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Navigator cylinders


Good afternoon everyone.  I have just bought a Navigator, or at least I thought I did.  I've been going through the bike, seems OK, never going to win any prizes for finish but seems to be mechanically sound if somewhat oily.

While checking the bike over I noticed that I could slip a steel rule between the cylinders, rather worrying that, I thought Navigators had a one piece cylinder block. Comparing pictures of the Navigator and Jubilee cylinders it looks like the engine has been fitted with two Jubilees cylinders. 

So, what does this mean?  A smaller bore is the obvious thing.  What about con. rod length?  How can I tell if the engine has Navigator or Jubilee heads.   The crankcase are Navigator, it has a "19" stamped adjacent to the engine number which, I believe, identifies it as Navigator.

The rest of the bike seems to be Navigator, forks and brake are the right ones.

Any comments would be most welcome



Hello I too are in the process of dismantling the Navigator Engine and now have the barrels of and there 30 over size and one peace barrels So you have Jubilee's barrels try sores a set from the club along with pistons that may do an exchange if you ask or context our Dan. The light twins Man yours. Anna j


John, as Anna says above if your engine does have separate barrels then they are from the Jubilee - another check can do to be absolutely sure is to count the number of cooling fins on the barrels.  The Jubilee has 6 and the Navigator 7.   As your crankcases are stamped 19 then they will be from a Navigator - the Jubilee cases are stamped 17.   The Jubilee stroke is 44mm (1.7323") and bore is 60mm (2.3622") and the Navigator stroke is 56mm (2.205") and the bore is 63mm (2.480").   That means the crankshafts are different and so are the conrods.   The Navigator rods are slightly longer at 4" 5/16" centres and slightly wider at the small end than the Jubilee but I'm afraid I don't know how long they are - perhaps some one who has a set of Jub' rods can measure them and let you know.   Although the crankcases are the same casting the holes for the cylinders will be wider on a Navigator to allow for the wider barrels.   The camshafts are different between the two engines too.    The cylinder heads are again the same casting and the valves are identical but if the cylinders are the earlier ones with the top spigot then they will be machined to fit the appropriate model.   The carb on a Jub' is an Amal 375/43 and the Navi' uses an Amal 375/48.   The other difference is the gearbox and rear wheel sprocket.   The Jubilee has 17 front and 55 tooth rear sprockets and the Navigator 19 and 52.   In my experience there is a significant difference in performance.   With a light weight rider (sadly no longer!!!) a healthy Jubilee might top 70 or a little more on a good day where as a Navigator feels noticeably more spritely and a good one will make 85 or perhaps a little more if you believe the infamously inaccurate speedo!   How does he bike run and perform?   If you want to know more about the implications of a Navigator engine which has been converted to run with a Jubilee crank, barrels, pistons and heads I would ask the club's Lightweight expert, Andy Sochanik - his email is:    

I hope the above is helpful.   All the very best of luck and let us know how it goes - and perhaps send us some photos!  



Hi John

Nicholas beat me to it, but I agree, first thing is to double check engine and frame numbers,  the frame number is usually on the rear footrest mounting or the gusset next to it. As has been said, 17 is Jubilee and 19 is Navigator.  The Navigator has a slightly longer stroke so what ever has been done (unless it’s just changing the engine number!) would not have been an easy fix. 

The stamped numbers on the heads are different between Jub and Navigator, but can’t recall them at the moment, same for the Cams.  

What I don’t know is whether you can put jubilee rods, pistons, push rods  and barrels on a Navigator bottom end. The exhaust down pipes and head steady are also different.  It’s not a common conversion if you can! 

Andy the club lightweight guru might know, but some pictures would help!






Hi All,

I'm in contact with the seller of this bike. He sold it as a Navigator - because thats how it was sold to him. Its been well put together by somebody who clearly knew what they were doing. I have seen pictures of the bike & the only external clue is a piece of packing under the head steady that shows that the engine is not as tall as the original 350cc unit.

This conversion to 250cc is possible, as the Crankcase, Timing case, Primary chaincase & gearbox are all common (except 17T on g/box sprocket vs 19T on Navi).The items that were changed are all to do with the engine:

Crankshaft, con-rods, pistons, barrels, heads, Inlet manifold & carburettor.

Although the cams are different for the two engines - its possible to use either set.

The cam followers are the same (not the pushrods) and all the 'furniture' on the cylinders heads (valves, rockers, etc) is all the same.

In addition, the exhaust downpipes would need to be changed as well.

Its possible to return this to a 350cc again - but all the above would need to be changed.

I think you need to talk to the seller (who was also duped) to find the best way forward.



I suppose the question is, have any of us got any Navigator top end bits we could sell to John so he can convert it back to a 350. 

Im partially interested if it has 350 con rods and cams, it would be interesting to know how a jubilee goes with those fitted?! If indeed it is possible ........ does it need a spacer under the barrel or is the comp ratio increased?

I have some con rods somewhere and will have a look!


Hi Dan/All

You cannot mix 250cc & 350cc components without a lot of engineering work.

40 yrs ago (or more) I helped a lad step down his Navigator to a 250cc for him to pass his test on. I checked my records & I don't this is the one, so at least one other person had the same idea.

To return this engine to 350cc, you will need (minimum): crankshaft, con-rods, pistons, barrels to match, cylinder heads (valve gear all the same), push-rods, inlet manifold, carburettor and exhaust downpipes - oh, and a 19T gearbox sprocket.

To identify the con-rods - (as they look very similar) - hold them together side-by-side. If you have one of each - you will spot it, as one will be slightly longer than the other; if you have two jubilee ones - place the big ends together, and there will be air between the small ends & if you have a pair of Navigator ones, they will touch at the small end, as well as at the big end.

To identify cylinder heads - look at the numbers cast in to side top at rear, where they would face each other. The number is not important - but does it have an 'X' at the end? If so, that is for a Jubilee. Next, the heads without an 'X' can be for Electra or Navigator - depends if the barrels have a spigot or not - the heads are machined differently to suit.

Contact me if you have any doubts/questions on lighttwins@nortonownersclub.otg


Camshafts - how to tell apart:

Jubilee cams are marked I7 & E7 on the end away from the drive gear, whilst Navi & Electra cams are marked I5N & E5N on the end.


Thanks for all the background information Andy, it's a fascinating hybrid but a major job tracking down all those parts and then doing a full engine strip and rebuild to return it to 350cc spec again - but worth it from the look of the rest of the bike and I'm sure John could recoup some of the cost by selling the Jubilee spares on.   Lets hope whoever did the conversion retained the Navigator cams!   By the way is there any noticeable difference in performance or need to change the ignition timing if you use Jubilee cams in a Navigator engine?   I've often wondered given the availability of NOS Jubilee cams but rarity of Navigator cams.



Our dear departed John Hudson once told me that an easy way to pep-up a Navigator, was to use Jubilee camshafts - the valve timings are slightly different. He did not say (or I did not ask?) if this affected ignition timing.

For the technically minded, I have access to both sets of valve timing angles/lift if you are interested. The Electra used Navigator cams, timed differently by positioning the keyway on the crank in a different place.


I am building a Navigator engine and would like to know the gap between the barrel and cylinder heads i.e. spigot height as the heads have been skimmed can anybody help.

Sorry, Duncan - missed your posting.

You would have been better off to writing to me at - or starting a new thread.

Never mind - hope this gets to you before you make any rash moves!

If your head looks like its been skimmed - its for an Electra (or very late Navigator). The earlier engines used a spigot at the top of the barrel (looks like a cylinder liner jutting up). This was to locate the head & gasket and provide protection from the intense heat & pressure of the burn in the cylinder. Later on, around 1964/65, the powers that be decided to save on machining costs and do away with the spigot - the upshot is the barrels end up 2mm taller.

To compensate therefore, 2 mm is removed from the cylinder head to match.

Just ro confirm what you have, look at the number cast into the head, just below the rocker box joint. If its a 5 digit number without an 'X' - it is for an Electra/v late Navi.

If your cylider block has a spigot - do not remove it. Find a pair of heads to match - ideally with the larger 2BA clamp screws - the earlier 4BA ones were not strong enough.

clamp screws


Good morning all.  I started this thread when I bought a what I thought was a Navigator, turned out to be a hybrid, it has a 250 engine.  I returned the bike to the seller, a shame really as I thought it was a decent bike, but I wanted a 350.  

Since then I have bought another Navigator and yes, it is a 350 this time!  Better cosmetic condition than the hybrid but not as original although I am not too bothered by that.

Looking forward to taking it for a ride as soon as I given it the "once over" in the workshop.  I'll convert to 12volts so I can fit electronic ignition as well.



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