infamous D connecting rods

So i was digging my donor engine out of the corner of the garage in preparation for a rebuild, and when i look at the connecting rods I noticed that one had a "D" mark on it. I have read enough about the horror stories about these rods that I do not plan on using it, however only one of the two rods in this 75 eninge had the "D" stamp. :?



The other one does not have the "D" marks:



Is it possible these are both original, or is the non-d rod most likely a replacement? I plan to have this and my other good rod checked when i get the crank worked on, but is there anything I should be concerned about with two different generation rods? (one out of a 73, and one out of a 75)

mjfriesen said:

is there anything I should be concerned about with two different generation rods? (one out of a 73, and one out of a 75)

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Not as far as I know, as all Commando rods are basically the same part, although the casting "R**R" (or R*R*R?) numbers on the "good" (non-D) rods are often different.

L.A.B. said:

mjfriesen said:

is there anything I should be concerned about with two different generation rods? (one out of a 73, and one out of a 75)

Click to expand...

Not as far as I know, as all Commando rods are basically the same part, although the casting "R**R" (or R*R*R?) numbers on the "good" (non-D) rods are often different.

Click to expand...

Thanks for the info!

As a side question - any recommendations on a brand of 1/4W socket to purchase to undo the rod bolts? I had a fairly cheap and poorly made socket that i ground down so it would fit, and it managed to survive long enough to get one con rod undone.

I think the only trick to the rods is making sure they weigh the same.

Most important rod mod is to remove any swarf from the bolt seats
and make sure bolt head does not shave down a bit of swarf to
prevent square full seating. Bolts often need a head clean up.

Next is the outside finish, smooth and compacted as practical.

3rd is the bolt nip up done by stretch IIRC .007" here.

4th is mix-matching-milling rod, piston and pin to equal close as practical.

Interesting mis match of rods, did one of a double D rod set let go and better
rod put back or was a weaker D rod put in by better rod failure or
factory grabbed what was a hand? There was known sabbatage
in the later days.

hobot

I use a 'King Dick' 3/8" drive socket. I don't remember ever having access problems. A 3/8" - 1/2" drive adapter avoids the need to buy a 3/8" torque wrench.

79x100 said:

I use a 'King Dick' 3/8" drive socket. I don't remember ever having access problems. A 3/8" - 1/2" drive adapter avoids the need to buy a 3/8" torque wrench.

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Thanks for that info. I have a really cheap no-name whitworth socket set that i almost threw out as soon as i bought them. they were a nice price, though! :roll: The set is now short a 1/4 inch socket. Fortunately i have been able to make due with a couple really nice open and box end wrench sets in the past. not for this job, though.

hobot said:

Most important rod mod is to remove any swarf from the bolt seats
and make sure bolt head does not shave down a bit of swarf to
prevent square full seating. Bolts often need a head clean up.

Next is the outside finish, smooth and compacted as practical.

3rd is the bolt nip up done by stretch IIRC .007" here.

4th is mix-matching-milling rod, piston and pin to equal close as practical.

Interesting mis match of rods, did one of a double D rod set let go and better
rod put back or was a weaker D rod put in by better rod failure or
factory grabbed what was a hand? There was known sabbatage
in the later days.

hobot

Click to expand...

Thanks for the tips about rods - i guess it goes without saying not to re-use rod bolts? Or does it?

I was kind of curious about how they got there myself. this was from a parts bike my brother and i picked up about 22 years ago from a BSA guy that bought it as a package with a lot of BSA's. He just wanted it out of his garage, and let us have it for just about nothing. I have no way of knowing how long it was in "parts bike" state, so I always guessed it had very few miles on it and was relatively untouched. I do know it was smacked in the rear wheel area pretty hard, because the rear of the swingarm is bent to the side a good 1/2 inch!

British Tools & Fasteners?: http://www.britishfasteners.com/mm5/merchant.mvc

A 3/8" square drive set is often more use for this sort of work than a 1/2" set, as the thinner walled sockets allow better access.

Abingdon King Dick are high quality tools.
http://www.kingdicktools.co.uk/

My '77 Mk3 had one "D" rod and one good rod.
I think it was built with what ever was left in the reject parts bin.

graeme

hobot pretty much said it all about the care and feeding of your connecting rods, spoken like an expert. I would add only that if you are at this stage and level of an engine rebuild I suggest that you find a local service that can bead blast your rods' beams and shot pean the caps; enexpensive and quick.

In the event that your paternal set of rods was the result of a previous engine catastrophy (look over your cases), you may want to get all your final assembly crankshaft components magnafluxed and balanced, again inexpensive and quick. If you pay attention to your project at the same depth as your questions, you can easliy build an engine that is more reliable than what came out of the crates, faster too.

I am about 3-4 weeks away from building my second engine, a 750 this time; I can't wait. I'm sure that your project will be illuminating and quite entertaining.

RS

Polish the rods and make sure that they are absolutely smooth. I have not had a problem reusing rod bolts (yet... ). I do use new nuts everytime.

The balance figure I was given (between rods, pins, and pistons) was 1 dollar bill. It was a long time ago, nobody knew anything about grams.

Greg

Of all the fasteners to take no risk of unknown history or state are rod bolts/nuts.
Find chart or real Nortoneer and get the stretch spec for nip up.
My notes show .007" but 25-26 ft/lb only gives .006to .0065.

A unseen fracture line can develop that exposes the innards to
the corrosives and etch away till takes out a whole engine,
just lolly gagging along 50ish almost coasting, tinkle-tinkle, SIEZED.

I'm about to get act together to put 750 back this happened to
2 year ago.



Can ya see which bolt let go first and which gave up doing its duty?


hobot

Thankfully, I've never suffered a catastrophic failure in 35+ years, over 140 bikes owned; 10+ personal ovrhauls and 17+ client overhauls.

Others definitely have different experiences, but believe it or not, I've always gone strictly by the book (fully enough, it's been a mix of Haynes, Clymer AND original shop manuals) and built conservatively. That includes tolerances, torque specs, lubricants, etc.

I got away with aluminum rods for many years. But I wouldn't do it any more - those rods are just too old and risky. Aftermarket alum rods are way too heavy on the small end and add to vibration. The standard Carrillos are also too heavy on the small end. New factory rods are more expensive than Carrillos. That's why www.JSmotorsport.com went to so much trouble designing special bushless Carrillo rods requiring DLC coated pins that are lightweight on the small end.

hobot said:

Of all the fasteners to take no risk of unknown history or state are rod bolts/nuts.
Find chart or real Nortoneer and get the stretch spec for nip up.
My notes show .007" but 25-26 ft/lb only gives .006to .0065.

A unseen fracture line can develop that exposes the innards to
the corrosives and etch away till takes out a whole engine,
just lolly gagging along 50ish almost coasting, tinkle-tinkle, SIEZED.

I'm about to get act together to put 750 back this happened to
2 year ago.

Can ya see which bolt let go first and which gave up doing its duty?

hobot

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Hobot, I don't know your real name but I'm absolutely sure that you are in some way related to the famous fictional hero 'Ernest Thrasher'

I've seen some blown up engines but there's usually been a semblance of a suggestion that the riders at least eased off once a rod had exited...

Keep up the good work.

To set record straight, hobot engines have not been damaged by hot roding them.
Above photo is 2nd blow up of Trixie.

Trixie 1st seized just riding timidly on newly acquired Combat, flowing with
traffic, examined remains to find old style oil slotted pistons and and
**Aluminum** head gasket that had finally deformed into the bore
so weak pistons were rubbing against gasket and picked me to let
go on. What could i have done to avoid but complete tear down
on fine running engine. I've photo's looks about as bad as one posted.

2nd seizure was after switching to Ms Peel no longer needed pistons and cast iron
barrel and rods. But only used new rod nuts not bolts thinking to
get on the road and never intending to hard rev ole Trixie.
300 miles later, coming home from work, about coasting watching
fields of yellow flowers that prefect matched Trixie for photo
spot, heard a half second of tinkle sounds then silence but for
rear squealing locked at 50 mph. Clutch pull gave free rolling.

Ms Peel Conbat first rebuild I wrung the snot out to 7600 in 2nd at
times, in turns btw pulling sideways wheelies not boring straight line sprints.
She was built to take this over 5 years and did.
But what did this engine in was a stuck throttle on start inside
shed - tach needle disappeared for a hand full of seconds,
but I rode her another 2000 miles - though down on power
afterwards above 5000 rpm/110mph.

Fate is the Hunter.
hobot