4G Conrod Advice

Singles, twins and fours.
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paul.wirdnam
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4G Conrod Advice

Postby paul.wirdnam » Sun Dec 09, 2018 10:04 am

I've started restoring my '39 4G engine (viewtopic.php?f=40&t=7151) and I'm at the point where I have to make a decision about the big ends and conrods. Here is what I think I know:

1) All my conrods, including my spare ones, are white metal ones and look to be in good condition
2) The bigend journals on the cranks look nice, shiny and are std size but are oval
3) I've read the AOMCC info sheet on their replacement shell-based conrods and, if I understand correctly, these can be used as a direct replacement for the white metal conrods on the 4G engine.

So the decision I'm trying to make is whether to stick with the original conrods and have them re-metalled or go with the new club ones....either way, I still have to have the journals reground. Cost might be a factor, especially as I probably wont ride this bike much (I'm going through a "singles are best" phase). I'll probably use Saunders to do the work as I know they do white metalling and they can also repair the thread on the end of the drive shaft at the same time. I'll phone them tomorrow and get a ball park quote.

Views? Comments? Please....
'28 Model B (future project), '30 Model F, '31 SF31, '35 4F, '38 VB (abandoned project), '39 4G (current project), '46 VH, '48 KG

Paul

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chris.shearwood
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Re: 4G Conrod Advice

Postby chris.shearwood » Sun Dec 09, 2018 1:24 pm

At the time I put my 1946 Sq4 together new conrods were not available (except for the Carillo ones) so I installed the old ones that had an unknown number of miles on them. When I would be out on that bike with a nice stretch of open road ahead of me I was usually thinking about those seventy year old rods: Is one of them about to suffer from metal fatigue and let go right now? Since replacing those with new club ones I no longer have that particular worry. Expensive but worth it for me.
1946 4G, 1950 NG and 1951 VH

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Re: 4G Conrod Advice

Postby MARK.WALSH » Sun Dec 09, 2018 4:01 pm

Hi
I agree with Chris - when your bombing along you don't want to worry about a rod failure. These engines sound so busy when your moving along at speed and new rods will give you the confidence to use and appreciate the finer points of the motor
Mark

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paul.jameson
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Re: 4G Conrod Advice

Postby paul.jameson » Sun Dec 09, 2018 10:13 pm

I think you may well find that new Club rods are cheaper than re - metalling the old ones. As the others have said, if the difference is not greatly in favour of the old rods, the new ones make more sense. They fitted perfectly easily in my 4G and the resulting engine is very smooth.
Paul Jameson
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.

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Re: 4G Conrod Advice

Postby malcolm.johnson » Mon Dec 10, 2018 11:03 am

This is a subject that I will have to address in due course and I understand that the club spares scheme gives a part exchange of £25/rod, for old style rods. I presume the club then sells these "old" rods on, but to who, if it is such a bad idea to use them? Is connecting rod failure on a Sq4, more due to lubrication failure/blocked oilways rather than metal fatigue? Have many owners had old rods fail in a newly well cleaned and assembled Sq4 engine. I do not ride old bikes very hard, but if I did new rods may well be a sensible plan.

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Re: 4G Conrod Advice

Postby roger.fellows » Mon Dec 10, 2018 4:21 pm

I think it's an incentive so that they get binned according to Bruce.

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Re: 4G Conrod Advice

Postby john.mitchell » Mon Dec 10, 2018 5:57 pm

Absolutely, Roger. The club scraps them and it obviously cost money to do that but it saves a lot of crankcases
John Mitchell
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Re: 4G Conrod Advice

Postby malcolm.johnson » Mon Dec 10, 2018 8:55 pm

That is a very commendable incentive. Does this same advice also apply to the later Sq4 rods with separate shell bearings? When I rebuilt my first 4G, it had the later rods which I carefully checked, polished and refitted with new bearings on a re-ground pair of crank shafts which obviously were thoroughly de-sludged. Several hundred miles later, still runs well and I am reluctant to strip again just yet ... or lose too much sleep over either!

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Re: 4G Conrod Advice

Postby nevhunter » Mon Dec 10, 2018 10:07 pm

The choice is yours. You have no way of knowing the history of the rods in your motor, and you have no way of determining the fatigue state of them. You can have them crack checked but that's not particularly assuring with an alloy rod which does have a definite life. As I said in another post JAP speedway rods were only run a season and scrapped, and many years ago when they weren't as old as they are now, well rebuilt Triumph engines were mostly fitted with new rods, if you wanted to use it with some vigour. The decision is probably easier with the metaled ones as the cost of remetalling is about the same as a new one, believe . Good slipper bearings will outlast poured metal ones as well, and undersizes are easily coped with if they are made to those sizes. It's a good idea to have semi finished shells which you bore in the rods to the required undersize, as required. This controls your clearances better. Another way is to fit the undersize shells and measure them and then grind the crankpins to suit. You might/will remove more metal from the pins that way, which is not desired. Nev

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paul.wirdnam
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Re: 4G Conrod Advice

Postby paul.wirdnam » Tue Dec 11, 2018 8:55 am

Thanks all for your advice....new club conrods it is. Now, if only I could get hold of Bruce....
'28 Model B (future project), '30 Model F, '31 SF31, '35 4F, '38 VB (abandoned project), '39 4G (current project), '46 VH, '48 KG

Paul


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