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I got a few rods where the big end hole is out of round. I recently read wthat they can be reconditioned by milling a small amount off the cap joint surface, bolting them up and rehoning to size.

What are the drawbacks to this operation?

Bob. Up here, it's called sizing. I had a couple of pairs checked for being straight and then sized. I guess this is to account for slight out of round from miles of use. And then as you describe. Many sources in these parts recomend the procedure and the shop says they don't have to be re installed in necessarily the same side or direction.....so they tell me. Cost was about 15 dollars per rod. I don't know of any drawbacks but I would imagine someone will. Cheers, Wilf.

Just did a set that One was .001 out of round and one was .005 out. Definitely needs to be checked. But the problem is the cranks vary so much on size that you cant just hit a certain measurement when re honing to size. So you really need the machine shop to hone them small and check them then tell them how much you need to open it up. Kind of a PITA to do it right.

If the rod is oval,IMHO,it's time to replace them,especially if they are to be used in anger!!! Dick

Haha i agree with Dick, If they were cheaper that would happen more....but sometimes just have to make do.

I read an article by John Healy wherein he mentioned that this should never be performed on aluminum rods. I like the way he stated his point. I guess he won't mind if I quote him:

"The typical process of grinding the connecting rod cap and re-sizing the rod's big end, routinely done with steel rods, is inappropriate with aluminum. That little bit of stretch is the only warning the aluminum rod will give you before it breaks. If you measure the length of the rod, the big or small end eye, and they have stretched or gone out of size or round, cut the rod in half before it does the same to your crankcases."

(From Vintage Bike Tech Tips, Winter 2009-2010, p. 27)

I just made a contribution to my aluminum recycling bin.

That's too bad. Someone could have made good use of those rods probably. I've been sizing rods for a long time and will continue to do it. The only rods I've seen break are broken for some other reason than weakness of the rod. Folks you need to have a machine shop that can steer you in the right direction.
Of course, I wouldn't tune a Triumph to make 70bhp either. Its just not a good platform for that kind of treatment. If I did, it would likely have something other than a stock rods in it.
Billet crank too, but now I'm getting rediculous.
Bill

Bill:
You see, it is the cranks that break in those 70 hp motors, NOT the stock rods. Not that is, if they haven't stretched the aluminum beyond their modulus of elasticity. The only way you can tell if you have done this is by dimensional changes.

I have run those 70 hp motors for a lot of my life and used STOCK rods. The last iteration of my 750 did use Nourish rods, only because Triumph didn't make large journal rods. In fact with a little effort you can break a stock 750 twin crank with the modest horsepower the bike came with.

Bill, you need a machine shop that doesn't have a "boat payment."
John Healy

Sure a nos rod would be great but far and few between these days.To many rods out the I have no clue what they have been through to use in a new 70 hp motor.I have a stock rod that cost me big when it blasted through the cases but like john said on a street motor may have been fine.Learned real fast what high heat does to aluminum over time.

Um, my machine shop guys make dragster payments actually.
Bill

Yesterday, I was re-reading that Vintage Bike article I quoted above, where John was discouraging people from sizing their alloy conrods. I'm trying to get going on my project bike again (1964 TR6), and finally measured the big ends on the rods (torqued to spec without the bearing shells). Using a dial bore gage, I checked the diameter at various points around and across the big end bores. I found the spec diameter of 1.770 at some points, but found variation up to 1.7705 on one rod, and up to 1.7708 on the other. So, how much deviation is too much?

I have a friend with some shop equipment who routinely sizes Harley and Jap bike conrods, and gets them to within .0001 measured variation. Another guy who has rebuilt many Triumphs told me not to worry about the +.0005. He said the +.0008 was sort of "out there" but probably not a problem.

As always, I seek your opinions.

I looked into that and was told by my machine shop that the problem is the rods are aluminum and the caps are steel.
that dissimilar metals issue is impossible combo for a rod honing machine to do.

So it would seem. You guys that do this stuff all the time -- what rods are you using? I have heard that the rods available today come without the small end bush. I did not realize it, but was told today that the T140 never had a small end bush from the get-go, and they did well, so the replacement rods for the 650's have gone the same way. What sort of variation would one expect when measuring the big and small end bores of a brand-new rod? Anybody checked one before installing it? I'm almost inclined to pretend I haven't measured mine, and to install new bearing shells and put it back together. To me, .0008" doesn't seem too far out of round, but my friend seemed to think it was trash.

If I knew I was going to add some value, I guess I wouldn't mind pouring some more hard-earned cash down this rat hole. I'm just wondering if new rods would be any better than what I have?

[Edit]: I think I just got my answer by stumbling onto an old post by our moderator. I'm gonna reuse my old rods.

Re: conrod bushings

The torque you use can make a big difference to the shape of the rod.
Early bolts,26 tpi,are meant to be 28 ft-lbs (dry).
Bolts supplied after June '69,with 5/16" UNF x 24 tpi,are meant to be 22 ft-lbs (dry).These nuts have 1/2" AF hexagon.

Tightening dry nuts onto dry bolts is a fast way to damage threads.torque measurement is not the best way to measure bolt tension.Measuring bolt extension is a better way to measure bolt tension.You should be getting 0.004"-0.005" elastic bolt extension,when you tighten the nuts.

I prefer to lubricate the threads and rod caps (where the nut sits) and measure elastic bolt extension.Check the bolt tension again by measuring bolt extension.It sounds like you're close enough,and with correct tension it may be even better.

Polish the rods,with all polish marks running vertically,and carefully inspect them for cracks.Do what you can to check that the big-end eye and small-end eye are parallel.When the rods are on the crank,you should easily slide an 11/16" shaft through both small ends at once,and their centre distance should measure 3.375".

If you decide to replace rods,they got heavier and stronger from engine no DU 47006.All standard 650 Triumph rods have a bronze small end bush.

Thanks, Pete. I do have the earlier bolts, and I torqued them to 28 lb-ft (dry). This gave me .004 to .005 stretch on all four bolts. I see your point on lubing the threads and going for the proper stretch. I plan to follow your advice here and from other posts re: checking with the 11/16 shaft, and also trying to shim the crank as needed to center the rods on the bores. I am not planning to check (or have a shop check) my crank balance factor, since it's essentially going back together as it was. I suppose the newer, heavier rods would make that necessary?

Well it seems I have people thinking about their rods. This cannot be a bad thing!

So while we are looking at our rods lets check to see if the wrist (gudgeon) pin is parallel with the big end. This can happen because the rod is bent or the wrist pin bushing was replaced improperly. The typical limit for this is .001" over 12 inches.

Why should we interested in this. If the pin isn't parallel with the big end eye (crankshaft) the piston will be sitting in the bore at an angle. This creates three problems. 1. The sealing faces of the rings will not be parallel to the bore causing break-in problems and heavy oil consumption. 2. You create a little engine that drives the wrist pin from one side to the other on each revolution of the engine. This puts a lot of stress on the circlip. This can cause the aluminum holding the clip to fail leaving the pin to push out against the bore. 3. I have seen them so bad that if the piston was running parallel to the bore there would be plenty of clearance as the piston warms up. But now that the rod is bent the area just above the top ring on one side of the piston and the opposite lower edge is rubbing. This can lead to a hot spot and detonation or pre-ignition.

So here is where I start to test my friendship with all of my Triumph friends. In the Harley world (steel connecting rods) it is recommended to use the crankcase mouth to check for a bent rod. So a lot of Triumph mechanics read all about this in "Modern Motorcycle Mechanics" and think they can do the same with their Triumphs. Well, yes you can. That is if you have previously "blue printed" your crankcase mouth. You see, Triumph crankcase mouths are seldom parallel to the crankshaft.

It doesn't matter, until it matters...

Early and later 650 unit conrods - When did the change to the larger beam occur? And what are the actual measurements of early and late?
Thanks, Mike

Mike, as Pete said above, the conrod change commenced at DU47006, which was a running change in the 1967 model year. I read somewhere on here that the new rods were about .100 thicker near the small end. Someone on here will probably know for sure. If I knew exactly where to measure, I have my original skinny ones in a box and could give you a dimension.



John, I hate to admit it, but I've been told by someone who works on them routinely that the same could be said about Harley crankcase mouths. He has a Sunnen inspection jig to check the conrods for twist and/or bend. They can check perfectly on the jig, and be off relative to the cases. Shocking, I know.

Ray

Measured 1" below the small end,0.650" width vs 0.550" width.

I check the rods with accurate mandrels.I'd pass them at 0.002" out of parallel in 6" or 8".If they're within 0.002" in 10",I'd say that's prettty good.New rods sometimes aren't that good.

Thanks, Pete