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Twice the charme!


Good news and bad news:

Good: I found the reason for the leaking gearbox on my Navigator: the bottom screws were too long and bottomed out withouth tightening the gearbox cover. Shorter screws solved the problem.

Bad News: Another seized piston! This time the left one. I was doing 60mph when the engine sounded stressed and I pulled the clutch.

damaged piston


There is a cylinder liner in the barrel but on the right side. No damage on this side. I had the barrels rehoned after the engine seized the first time. so there should have benn a bit more clearance.The piston size is now 63mm+ 0,10. There are no other oversized pistons available as far as I know. Except one +0.20 piston on Ebay.

Will find out what to do after I spoke the the man in the engine shop.


Hi Uli,

Sorry to see your seized Navi piston. What a nightmare!

Please check your ignition timing. The correct figure is 24 deg BTDC (Before Top Dead Centre) fully advanced. Check this with a strobe. You will see other figures quoted in various manuals BUT 24 deg is the correct value for a Navigator.

Make sure you find Top Dead Centre accurately - using a piston stopper. If you try to judge TDC by feel - you could be as much as 5deg out. The Navi (& its sisters Jubilee & Electra) have very short stroke engines and the timing has to be spot on!

See instructions attached - if you still have points.

If you have electonic ignition, follow the setup for finding TDC, then follow your makers timing setup instructions.

In both cases - you must check the setup with a strobe - revving the engine to check that a) the advance curve is working, and b) that the advance is not going higher than 24deg.



That's really grim news after all your hard work Ulrich, so sorry to hear it - as we've discussed before Andy's advice over timing is absolutley crucial and I know you have said you had set the timing to 24 deg.   However, strange it has seized on only one side which might point towards a clearance issue.  Another thought - how many miles had you done since your last top end rebuild?   I would be nervous about sustaining 60 mph for any length of time on a freshly rebuilt Navigator engine until I had at least 500 miles on it ……but may be that is me just being over cautious having had similar seizures in the past due to over enthusiasm and impatience as a teenager!   Good luck with sourcing OE (Hepolite) oversize pistons which are difficult to find, there are modern manufacture Australian ones from JP Pistons available for about £200 a pair on ebay and elsewhere but I've seen mixed reviews.   Perhaps someone can provide definitive advice based on personal experience?    Nick



I just completed the order of one new piston and rings  form the NOC shop.I checked the timing and it is at 24° BTDC. We can use the same size piston . There are no mark in the cylinder, just aluminium sticking to it.

I was at the engine shop today and showed the damage to the owner.

He suggested more piston clearance.

We are now at about +.20 because of the rehoned cylinders after the first seizure.

Will report when the piston is here.


Hi Ulrich,

Which piston seized? 

It is fairly common, after high mileage, for the sludge trap in the crankshaft to become so full of sludge that the left hand big end journal oil feeds get blocked. No oil is thrown onto the cylinder liner and the piston will seize.

If the piston that seized was on the left side, and you did not clean the sludge trap in your rebuild, this MAY be the answer.




The same said our local Norton engine specialist to me. Looking forward to another engine teardown... I left the bottom end as it was because I felt no play in the big ends.

In reply to by ulrich_hoffmann


Hello Ulrich, when you come to do this, I was amazed at how much hard sludge there was in the trap. I used a wire to chip it out, then pipe cleaners and parafin until there was no evidence of any sludge, I did have to take the crank out to do this though as I cleaned from the journals in. You may find your big end bearings are scored as well. Hope it wporks out for you.


Hi Ulrich,

I've done at least three different lightweight cranks. In all cases the aluminium cover screw on the end of the sludge trap refused to move with a broad screwdriver to the shallow slot. The slot is only of use to insert a new plug.

Grip the crankshaft in a large vice on the flywheel.

I drilled a 3mm hole through the centre of the plug. Then two more 3mm holes along the slot. The slot helps locate the drill. Then use a needle file to remove the bridge between the holes and insert a strong large flat bladed screwdriver. If is has a square shaft or a driving hole in the handle all the better. You may need a Mole-grip on the screwdriver shaft.

If that doesn't work, lengthen the slot until it is at the top of the threads, don't damage the threads of the crankshaft, and try again. Hopefully the cover screw will collapse inward a bit and lose it's grip on the crankshaft.

On a Jubilee, which has a smaller diameter screw cover to Nav/Electra, I had to resort to another pair of 3mm holes, like a number 5 on a dice, and make two slots, elongated to the threads. I was able to punch the four quarters of the mutilated screw inwards and to freedom.

Then I used the strong long screwdriver to chisel and remove the sludge. As Andrew says this will be compacted. Be brutal and ensure there are no pieces left inside. The sludge always collects more at the far end, the left end, where the oil flow is lowest.

If you have a parts washer, use it to thoroughly clean the oil passage from the timing side oil feed (end of crankshaft) to the sludge trap. Or an airline can be used. Bits of aluminium may have fallen in here from all the drilling and filing. A trick is to use a grease gun on the oil feed to fill the oil passage with grease before you start drilling the aluminium. Then, at the end, push in more grease to drive any swarf into the sludge trap where you can remove it. Then clean out the grease with paraffin and pipe cleaner. (or parts washer)

Clean, clean, clean.

Check your new cover screw fits. Apply a thread sealant and screw it fully home. 

Norton secured the screw cover by deforming the thread after insertion.

I apologise if my responses on this forum sometimes seem a bit lengthy, but future readers may not have the experience you have.

Please don't lose your enthusiasm for the Navigator. I'd love to see you write something in Roadholder. 

Best wishes



Need more pics to confirm, but seizing on the side with nodules on the piston top suggests a 4 cornered seizure from pre-ignition heating the piston. The 4 corners are the points where the piston is thickest and the more material the more expansion ie either side of the pin on both sides.


Today I started to pull the engine form my Navigator.

Navi sans engine



I came up to this stage. Are the nuts on the camshafts right- or left hand  thread?

Timing side




The camshaft nuts are right hand thread. If you are going to clean the sludge trap.

Remove the intermediate pinion. Slides off.

Do not remove the camshaft nuts. Not required.

Remove crankshaft pinion on timing side. Special puller is good to get into space by 3-sided washer.

Remove primary drive cases and contents.

Remove all fastenings between the crankcase halves.

Separate the crankcases and the camshafts stay captive.

Probably more controlled if you drop the gearbox internals out before splitting the crankcases.

The crankshaft can now be removed. Sometimes the timing side bearing is tight on the crankshaft. Don't damage the timing side end detail on the crankshaft by hammering. This is important for a good oil feed seal. Use a tube to push on the shoulder at the top of the left hand thread if necessary.

Looks to be going well.



Today I borrowed the three jar puller for the crankshaft half time pinion and took the engine apart. I left the camshafts in but noticed excessive play in the timing side bushes so the had to come out also.The nuts were very tight but in the end they loosened up and I could pull the pinions.

Navi engine


The gearbox was taken out as a cluster. I noticed a loose clutch operator lockring and also a loose bolt for the gearbox plunger. Also the layshaft bush fell out and it also has excessive play.

In the left side engine housing I found a bolt with a small hole acting as the crankcase breather. The part NO.23367 in the NOC shop crankcase breather surely looks different !

breather alias

The big ends also have too much play. The crank journals need a regrind and oversize shells. The sludge trap plug came out easily. Not much inside. I flushed it with my parts washer.

sludge trap

big end shells

Are the bushes for the camshafts available somewhere? Also the layshaft bush? Can´t find these in the NOC shop.

So in retrospective it was good to open the engine once more find all those faults

I guess it will take longer to get the Navigator back on the road!






The piston failure is not down to a blocked crank then, So some more carefull detective work is needed. What piston/bore clearance  have you been using.?., If your engineer is a car guy he may be not familliar with aircooled motors.

Hi Ulrich - for the closed type (top hat shape) layshaft bush I could only find a stock at Norvil, don’t know about the other bush though, sorry. You could try Russell Motors as well, they’ve been closed for the last 3 months but had a courtesy phone call from them (after my parts request letter sent in April) last week saying they’re due to re-open next week (I think).



hello now everyone as gone thought everything they can think of that the cause of this unfortunate incident  but as any of you come across the oil strainer filter in the sump plate As I have been dismantling my  350 Navigator I found the sump strainer filter to be partially blocked by red Hermerite  gasket sealer  on further inspection, I find a layer of thick oily gunge that was also blocking the oilways  you should check this part of the sump plate is clean  If the oil pump cannot pick up oil  you get a seizure somewhere,     yours  anna j 


Hi Ulrich,

Trust the rebuild is going OK. Have been hoping all avenues have been covered with the comments above but one thing nags.

I have reconditioned five or six lightweight oil relief valves. Wrote an article about it for Roadholder a few years ago. RH333 Page 6 Sept 2015.

It is not like the the oil pressure relief valves in heavyweights which have a ball against a seat. Not prone to the same problem.

You should check that the Navigator pressure relief valve is working, if you did not do it at the rebuild.

Very common for particles in the oil, like carbon, to seep between the close fitting piston and bore of the relief valve. The piston jams in the bore and is not pushed back onto the seat by the spring. Oil then spills continuously from the high pressure oil supply before it gets to the crankshaft. Of the handful of pressure relief valves I inherited, the MAJORITY were in this condition. It's easy to check the valve is clean and piston free by separating the valve body halves.

No surprise that the Norton Twins workshop manual mentions the importance of checking the lightweight oil pressure relief valve in its opening pages, but how many of us do!!

And crankshaft oil seal in good order.

And conical rubber seal on oil pump.

Let us know how you are getting on.




Thansk Peter for the hint!

The engine is totally apart right now. A friend of mine made a new oversize bush for the layshaft. The old one turned in its seating and was totally worn out.

Not pressed home on the pic!

layshaft bush


Also a new crankcase vent tube with a bigger diameter was installed. Some PO drilled in a bolt with a 3mm hole in it. I will fit a small breather valve so that air  can get out of the crankcase but not in!

The old vent hole towards the sprocket was already blocked.

vent tube



Crankshaft needs regrinding to fit +0.10 shells. I am waiting for the parts from Norvil. New lightweight bearings, big end shells and a gasket then and some other thingies..

And the barrels and cyliner are ready to be installed I will pick them up when I will bring the crankshaft to the engine shop.



Hi Uli, 

I got my camshaft and other bushes from Russel Motors. Very reasonably priced but I had to keep calling until someone took my order. I had to wait  a while until he was "Going to the warehouse across town" to find out if they could supply them. Worth the wait though.



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