Weight of Navi Piston

I would not describe a JP piston as an alternative, its a last resort only !!

Just been in the shed and weighed a few Navigator pistons (and a few others for comparison) used +20;  246.8 & 236.8g.   +30 no rings; 232.8 & 232g.    +40;  258.5 & 257.5g.

new +30;  251.9 & 251.2g    +40; 254.5g

used jubilee no rings 229.2g and just for kicks; Kawasaki zxr600 piston 67mm (Electra 66mm) 201.3g, CG200 63.5mm (Navigator +20?) 188.3g.

Hope that is of interest,

Barrie.

There are 3 issues with JP pistons.

1. In order to keep costs down, for the typically small batches they produce, they cast a limited number of base castings. Hence most pistons they make are over weight, for single and twin cylinder engines without balancer shafts you end up with higher vibration.

2. Their rings are useless, too stiff and take far too long to bed in. They also go for thicker rings so it's difficult to get decent alternatives.

3 Quality is suspect, missed machining operations are not uncommon.

I had similar issues with our Atlas , in the end I re-used the old Commando pistons ,even so I found the pistons were much heavier than the orriginal low comp ones and most of the added weight was in the steel  gudgion pins  which were about double the thickness . Also the seemingly identical pistons were different to each other. I have a beautifull set of 100 year old cased  lab scales (found in a skip) which can weigh a human hair so had fun with that!.

Andy/all: was there ever an "official" /factory balance factor given? If there are piston issues that cannot otherwise be resolved, is a crank rebalance feasible/practical?

A crank rebalance back to the right % will get the vibration back to the right direction but the pistons are still heavier so against the right % balance but lighter pistons the vibration will be higher.

Static balancing sets the % balance, this was decided by the factory and dictates the direction the vibration goes, the piston and 1/2 of the conrod is always out of balance the % just directs it somewhere between fully up or fully horizontal and found by trial and error test rides. You are looking for a direction that reduces the resonant vibration in the frame and the attached parts. This is done on knives or rollers and the crank assembly has weights added that make the % of the out of balance weight and then balanced so it stays in any position. The crank assembly is still unbalanced by the same weight but in a fixed direction set by the %.

Dynamic balancing is when the crank is spun, but on a permanently out of balance crank this only treats any side to side variation, not the out of balance weight of the pistons and 1/2 the conrods.

All the UK factories only did static balancing with the notable exception of Royal Enfield and the Interceptor twins. So you can get a good result from having the static balanced checked and a dynamic balance. How much better is a lottery both for the static and the dynamic due to the variation as it left the factory and what changes have been made.

The lighter the pistons the less out of balance the crank assembly is, the % only sets the direction so the cycle parts do not resonate too badly.

Andy,

After a long search I have just managed to source a new set of original Navigator +20 thou Hepolite pistons.   If you still need more data I will weigh them tomorrow and let you know.

Nick