Con rod damage

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Colin_McKellar

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Aug 11, 2008
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Further to my previous postings about the problems that I have had when my newly rebuilt Mark 3 engine after it broke up one of the main bearings and put a hole in the crankcase I have finally got round to getting it stripped again.

What I have found so far is the right hand piston with a small piece out of the skirt at the back, both conrods are rough with tiny nicks in them on the back but I am told that these can be polished out. What bothers me more is the right hand rod has been hit by one of the loose bearings and has an indentation for a couple of millimetres on the front for about a centimetre long.

What I am wondering is how much damage can a con rod take before it needs to get replaced. In an ideal world I would just go out and buy new rods, pistons and bearings but this is going to have to be done on a budget as my boss has put me on a 3 day week so the mortgage comes before Commando parts.

Either that or someone make me an offer for the whole lot!!
 
Hi Colin,
of course that will cost you more money , but if I was you , I'll go for changing the rods, have a look as they were from a MK3
they could be the "D" rod (should be stamped on) and so one more reason to change them.........
 
Hi Colin,

I wouldn't take a chance, replace them!!! I have some if you need a pair (everything I have now has Carrillos!)
 
Hello Collin,

Take Seeley920's very kind offer.
Don't ask what amount of damage a rod can accept before failing.
Ask how much damage a rod can do.

Being Mk3 watch out for these "D" rods. I had only one of these inventions of the devil in my 77 Mk3.

Con rod damage


Con rod damage


Regards Graeme
 
My rods were replaced a few years ago during a previous rebuild. There is only about 1,500 miles on them.

I will try and get them off the crank at the weekend and post a couple of photos.

I appreciate the offer Seeley920 and I may take you up on your offer sometime soon.
 
Replace them. Even the nicks from dropping on the crankcase mouth or catching on a stud can provide a starting place for a fracture.

It's a shame but just not worth the risk.
 
recently I got so frustrated with my lack of progress that I went out and bought a 1987 Yamaha FJ1200. What a difference to the old Norton. Not only did the whole bike cost less than the bits I have already bought for a non working bike but it goes really well and still does over 50mpg.

I could buy an engine for the Yamaha for the same price as one con rod for the Norton!!

Has anyone tried to shoehorn an FJ engine into a Norton Commando frame before? lol

Imagine 130BHP in that frame with those brakes!!
 
what if one were to polish out the rod then shot peen it? i don't have any first hand experience with broken conrods, but is there seriously that much damage that could be inflicted from a few nicks? please don't take this as being an ass by any means. i would like to truly understand this and whether or not this is just old thought that has continued to be passed along as fact. just like the controversial oil and engine break in threads, they learned it from this guy, who learned it from that guy, etc......
 
Colin_McKellar said:
recently I got so frustrated with my lack of progress that I went out and bought a 1987 Yamaha FJ1200. What a difference to the old Norton. Not only did the whole bike cost less than the bits I have already bought for a non working bike but it goes really well and still does over 50mpg.
Click to expand...

Yeah, but it's still only a Yamaha.
 
Can you post some pictures? Usually when a piston skirt gets a piece knocked out it is because a rod bent enough to allow the skirt to hit the flywheel. What flew out the crank case? I'd think if a main bearing went to the extent rollers were exiting the crankcase your rods could easily have been bent or twisted, it's hard to imagine that's all the damage.

You can imagine if you took the rollers out of the bearing how the crank could wobble around till it stopped rotating?
 
I just read your previous thread on this subject and have a better picture of it all. My sympathies friend.

I like lab's suggestion on the original thread that a roller was knocked out of the cage during assembly and it was getting banged around like a ping pong ball inside. The crank would still be held in place and the damage to the piston, rod and case is conceivable.
 
Hello Collin,
I understand your frustrations with your Norton. I'm building a Mk3, my first Norton, (and first English bike) and it's very frustrating and off puting when you start finding all the things that are built second rate with no pride what so ever.
(for all the Norton regulars I appoligises for hanging crap on these things) But I've built European bikes of the same age and older and I thought they did some stupid things. But not anywhere as bad as my Mk3.

Yes you can buy a Japanese bike for way less and it'll be reliable and go really well, but it's not a Norton. Or an old Ducati for that matter. But it is a nice old thing, FJ1200's are nice to tour on and go very well.
What kept me going with this piece of poorly made brunelian nightmare was it has character. (you can define "character" however you like) And I figured if I thought of it as being designed in the early 50's it wasn't that bad.

My rebuild seems to go from one thing that's rubbish to the next thing that's an insult to machining, etc.

Perservere, and from what I've encountered so far do what's right, not what the manuals say.

I've found the price of parts very reasonable, compared to old Ducati bits, and easily available. Watch the quality though.

Gavin, a nick or a deepish scratch can be a week point on anything that is stressed. they can be polished out but do you really know if the structure of the material has been altered deeper than you could polish.
I've seen the damage a broken rod can do. It's not worth the risk in my view. Mine had a "D" rod, only one though. Bought a really good second hand rod for $135.00. I know I should have replaced both with new ones, but it's not going to be used as a race bike.

Collin, look at it this way, now you've got a Yammy that's getting harder to get, and a Norton that you can take your time building when funds or bargain parts become available.

Good luck,
Regards Graeme.
 
Con rod damage


Con rod damage





I'm hoping that the two images of the rods come out OK

______________________________________________

(Edited by the moderator = photo codes corrected )
 
Collin, they look like "D" rods, look at the Norvil site, see tech issues, then "D" rods "Demons be Gone"
Graeme.
(sorry)
 
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