Dynamic balance ?

Why bother? The model 7/88 engine ran quite smoothly, as standard. Why waste money "improving" a machine, that will never be faster than a modern 250?, and, may well shorten it?s life.

Hello crankshaftbalancing is best done by Steve Manely Middletonengineers or SEP engineering . at the cost of around £130 pound notes but having saidthis is really up to you at the end of the day The Norton Owners Club members can only speak from their own experiences and there for only guideyou and cannot be made accountable from any endeavoryou may encounter has it your decision in the end yours anna j happy riding

I did not mean balancing, itself,would shorten engine life, but many people try to boost the performance of old engines, in some cases well above what was intended. The natural tendency is to ride faster, which increases wear, and tear, on components operating above their designed limits. Unless you are into classic racing (which a model 7, or even an 88, were not designed for), spending an extra 130 pounds will make very little difference. That said, what do you know of the engine?s history, and general condition? I see you are enquiring about new pistons, so, is a complete rebuild required? If so, it is doubtful if you will find pistons the exact weight of the originals. Since a rebuild will (hopefully) a one off exercise, it would be worth taking the pistons, crank, and con-rods, to be balanced together.

Ni Marcus,

Just thought I should add a few points. A parallel 360 degree twin is much the same as a single with regard to balance but with the added complication that one cylinder is sucking and squeezing while the other is banging and blowing. Modern designs utilise a secondary balance shaft to help damp the vibration but it will never be the complete answer and of course with a Model 7 you've got no chance. Nortons used a balance factor of somewhere in the region of 50%for most of their touring bikes, but if you read the road tests of the day they very often mention a period of vibration around 55 mph. Dynamic balancing is an elaborate versionof having your car wheels balanced and that gives you the opportunity of making the smoothest period of vibration happen at the speed you use the most, i.e. your cruising speed. What you willprobably find is if you have the best balance at say 60mph the bike might be rough at 30 mph. You will probably find the bike will be smoother than it is now, but don't think this is a cure all process. If you do decide to go for this decide what speed range you most ride at and work out what engine revs that equates to in top gear and ask the balancer if it will be possible.

Regards, Richard.

Previously marcus_bloomfield wrote:

Previously richard_cornish wrote:

Ni Marcus,

Just thought I should add a few points. A parallel 360 degree twin is much the same as a single with regard to balance but with the added complication that one cylinder is sucking and squeezing while the other is banging and blowing. Modern designs utilise a secondary balance shaft to help damp the vibration but it will never be the complete answer and of course with a Model 7 you've got no chance. Nortons used a balance factor of somewhere in the region of 50%for most of their touring bikes, but if you read the road tests of the day they very often mention a period of vibration around 55 mph. Dynamic balancing is an elaborate versionof having your car wheels balanced and that gives you the opportunity of making the smoothest period of vibration happen at the speed you use the most, i.e. your cruising speed. What you willprobably find is if you have the best balance at say 60mph the bike might be rough at 30 mph. You will probably find the bike will be smoother than it is now, but don't think this is a cure all process. If you do decide to go for this decide what speed range you most ride at and work out what engine revs that equates to in top gear and ask the balancer if it will be possible.

Regards, Richard.

Hi Richard,

Thanks for the info, I will think it over some more, while your there, any idea if iron head and alloy head have the same pistons? I have alloy head 54, can only find good iron head pistons,

Marcus

Hello in answer to your query its NO they are NOT the same compress ratio, the ironheads were 6.5.1 and alloy heads were 7.5.1 and pistondesigns were both different has the very early piston had a split the piston shirts and strengthen bosses wherethe gudgeonpin fits the model 88 of 1954 has a high piston crown and has drill holes around the oil ring and no split shirt and are lighter and in 1954 there were 9.1 pistons has an optional extra for a cafe racer or racing the choicewas yours but now with GPM Piston and JP pistons only readilyavailable the GPM are the heavierof the two sets and you must have the barrel bored to the pistons dimension and use the GMPpiston rings you cannot mix up the piston rings with Hepiloite or BHB Or wellworthy or Hap Jones or Novo -covo has the radiuses are different and ring end gaps should no more than 8 thou, and piston to bore clearance no less the 3 thou and you must have a good cross hatch in the bore. now have fun yours anna j

Hi again Marcus,

Thanks to Anna for the info. As she stated piston weights willvary from one manufacturer to another nowadays and even the originals will vary with over-sizes but within a small tolerance. Also, as Anna said the later pistons will be heavier if they are high compression. If you do go for dynamic balancing this won't be a problem as the balancing machine works out the adjustments. If you statically balance the crank and have the original pistons and some digital kitchen scales you can make the calculation with a pocket calculator to keep the status quo, but if you want to modify the balance, it's a guessing game. As a rule of thumb the faster you ride, the heavier you need the counter balance on the flywheel (effectively making the pistons lighter)from the standard balance factor.

Regards, Richard.

don't know if this will help in anyway

http://www.classic-motorcycles.co.uk/cgi-bin/ecom.cgi?Command=ShowProduct&db_pid=4713

Barry: That piston shown is indicated to be Left Hand, I think. The only right hand one I saw was for a +.030 bore.

sorry about that Michael i just saw it said twin so assumed that would be for two pistons of the size required.

Barry

I agree with John Shorter. The model 7 with it's low performance is relatively smooth as standard particularly if you have the cast iron head as this "ton" weight acts as a "mass damper" so heavy it naturally resists high frequency vibration.

In one of the monthly mags (classic Guide I think) Ian Legg rebuilt an iron headed M7 and had the crankshaft dynamically balanced. The photo showed it had the treatment and I was amazed how many huge holes had been drilled in it making it look like Swiss cheese. As I like to keep things as they were historically, that was far too much of a corruption to accept.

Les

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Previously les_howard wrote:

I agree with John Shorter. The model 7 with it's low performance is relatively smooth as standard particularly if you have the cast iron head as this "ton" weight acts as a "mass damper" so heavy it naturally resists high frequency vibration.

In one of the monthly mags (classic Guide I think) Ian Legg rebuilt an iron headed M7 and had the crankshaft dynamically balanced. The photo showed it had the treatment and I was amazed how many huge holes had been drilled in it making it look like Swiss cheese. As I like to keep things as they were historically, that was far too much of a corruption to accept.

Les

Hello LES this Cheese looks more like jasburg Dutch cheese to me it lovely have fun yours anna j