Rebalance after oversize pistons

Previously les_howard wrote:

The heavier the reciprocating mass the more energy is created that can cause vibration. To maintain the existing balance factor you will theoretically require extra mass to be added to the flywheel to counter balance the heavier piston or more metal removed from the big end side of the flywheels (drilled out) . If the original BF is say 66% then 22gms x 66% should be added= approx 15 gms alternatively if drilling the crank this weight has to be removed and this is likely to be the best method.*

However this will not guarantee that the engine runs with the same amount of vibration at the same revs as the total amount of unbalanced energy has increased in the secondary mode fore and aft (right angles to the stroke). Sometimes a different balance-factor can result is LESS vibration at mid revs, so you might end up with a smoother engine at normal riding conditions, you just never know!

I would calculate the extra mass as a percentage of the original total mass (rings +pin included in both measurements) to get an idea of what the real world difference is.

22gms does not seem to much in my opinion but it is true the lighter the piston the less potential of vibration there is at maximum revs.

Bottom line.... I personally would just fit them ride the bike and forget. But then again you might wish to spare no expense and get the whole engine bottom end balanced but before doing so make sure both rods are identical in weight too...it's your choice. Most people who have a rebore don't even think about rebalacing but the pistons, even if they are the same make, are going to be heavier and years ago many owners fitted big bore kits without any thought of rebalancing .......

* I wonder if you could do the work yourself?

If you were to drill out the required weight dead centre from the flywheel and weigh the swarf, (with modern digital kitchen scales) you could remove the required weight and leave the static balance balance of the crank unchanged ....this is just conjecture for discussion.

Les

Hi Les,

thanks for your very clear and easy to understand explanation. I had assumed that either the piston would have to loose some mass or I would have to add some to the flywheel - not desired as it increases the overall amount. Interesting comment about possibly being less vibration, especially as I do not feel inclined to push the performance envelope.

I had not thought to remove weight from the flywheel, seems like the obvious solution. I will have to look at that. Food for thought.

I am encouraged to do as you would and just fit and forget, funds can easily be redeployed to another part or parts. It seemed a good time to consider this properly though.

Thanks

Andrew

Previously lance_crossley wrote:

Hi, Andrew.

I had my 650SS re-sleeved back to standard bore last year (from +60) but, unlike you, didn't think to check the new piston weights until after the re-build. I had terrible vibration and on stripping the engine down again discovered the new pistons were around 70 gms each heavier than the old ones! I managed to get the outfit who did the re-sleeving to reduce the piston weight by some 30 gms each and this did improve things. However, in spite of the comments in my article for Roadholder, which you will be reading next month, that I would live with this I have subsequently stripped the engine again and fitted another set of pistons from RGM which are much lighter. In fact they are around 25 gms each lighter than the old ones and the vibration is much improved. At 11 gms each in your case I would simply rebuild.

If anyone would like to buy some heavy (381 gms total for each piston, rings, pin and circlips) standard 650SS pistons which have only done around 700 miles make me an offer!

Thanks Lance appreciate your observations, 70gms per piston sounds quite a lot, for me the piston is a lightweight at 230gms incl pin and rings, so my difference is a mere 5% or so. I felt this was significant but have no experience. Knowing where to stop is a tricky judgement so all others comments are a great help.

Previously patrick_mullen wrote:

Over the last 25 years I have had three different Jubilee/Navigator engines bored and fitted with new pistons by professional engineering workshops. None have ever mentioned re-balancing the crank and I have never noticed any obvious problems with vibration. I never even thought about weighing the pistons! So assuming you have not already split the crankcases the simple answer is to fit the pistons and try it. If you are splitting the cases then you will probably want to fit new main/big end bearings as well etc, etc. i.e it can become a money pit with very little gained unless you have the equipment and knowledge to do the work yourself.

Patrick

Hello Patrick,

crank is split now and glad I did, sludge trap was very gritty and the big ends - despite feeling tight were very poor .Fortunately the journals look ok. As oil had a lot of grit in it I decided to replace the main bearings in case there was contamination there. As it happens they looked ok but I found that the timing side of the crankshaft (I think it was that side - the ball bearing side) had been spinning in the bearing. In addition there was a gasket between the two crankcases that looks like a Tesco weetabix box - happy days. Not sure there should even be a gasket. Mainly split the case to check out the layshaft bearing which looks like was caught in time as was a loose fit - must have been so close to seizing. The gearbox bearings are also grim. Very interesting project and learning so much but without some very good friends, a lathe and patient wife I might be in tears. Currently looking to add a good bench drill and grinder. This site is a wealth of info and you folks are very generous with your knowledge.

Regards

Andrew