I'm fitting +0.020" pistons to my navigator, not so much because of wear but corrosion on the standard bores.
I've bought a pair of "JP" pistons, all I could find. Each of these are some 50 grammes heavier than a standard +0.020" piston. Therefore the need to check the balance factor.
Anyone know the balance factor for a Navigator engine please?
I cannot say I'm impressed with the quality of the JP pistons. I can reduce the weight of the pistons a little with the usual modifications. The gudgeon (wrist/piston) pin looks like its been drilled with a blunt drill! No tapered bore either. More work to sort that out. And, why the centre hole in the crown of the piston?
Balance Factor
Garrett lists the balance factor for a Navigator as 70% (same as other twins)
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Ian, thank you for the…
Ian, thank you for the information. I'll use 70% when I balance the crank, I'll also "work backwards" to check what the crank has been balanced at.
Kevin, the JP pistons are the same in the important dimensions and crown shape. I do not know what pictures you have been looking at, if its F W Thorntons then they just use generic pictures.
I tried several different dealers for pistons, seems they all supply the readily available JP piston.
I've attached a photo of the piston for comparison.
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JP Pistons - beware
Hi all,
JP Pistons have a different factor of heat expansion - they are made from an alloy more suited to water cooled engines. Your re-borer will need to factor this is in when matching your pistons to the cylinder bore & allow extra room for expansion.
You or he will need to do your homework on this one - I have no figures for using 'water cooled' pistons in an air cooled environment.
Also, you will need to rebalance the crank as, as you found, the JP Pistons are heavier.
NOC Spares have +10 Hepolite pistons in stock - that surely is the way to go! Proper 'air-cooled' pistons with no need to re-balance the crank!
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The clearance JP give for…
The clearance JP give for the pistons takes in to account the higher expansion alloy used.
I would have used the +0.010" pistons from the club if it had been possible, problem was corrosion. I've bored the cylinders ready for honing and there are still some corrosion marks. I think they will be okay after honing, if not I'll re-sleeve back to standard or +0.010" or may be Electra pistons?
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Rebore Navi for Electra piston?
Hi John - sorry, missed tour reply.
Reboring your Navi barrels, going out to Electra size (66mm, rather than 63mm) is a no-goer. The Electra barrels are taller (to the height of the spigot) and fatter at the bottom. The crankcases are opened out to accomidate this, whilst the flywheel is thinner to accomodate this.
Sadly, you would need to procure specific Electra barrels (and heads!!) to do this.
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Electra pistons
Andy, I agree that boring Navigator barrels to take Electra pistons is a non-starter.
But I think there is a way. Sleeving navigator barrels looks like it might work with each sleeve offset outwards with the crankcase modified to match. Then the flywheel would not need thinning, the cylinders could be cut away in this area, they are not doing anything down there! The conrods look like they could accommodate the offset, ~ 1 to 1.5mm.
For the head joint there seems to be a couple of options, no spigots, depends how far up the cylinder the top ring goes, or eccentric spigots.
This has been at the back of my mind since dismantling the Navigator engine, I've only been measuring with a rule but so far no show stoppers. I'm not intending to do this with the engine I'm rebuilding, I'd like to find another engine.
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Hi John.
I'm afraid that I don't know about the balance factor, but surely the piston head shape is wrong. I've been looking at the various shapes on their website and as far as I can tell, nothing matches the proper thing. Even 1C doesn't come close. I think I'm right when I say that the Jub and Nav pistons have basically the same head shapes?
Having said that, I ordered Jubilee pistons from Sheffield Classic M/cycles on 01246 290021 on 9th April and I'm still waiting to receive them. Getting worried!