Con rods

Not open for further replies.
Jan 10, 2006

Some of you, specially Les, who I'm very grateful to, may remember me.

I broke down my engine badly and decided to do a total rebuild.

As everything began with a broken conrod, I decided to buy what seemed to be the best choice for me and my wallet. I was very scared about Norton conrods after that, so I bought a pair os SRM rods.

When I got to place them on the crankshaft and put the crankcases back together, I tried to move the whole thing just to check it turned correctly and found out that they touched the bottom of the crankcase... I opened it all again and used a dremel to remove just a little of the case in order to let the crankshaft and rods turn correctly. Now it goes, but I told this to my mate and he asked me if I had thought that the conrods and the cases would expand a little when the engine's hot, and they could hit back again and break. Now I'm thinking if I sould open it back before placing it on the bike...

All the best regards to all of you, guys!
Hello again sparkplug.

Glad to hear you have been able to make some progress with your Commando. I have no experience of the SRM con rods, I believe they used to sell the US MAP rods but recently they have started to manufacture their own con rods.
I think I remember hearing elsewhere that there was some minor clearance problems with the MAP rods but I cannot say for sure.

If you have removed enough metal from the cases so there is a small amount of clearance then I would guess that should be OK, and the alloy cases should expand outwards at a higher rate than the crankshaft/con rods, so I would think that the clearance between them would increase slightly as the engine heated up?
Thanks for your answer!
Yes, I finally found a guy who used to run the Norton, Triumph and BSA official service in Madrid back in the 70's. I knew him before I found out he was the right guy to ask this favour to. He's been really cheap and has prooved to be really honest and professional.

The engine's been completely done and I'm about to install new vernier isolastics and to place new bushes into the swingarm. I'll finish the gearbox and put everything back together in about 2 weeks. It's been a year now I can't ride my Norton and I've missed it more than I ever thought.

I'll be back on the road very soon!!
I'd be very careful that there's enough clearance. Remember that it's not really thermal expansion you have to think about, the crank flexes in use. That's why superblend bearing are used, to allow the crankshaft to flex a bit.
I don't know how much they flex in normal use, maybe someone here does.
You should also check that the rods are the same width as the standard ones as some aftermarket rods (M.A.P. for example) require the clearance slots at the bottom of the cylinder barrels to be widened.
I've put the pistons and the barrells together and it seems it turns fine. I imagine I've done it right about the width of the clearance slots of the cilynder barrels because, otherwise they would hit the conrods... Am I wrong? :?

You're very right Pommie John, That's another very good point I need to know... How much would superblend bearings let the conrods approach the crankcase when the crank flexes?

This afternoon I'll close the engine with the cylinder head.

All the information and advices are much appreciated!
Just in case someone is interested, here's what SRM answered about this matter:

"on the pre-unit Triumphs this also happens, but i have not seen it on a 850 commando.
No problem to machine some metal, but leave at least 1.5mm to 2mm clearance"

Hope they're right!
Not open for further replies.