Author Topic: Con rods  (Read 767 times)

Offline Mosin

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Con rods
« on: 22.08. 2017 16:56 »
Hi,

I have a bit of lateral play in one of my con-rods and I am wondering if it is too much and I am going to have to split the cases and start replacing bearings etc, or whether it falls within acceptable tolerances. (there is no 'up-and-down' movement at all).

I have made a little video to show what I mean which you should be able to access by clicking this link:

https://www.dropbox.com/s/nj97ql3wpm716d6/2017-08-22%2016.35.27.mp4?dl=0

Any thoughts gratefully received.

Simon
1960 A7 Shooting Star
1959 D3 Bantam
1994 Triumph Trident 900

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Online Greybeard

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Re: Con rods
« Reply #1 on: 22.08. 2017 18:10 »
The Forum gurus will add their tuppence-worth later. I would not be happy running the bike knowing that there was that much movement in the big-end; I mean, how long is it going to last?! The shell must have a convex surface to explain the rocking yet with only a little up and down play, so not much actual bearing surface left, (see my quick sketch).

My feeling is that as you have got the old girl stripped that far you should open her up and check and clean all of the bottom end properly. While you are about it clean out the sludge trap.

This may be the first time you have discovered the problem of knowing when to stop when owning a classic vehicle. When I started working on my machine I was only going to do a quick fettle but I ended up spending several thousand pounds. Mind you I now think she's worth it.

Online Black Sheep

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Re: Con rods
« Reply #2 on: 22.08. 2017 19:11 »
Yes, too much play. As the engine is out, a bottom end strip is the answer. If you are looking at a rebore as well, may I suggest lower compression ratio pistons? It won't make any noticeable difference to the performance but it will mean no worries with modern day reduced octane petrol. A nice bike to have!
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Online muskrat

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Re: Con rods
« Reply #3 on: 22.08. 2017 20:47 »
Yep, time for a freshen up.
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Online RDfella

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Re: Con rods
« Reply #4 on: 23.08. 2017 18:51 »
Can't open the link, but you said lateral play - ie side to side as opposed to up and down. This is not a sign of big end wear and there is no remedy apart from finding a conrod with a wider big end (unlikely). Is there a difference between the early and later rods width-wise? Not aware of it.
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Online Greybeard

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Re: Con rods
« Reply #5 on: 23.08. 2017 22:59 »
Can't open the link, but you said lateral play - ie side to side as opposed to up and down. This is not a sign of big end wear and there is no remedy apart from finding a conrod with a wider big end (unlikely). Is there a difference between the early and later rods width-wise? Not aware of it.
The video shows quite a lot of wobble on one rod, rather than lateral movement, hence my sketch.

Offline BSA_54A10

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Re: Con rods
« Reply #6 on: 24.08. 2017 10:37 »
I usually slip the gudgeon pin back in the small end.
If you can twist it then the bearings are shot as per the diagram but if the rod simply slides left right it is fine provided no up & down movement.
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Offline coater87

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Re: Con rods
« Reply #7 on: 24.08. 2017 11:12 »
 I agree as far as a tare down goes. You are this far into it, might as well finish.

 That way when you are done, you know exactly what you have and you also know the sludge trap is not plugged solid.

 Just the side to side comparison in your video would make me want to investigate. The timing side looks OK, the drive side is much different. That amount of wobble has to come from somewhere, and its obviously not the bearing sliding side to side on the journal.

 Assuming a PO didnt do something stupid, you have a lot of clearance somewhere on the drive side. Pulling up on the rod might show you when something is completely shot, but your not going to feel a few thousandths extra clearance especially if its still cushioned by oil in the bearing.

 Taring it down costs nothing but time. After its apart is when the bill starts to roll! *dunno2*

 But I really believe not looking into that will cost much, much more later.

 Lee
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Offline bikerboy

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Re: Con rods
« Reply #8 on: 25.09. 2017 01:43 »
I have to go with greybeard and muskrat on this. You are about 15 minutes away from finding out whats causing that and there is no way I would complete that rebuild without having the rods off. A few more nuts is all it will take.

I have to say the pistons look as if they need a bit of work as well I would want them cleaned and measured before I used them for sure.

I dont mean to patronise but IMO if you even need to ask, going by the movement in that conrod, then you should make sure you get a decent workshop manual and follow it to the letter