part numbers on MkIII con rods

Hi -

I have a '75 MkIII that needs an engine rebuild. I bought it recently - beautiful physical condition but an unknown condition engine that didn't run well. I got it running, but after about 200 miles I seized a wristpin and blew a hole in a piston, so I have my winter work cut out for me. I'm going to need pistons and 1 con rod. I have some questions about part numbers, as I'm not sure what con rods will work in the Mk III. Its present rods are marked "NM23258 R5R2R", but only one is good. I've seen some other rods on eBay with "NM23258", but the 2nd half of the number is different. Can anybody tell me any detailed info about the stock rods used in the Commandos and what I can use to repiace the one damaged rod?

And if anybody has some used rods in very good shape, please give me a shout! Same for +.030 pistons - the jugs are 20 over now. Same for a cam - turns out the engine has a Norris D+ cam, which isn't going to be something I'm going to love for country road cruising - I believe it's a fuel/drag cam. I wondered when I pulled the head why it had fancy triple springs <g>>... Actually, it also might explain another detail - the bike came with a single VM-38 Mikuni, which seems to be a big carb for this bike - most of the conversions I've seen are 34's. And the carb was jetted very rich.

So now I'm looking for used parts to get back to a more modest engine that'll just be pleasant for a lot of sport cruising - no drag racing for me...

Thanks for any info or leads on parts - Peter

Hello Peter, the only advice I have is watch out for "D" rods, they can break any time.
There are 2 small "D"s in a circle on one side and 1 on the other side.
I've seen them for sale on ebay many times.
Regards Graeme

ptourin said:

I'm not sure what con rods will work in the Mk III. Its present rods are marked "NM23258 R5R2R", but only one is good. I've seen some other rods on eBay with "NM23258", but the 2nd half of the number is different. Can anybody tell me any detailed info about the stock rods used in the Commandos and what I can use to repiace the one damaged rod?

Click to expand...

Numbers you find which are actually cast into the part (and not stamped on) will normally be the part's 'casting' number, and will not necessarily be the part number. The actual part number for a MkIII conrod is 06-4896.

My MkIII's rods are marked "23258 R3R1R"

The early Commando rods were listed as part "NM25369" which still appears to be an equivalent part to the 06-4896 item?
http://www.nortonmotors.de/ANIL/Norton% ... 001&Part=1

Yes, I'm finding the con rod numbers confusing. As you said - the '73 and '74 parts books have the part number as NM23528, which is similar to the left part of the number stamped on my rods, but not identical. The part number in the '75 book changes to 06-4896. I don't have access at the moment to any earlier part books, so I don't know what the number was for earlier 750's. I've been told several times that the 750 and 850 rods are interchangeable - and in fact, our shop built a cafe racer from a bike with a blown engine, and did a rod swap with rods from a much earlier 750.

But I was also told that the MkIII rods are different and desirable, as they were shot-peened and are stronger - that might be why the part number changed in '75. I'm guessing it's going to be pretty hard to find a MkIII rod but I'm looking...

ptourin said:

Yes, I'm finding the con rod numbers confusing. As you said - the '73 and '74 parts books have the part number as NM23528, which is similar to the left part of the number stamped on my rods, but not identical.

Click to expand...

Actually, I said the earlier part number for the conrod assembly is NM25369, which is listed from the 1968 parts book to the '74 parts supplement.

I think the only difference between the MKIII rods and garden-variety 750/850 rods is the shot peening. The shot peening makes the rods more fatigue resistant, not stronger. So I would not hesitate to install a non shot peened rod in your engine.

I must add, that I have no way of knowing if the 23258 'R3R1R' rods fitted to my MkIII are the originals-or not. I do know that my MkIII's crankcases are not the original items.

Here's my non-scientific take on the matter:

In my shop, at this moment are the following rods on Nortons; 2 pairs are on 850s, 1 pair on an N15 (750), and the rest are miscellaneous 750-

ALL are marked with one or another variant of '23258', whether it be 'NM23258 RXX...' or 'D 23258 D'

Of the ones with the RXX... following the '23258', I have-

(3) R10R
and
(1) each-
R1R1R
R(?)R (illegible without magnifying lass and spotlight)
R6R1R
R42

As they all measure exactly the same, I'm guessing all that really matters is the '23258' number.

Yes, that's the way it's seeming to me - I'm guessing the "R..." part is some sort of casting batch designation. My bike is a '75 and that part of the number is R5R2R - the first digit is higher than most of the others I've seen. But it appears that all the rods are the same size. I'm guessing that the Norton part number changed in '75 because they shot-peened the rods, not because the casting was different.

Here's some information from the Captain Norton's Notes pages: http://home.clara.net/captain.norton/cnn3sec13.html

"I spoke to Les Emery of Fair Spares several months ago concerning high performance conrods for Nortons and his advise was to never use 850 rods. He said the only rod failures he had ever heard of (not related to oil starvation to the big ends) were the 850 variety. He said the factory changed the alloy in a cost cutting move. Can any metallurgists on the list confirm this? My workshop manual for both 750/850 lists the different rod material designations BS.L83 or 2L65 or L77. It does not state what specification goes with what conrod."

All this got me to thinking, and I realised I had only heard of two instances over the years of conrods just breaking for no apparent reason and they were indeed 850s.

(Fair Spares = who now trade as Norvil)

(as this information is at least 13 years old, then perhaps the "850 rod failures" had not been directly linked to the 'D'-rods at that time, however that is mere speculation on my part?)

ptourin said:

I'm guessing that the Norton part number changed in '75 because they shot-peened the rods, not because the casting was different.

Click to expand...

All the old NM, NMT, etc. part numbers appear to have been superceded by the time the MkIII parts book was published, so the change could have been nothing more than a part number update?

Andover Norton lists both the old NM25369 conrod assembly number, along with the 06-4896 number for the MkIII conrods?

http://www.nortonmotors.de/ANIL/Norton% ... 001&Part=1

NORTON 850 MK III - 1975

Item: CON ROD ASSY. NM25369
Part Number: 064896

I once put Atlas rods in a Commando and the only difference I could see was the oil drilling to lube the piston. I drilled that hole since Norton seemed to think it a good idea. After peening, balancing, polishing etc. they worked fine.