for sure if they can be honed and bushed but I sorta doubt it.

Yeah... pretty bad score marks in there. I wouldn't run 'em.

Downeaster said:

Don't know for sure if they can be honed and bushed but I sorta doubt it.

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Don't think anyone makes bush's for 'em. The problem is you pretty much have to jig 'em to get a straight bore. That means removing 'em. And if you have to remove 'em, well....

Alright for sure. Sounds like a resounding yes to replacing them as far as bottom end rebuilds go.. where do you stop? It’s a 50 year old engine, so I get it owes nothing at this point, but the further I look in the more there is to be done and replaced. Where is a general point of stopping without rebuilding the whole engine? Sometimes it feels like I’ll be putting more into it than it’s actually worth.

Also, where is a good place to source new con rods from?

Well, it's not so much sourcing the rods, it's finding someone to rebuild the crank. It's a pressed together crank. Involves a 20 ton press, press plates, Vee blocks and dial indicators... a lead hammer. Experience doing split cranks helps too. Gary Hoos used to do 'em, but he retired. Not sure who still does.

Jim said:

Not sure who still does.

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If I’m not mistaken I think Gary handed everything over to Roger who you can contact through his FB page