con rods tightening on assembly

Hi , I am in the process of assembling the bottom end of my '73 850 and have come across a problem when attaching the con rods to the crankshaft . The crank journals have been ground to + 30 by a reputable local company but when torquing up the new con rods, with correct size shells etc I found that the movement of the the rods was tight with a notchy feel. I have tried hard to ensure that things have been correctly put together and cleanthliness observed. On disassembly I noticed the shells had scores inside of them - am I right in thinking they are now scrap ? Surely the rods should rotate freely and smoothly at this point ? Feeling a bit unsure /insecure at this early stage of what will be a extended project so would really appreciate any advice. Thanks in anticipation and Happy New Year.

sounds like you have a nick on the journal if you have scored the bearing,take it all back to the machinist who ground it and get him to assemble and torque it up :roll:

Make sure he knows about the .090 radius in the corners.

Have you got the bottom caps on the right way? Try reversing the bottom caps. i.e. rotate 180 and refit, torque gently gradually on each stud. Rod ends on a correct resize grind and matching size new shells must be smooth and consistent all around the big end. use a light grade oil or break-in lube. If that doesn't work, you'll have to remeasure the big ends with a micrometer in the centre and sides of the big end journal at 4 intervals around the big end. It could be a misaligned grind, not good.

Mick

I would think the machinist would have marked the shells with a marker to ensure they go on the journals right side and the right direction.


good luck

The bottom caps normally have a line stamped across their edge that goes onto the mating side of the con-rod, so it should be easy to see whether rod and cap were originally together and with the same orientation, this is important as they are machined as a matched pair, also make sure the securing tabs of the upper and lower shells are both on the same side. You can generally see the outline of where they were on the mating faces of the rod and cap.

Thanks for your advice. Mick - regarding your comment on the end caps being on the right way round - should the locating grooves for the shells be on the same side of the big end eye or opposite each other ?

Thanks Dave m - just seen your reply.

Did you ensure the shells and crank were well lubed prior to fitting the rods? The rods will be almost impossible to turn if there is no oil used when fitting.
Steve J

Hi I have had this problem with a 650 ss, The shells where too big, when bolted together they bulged..ended up filing the faces off...yes sounds "not on" but that was the fix...parts are poor today....and they came from a Norton supply company.

john robert bould said:

Hi I have had this problem with a 650 ss, The shells where too big, when bolted together they bulged..ended up filing the faces off...yes sounds "not on" but that was the fix...parts are poor today....and they came from a Norton supply company.

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What John is referring to is excessive bearing nip, and the most common cause of that is someone filing the rod or cap to give them less clearance, the bearing nip is essential to hold the bearings in place during running. This is roughly how I was taught to assemble a crank and rod assembly.
1/ check rod alingment
2/ check rod tunnel diameter
3/check bearing thickness, more of a check to see if they were all the same size
4/mount bearings in rod and tension nuts, loosen one nut and the bearing nip should push the rod and cap apart, 0.003-0.005''
5/torque nuts up and measure diameter with bearings fitted
6/ measure crank and and work out clearance/ interferance

As you fit larger undersize bearings they get thicker and dont conform to the rods as easy, excesse nip or distorted bearings shows up as rubbing not scoring like you described, excesse nip normally rubs on the parting faces
Check your sockets aren't pushing the cap as you torque the nuts up, if you have rubbing and not scoring give the rod and cap a hit with a plastic mallet on the sides and on the bottom of the cap, it will help to seat the bearings and align the rod and cap,if you have scoring swallow your pride and go back to the shop that ground the crank and get help

Long winded I know but short in the long term.

Thanks for all your comments I have managed to get the rods to rotate freely using a combination of your advice just a couple of other points I hope you can help me with. Once the nuts have been torqued do they have to be replaced or is it useage that determines the need to replace them? Do you recommend the use of threadlock here? Should the shells be replaced if they show any signs of scuffing/marking? Also - sorry I know this is a bit of of different thread- but there is a small circlip inside the timing side crankcase where the intermeadiate gear shaft fits through . Is it necessary to replace this circlip is its function to act as a stop for the shaft or has it anther role?. Sorry if these questions are a bit tedious to some contributors but I was always taught as a kid to ask if you're not sure. Thanks again. Teesrider.

The con rod nuts are single use lock nuts and should not be used once they have been tightened up to the specified torque. I normally use the old nuts to check clearance and then when satisfied with this I use the new nuts, the con-rod bolts can generally be reused but you should ideally measure them for any stretch outside of spec. before reusing them.

The circlip behind the intermediate timing gear shaft is only to establish the correct depth of the shaft, the shaft itself should be a tight fit in the bore and if not you might try some loctite bearing fit in there. The shaft itself should have the oil drain hole facing down as I recall.

Well what did the problem turn out to be exactly? You were a bit vague about the solution.

Russ

teesrider said:

On disassembly I noticed the shells had scores inside of them - am I right in thinking they are now scrap ?

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teesrider said:

Should the shells be replaced if they show any signs of scuffing/marking? A

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These two statements make the condition of the bearings sound questionable. In new condition they will last many miles. Once heat, oil contaminates and just too many miles have caused inevitable wear to the bearings their life reaches the end and damage occurs to surrounding parts. It sounds like your shells have a head start toward a short life. I'd replace them. It should be a long time before you have to get in there again if you do it right the first time.