3/8ths BSC conrod bolts

John Hudson in the NOC rebuild video said there was no reason to replace the con rod bolts, so unless yours are damaged I would reuse (as I have myself), one tip is to look at the underside of the bolt head and remove any burrs which could damage the alloy of the rod if the bolt turns.

You can read this thread starting at page 4 and review ZDF comments ref con rod bolts, Andover and Norvi and form your own opinion. RGM does not come into the discussion.

http://www.accessnorton.com/quality-spares-t19397-45.html

I was taught that rolled threads are considerably stronger than cut threads which made me look twice at Norvil's warning. One thing you don't want to happen is have a bottom end nut or bolt fail. For something so critical I would resort to the old adage 'You get what you pay for'.

The John Hudson Engine Video was made at a time when replacement conrod bolts were a bit iffy with regard to quality. John pointed out that original bolts were made to a very high standard and in his experience, even used bolts, were of far superior quality to most pattern alternatives then available. But.........only if the threaded sections was not stretched or damaged. Like all engine parts, time eventuallytakes its toll and the best course of action will be to replace rather than chance.

Having had the r/h big end exit through the crankcase, on a 750 Commando, if I were rebuilding an engine I would replace the bolts. Obviously a Dominator is not so highly stressed, but, why take the chance?

For the cost of 3x bolts I would think the club could arrange these to be tensile strength tested. There must be a member who has access via his employment that could take the bolts for testing....it only takes a minute or two to do each one although some form of holding the bolts in the machine might have to be made up first but this might not be necessary. These could also be compared to an original used bolt too which would be very interesting.

Les

No John....I mean one from Andover Norton....one from Norvil and one from RGM and one of your old ones...that you are about to throw away. ...and nuts of course too!

Les

I like the idea of having an emergency big end bolt!

The way I see it is as long as you are not going to rev the engine past it's normal rev limit (most of us won't get anywhere near that) and especially if you have a lower state of tune model the originals will serve OK. The crank, and con-rods are the originals so are these then to be discarded for new ones? One might say to use the original bolts is a false economy ....I do... but the nuts are new...Les

Sure is...at 88 quid plus postage I would want them to be made out of Titanium

This is why you need scientific proof of how sound the originals ones are likely to be ... and are the new ones weaker than the originals?

Les

I have so far (40+ years of ham-fisted brutality) torqued up big end bolts using a fairly short spanner to what feels right whilst using thread sealant. I have yet to suffer a nut coming off or a bolt breaking. You have to be a bit careful with torque figures. If the thread is stiff it requires a lot more torque to put the same strain on a bolt than on a lubricated thread. This will be why some manufacturers specify bolt stretch rather than a torque figure for big end (and other) nuts. Some tables specify different torque figures for dry and lubricated threads. A single torque figure ain't always right.

Hopefully with this posting there is an attachment showing all torque settings for the Norton 650cc and bigger engines. This is with thanks to the Andover Norton team who spent quite a fair amount of time getting the numbers correct.

That's a very neat list Phil...it just needs a few extra lines for the 500cc crank and big end bolts. (probably the crankshaft is probably the same?)

However, I have to admit to doing things the same way as Gordon Johnston does....simply by feel although big ends bolts I do use a torque wrench. If you fit spring washers on everything and tighten sensibly with the correct length spanner everything should be OK... noting that smaller nut size spanners are shorter and visa versa which automatically sets a lower torque or higher torque for the same hand pressure feel....This leads me on to point out that using a socket set where the wrench is one length but it fits all sizes of nuts can be a cause of incorrect torquing of fasteners, either over or under tightening.

Where there is a problem with the Norton Dommi engine is that some bolts on the crankshaft are impossible to get a torque spanner on, so I just torque up the ones you can get to with a torque spanner....then swap to a ring spanner and get a feel of how much I need to slightly move them again and duplicate this pressure on the shielded bolts. Fortunately they are locked by the tab washer or by centre punching.

Similarly on the cylinder head there are 3x nuts that you can only do by hand as there is no room around them.

Les

Hi Les........you are quite correct. The only missing numbers are the 15ft lb big end nut torquings for the Model 7 and 88 motors. The crankshaft bolt torquings are the same for all the twin engines.

A neat trick I was shown many years ago can be seen in the attachment. It works very well on those crankshaft and cylinder head bolts that can not be easily reached with an adjustable spring-tensioned torque wrench.

You torque up an appropriate size bolt in a heavy metal plate. You then add the spanner or ring spanner making sure they are as close to in-line as posssible. Next you down-tension the torque wrench until it stops clicking-off when just moving the bolt head. This has now compensated for the extra leaverage created by the length of the spanner to give a very close torque setting to that required.

I now use a torsion bar type of torque wrench on my cylinder head top bolts after discovering that they are far more accurate than the spring-tensioned kind.

That's an excellent tip Phil and if you want to calculate or to check the new figure on the torque wrench, its simply:

(Old length of torque wrench) / (New Combined length of Torque wrench and spanner) x Original Foot Pounds = New foot Pounds Setting.

Thanks Phil.

Les

That makes sense John. The effective spanner length you create with the different angles is the straight line distance between the nut and the point of force you apply to the end of the torque wrench making the actual total length of the spanner combination irrelevant.* To complicate things a stage further, as you need to apply force to a torque wrench perpendicular to its length (90 degrees) the resultant force is not acting purely as a turning force on the nut head as some of the force is either pushing or pulling on the head sideways but not turning it, the actual amount changing with different angles....this in both cases must reduce the torque being transmitted as a turning force....(methinks)

*Many years ago it was thought that by having curved lever arms to actuate drum brakes increased the effective turning force but it doesn't....it is just the same as a straight lever. You see this flawed theory on many old manually driven devices...eg: washing mangles, bicycle pedals etc.

Les

This 90* lever torquing has been argued about on this Forum before. The Mathematical principles are fine but let down by the Physics. As soon as you start pulling on the end of most torque wrenches they pull inwards before effecting the desired force and intended direction of that force. Thereby resulting in an actual lower resultanttorquing. The last thing you would want on crankshaft nuts and bolts.

I agree that you can not always get a 180 alignment of the wrench and attached spanner and the same issue as above alsoapplies to this method.

As pointed out, some experimentation is recommended to obtain accuracy. You could always check using Gordon's hand method...........which I was forced, like many others, to use in the 60s and 70s.

Hi Phil...just in case you thought I was challenging your extension method...I wasn't....I think it is a good sound method and can supply a pretty accurate tightening torque. I will use it sometime.

No....I was replying to Jon Marshall and simply explaining why, when the spanner torque wrench pairing is used at an angle way past a straight line set up, some extra difference occurs, but even so this can be accomodated.

Although I sometimes use a "click" type torque wrench I never seem to feel I can trust it and on occasions set them lower for fear of snapping the bolt by overdoing the tightening. I have often felt tempted to buy one of the torque wrenches with the dial gauge and pointer. This overcomes any stiction or friction in the click mechanism and gives better feedback by being able to watch the needle move up the poundage scale... Someone told me these aren't so good but can't see why.

Les

No problem Les........polite challenges are always welcome if they help lead to a solution to an interesting problem. Somewhere hidden within the historic files of this Forum are a couple of sound practical solutions to this Norton crankshaft nut torquing conundrum. One of them invloved a long cranked metal bar welded to a spanner. A spring force meter, like those used to weigh luggage, was then hung one metre awayand heaved at 90* to the bar until the required scale reading was reached. I don't think that there would be enough room in my garage for such apparatus to be operated.

I think that method would be quite practicable Phil.

So to tighten the crankshaft bolts you would have a total bar length of:

1 x foot pulled at 25 lb weight.

or

3 x feet pulled at 8.33 lb weight

or

1 x metre pulled at 3.44 Kilograms

Les