Resizing Connecting Rod Big Ends

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Stephen Hill

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I can't find any information about resizing the big end of Norton Commando connecting rods. I understand that for many other applications, when the big end is worn or no longer round, it is common to remove material from the clamping surfaces of the cap and rod, then rehone back to round and correct dimensions.
Because the stock Norton connecting rods are aluminum and the caps are steel, does this mean honing to size is not possible? In which case, is there any other method of resizing?
 
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Hi, this seems like a lot of work for a set of rods, why not just buy new ones.
 
Stephen Hill said:
I can't find any information about resizing the big end of Norton Commando connecting rods. I understand that for many other applications, when the big end is worn or no longer round, it is common to remove material from the clamping surfaces of the cap and rod, then rehone back to round and correct dimensions.
Because the stock Norton connecting rods are aluminum and the caps are steel, does this mean honing to size is not possible? In which case, is there any other method of resizing?
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I had that done on one rod by a local rebore/machining company in north London in 2002, but I don’t know exactly how far out of round it was to start with. I did use normal shells once it was done.
 
Re: "why not just buy new ones?"
Honing to size is a very common and inexpensive technique that has been used for years.
I am interested in a discussion about whether honing an aluminum con rod and a steel cap is possible.
I see there are some newer rod conditioning machines that use a boring operation, which might address the two different metals issue.
 
I worked for a company that started machining material made from steel and aluminium in the 50's, they were still developing machining techniques to do the job properly 50 years later when I left. The problem is the tool angles, feed rates etc are very different between the 2 materials and you end up with different finishes and sizes. Even with tooling geometry aimed in the middle does not help.
 
I had my BSA b25 rod re sized at seager engineering
They did a perfect job
 
kommando said:
I worked for a company that started machining material made from steel and aluminium in the 50's, they were still developing machining techniques to do the job properly 50 years later when I left. The problem is the tool angles, feed rates etc are very different between the 2 materials and you end up with different finishes and sizes. Even with tooling geometry aimed in the middle does not help.
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Honing may be a different ball of wax than boring, eliminating (some of) the tooling discrepancies.

Personally, I look at a stretched big end as potentially a symptom of a problem, not necessarily the problem itself.

"People have been doing it for years" is not really good justification for future practice.

Somewhat random thoughts, somewhat applicable.
 
t ingermanson said:
Honing may be a different ball of wax than boring, eliminating (some of) the tooling discrepancies.
Click to expand...
Problem with honing aluminium is the aluminium fills the pores so frequent dressing to reveal fresh media is needed. Which is a different issue from swarf chip production when cutting.
 
Slightly off topic but I've always wondered how the rod/caps were made. I can't imagine them being machined separately, but then it seems like it would be problematic to do them together.
 
I had no issues resizing that type of rod , we had a sunnen hone with plenty of honing oil , there is also a variety of hardened mandrels to keep your stones in good shape , we also had a Schou in the 80s

Schou Model 150-B fine boring machine which is made for conrods , it holds the big or little end at the top and then it has adjustable pins which centre your bore , it creates extremally accurate pin hole bores in the little end , easy to bore to .01mm clearance and the pin will glide through , nothing like a reamed Finnish where you get lobe-ing , and for the big end it can be bored or ground , the stone on an arbour moves around eccentrically while spinning very fast , with coolant , again accurate . hope this helps , give it to a good reconditioner . dont worry .​

​

 
I'm just a simple person and am wondering how the big end shells will fit if you bore out the big end. Can anyone enlighten me?
 
Nortoniggy said:
I'm just a simple person and am wondering how the big end shells will fit if you bore out the big end. Can anyone enlighten me?
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You are returning the con rod bore back to standard, the shells will fit exactly as they do a new con rod.
 
And if you think about it it makes your conrod centre's closer together! Maybe .005" shorter . :)
 
Stephen Hill said:
I am interested in a discussion about whether honing an aluminum con rod and a steel cap is possible.
I see there are some newer rod conditioning machines that use a boring operation, which might address the two different metals issue.
Click to expand...
Well they had to do it when they manufactured the rod didn't they?
 
They can be re-conned by a competent machinist. They are a bit touchy and can easily be made worse by someone with no experience doing an aluminum rod with a steel cap.
But personally, if they had enough use to need re-conned, I would be worried about metal fatigue in the aluminum and replace them.
 
When the conrod was first manufactured, the rod and cap were probably machined as an assembly. When being resized, the faces of the cap are probably ground, then the cap and rod are machined as an assembly. There would be no difference in the degree of difficulty. The rod and cap would be securely held while the machining is done.
 
I had experience of an oval rod in my 99 some 30 years ago . one rod when fitted with a new std shell would not rotate freely even though the std crank was well worn. I think it was because the bike had a partial seizure on that side . the degree of ovality was only a thou or so. I overcome the problem by polishing out the scores on the crank !. Has run like this for the last 30 years. I recently tried to obtain the correct rod bore size to correct the issue and no one seemed to know. I measured a number of used rods and a new old stock one , I settled on a measurement of 1.6" ( 1.5 crank) although the NOS rod was well out (probably why it was not used) . When I get my crank ground I will decide if new aftermarket rods are the way to go as 62year old rods are a bit sus !.
 
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