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Aluminum connecting rod with a steel cap

Joined
Nov 19, 2007
Location
marysville ohio
I have been working on Triumph motorcycles since the early 70s and have always wondered what they used to finish the big end what with the rod cap being steel and the rod aluminum. It is amazing how round they stay. I just measured 3 rods out of a Trident, last produced 45 years ago. 2 rods dead nuts round, 1, .0001 out of round! Any ideas?
 
I have been working on Triumph motorcycles since the early 70s and have always wondered what they used to finish the big end what with the rod cap being steel and the rod aluminum. It is amazing how round they stay. I just measured 3 rods out of a Trident, last produced 45 years ago. 2 rods dead nuts round, 1, .0001 out of round! Any ideas?

You think proper honing wouldn't do the job? Maybe a fixed-abrasive final lap?
 
There was an article in Machinery magazine detailing some engine...this was the 1960's
IIRC it had 1/2 aluminum and 1/2 steel inserts in the block casting, they went into details
of the final machining (main bearing bores IIRC) and how they did that operation.
 
lol.. The minute I saw the title I thought 'Triumph'.

I think the question is a good one, and I don't know the answer, but the difficulty lies in machining two quite different materials at the same time and having them 'behave' the same.
 
I have been working on Triumph motorcycles since the early 70s and have always wondered what they used to finish the big end what with the rod cap being steel and the rod aluminum. It is amazing how round they stay. I just measured 3 rods out of a Trident, last produced 45 years ago. 2 rods dead nuts round, 1, .0001 out of round! Any ideas?

Yes, on your bench, at room temp.
Interesting to see what happens at "operating temp".

Got a pix ? Where is the split line ?
 
69, they were all the same.

I was wondering if it was one of the modern trident / triple engines, .............but seriously Moonlight I'm surprised it's that close! ...............by then the rot had set in to the entire Brit bike industry and Triumph were no exception, clapped out tooling dating from the war (when the Gov't paid for it) and a general lack of investment forethought and arrogance ** .......which fed through to the workforce most of whom couldn't give a 4X

As to how it's out of round, my guess would be the 2 different materials, extra tool deflection etc on the steel when they were bored.

** Proven that the year the Trident / Rocket 3 was launched at least 3 years late, ...........the Honda CB750 was launched knocking the triple out across the board.
 
Aluminum and steel will cut at a different rate when honing I'm sure. So, machine say a batch of 6 rod and caps at a center to center distance of exactly 5" and hone the bore to exact size. Now re-measure the center to center distance and you have a correction value for production.
Wasn't anything really precise on those engines anyway. I cut my teeth on BSA and Triumph 250 singles with those type of rods. It was the first thing that came to my mind as well.
On the factory racers those rods were replaced, they ventilated the crank case. LOL

Mr Bridgeport
 
My first guess would be that they machined them in pairs, 2 rods at a time or 2 caps, and then matched rods and caps for roundness.

This sorting of parts by size for later selection at assembly is not unique. Years ago Smith & Wesson revolvers from the custom shop were tuned by selecting parts to match the frame dimensions, no shims involved. A friend had a Model 29 that was done that way and it was not only slick as glass but stayed that way despite heavy use by himself and friends who put many thousands of rounds through it..
 
It's hard to believe they could separately machine the cap from the rod then match it that closely.

Tridents.....I seem to recall the common lore was the center cylinder lacked the cooling of the outers and ran hot.
 
It's hard to believe they could separately machine the cap from the rod then match it that closely.

Tridents.....I seem to recall the common lore was the center cylinder lacked the cooling of the outers and ran hot.

I ran a spark plug washer with temp wires on all three cylinders of one we were road racing and did not notice any difference.
 
They were probably bored with a single point tool like a Devlig Micro Bore cartridge. Light finishing cuts would have very little effect on bore diameter or roundness with a sharp tool point. I've line bored cast iron blocks with aluminum caps using a lapped carbide tool bit with a very tiny tip radius and gotten decent results.
 
They were probably bored with a single point tool like a Devlig Micro Bore cartridge. Light finishing cuts would have very little effect on bore diameter or roundness with a sharp tool point. I've line bored cast iron blocks with aluminum caps using a lapped carbide tool bit with a very tiny tip radius and gotten decent results.

Interesting. What CI block uses aluminum caps?
 
I don't know about production blocks, but some race engines use aluminum caps....the theory is the aluminum is better able to attenuate the sharp blow that comes from combustion. I think this is mainly in nitro-burning applications. Then, most modern nitro engine are all aluminum...I think this was in years past.
 








 
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