revtothelimit - motorbike forum

Obviously I now have to decide if I need to get that inlet stem tube replaced, (They're pressed into the head, so I need to find someone to do it for me), or if I live with it. The exhaust side looks a lot newer than the inlet one.

I'm going to hold off on lapping the valves in until I make that decision.

Still pondering the best way to clean the burnt on carbon from the inside of the head and barrel (and piston), without damaging any of the surfaces.

Rockburner wrote: Tue Apr 01, 2025 7:58 pm Obviously I now have to decide if I need to get that inlet stem tube replaced, (They're pressed into the head, of need to find someone to do it for me), or if I live with it. The exhaust side looks a lot newer than the inlet one.

I'm going to hold off on lapping the valves in until I make that decision.

Still pondering the best way to clean the burnt on carbon from the inside of the head and barrel (and piston), without damaging any of the surfaces.

The only thing that springs to mind for removing carbon is...fire!

Unfortunately it's not going to be pure carbon so the remaining residue would be even harder to shift. Probably.

Stick the head in a pot with some oven cleaner (non caustic soda) for 24 hours. As far as the inlet guide...push the valve out far enough to clear the seat. If there is perceptible side to side play, it should be replaced...if it is very slight....it's up to your intended use. Any half decent machine shop should be able to remove and re fit.. a good one will re manufacture. I would be more concerned with the exhaust valve. You need to clean the stem thoroughly and see if it is still as stiff when refitted after lubrication.
I apologise if I am teaching you how to evacuate an egg

David wrote: Wed Apr 02, 2025 11:12 am Stick the head in a pot with some oven cleaner (non caustic soda) for 24 hours. As far as the inlet guide...push the valve out far enough to clear the seat. If there is perceptible side to side play, it should be replaced...if it is very slight....it's up to your intended use. Any half decent machine shop should be able to remove and re fit.. a good one will re manufacture. I would be more concerned with the exhaust valve. You need to clean the stem thoroughly and see if it is still as stiff when refitted after lubrication.
I apologise if I am teaching you how to evacuate an egg

Nope - this is stuff I've either never done before, or have only done once in my entire life, and then a long, long time ago. So all egg-sucking advice is appreciated.

The valve guides I can buy, I just need to find somewhere to press them in, (after removing the originals obvs). I would assume that would also mean new valves - but I'll discuss that with the place where I can source the guides.

I still haven't summoned up the courage to touch the timing gear...


I discovered Loctite SF7200 this morning so will try that. (It's probably the same chemicals as oven cleaner. )

Got into the workshop this afternoon, and took the decision to just fuggin do it.

Timing chain cam off easily enough, the two sprockets were well jammed on, but I eventually got them off with the bearing puller.

There's no keyway or anything on these, so the ignition timing needs to be done by "eye"...



The platform the mag sits in is pretty mullered....

Probably why it wasn't exactly firmly located....

It was also only held on by 2 studs.

Here's the Mag drive shaft - note complete lack of keyways...

But nice modern seal.
The foam on the Mag is to stop oil flying about from the timing case, because the hole the mag shaft pushes through is about an inch square....

This frigging engine mount bolt was the main reason I was swearing all afternoon....

The nut on the end was tight as hell and so I couldn't pull the stud out of the plates on the other side.


When I was talking to the guru the other day: he mentioned the "bible" for JAP speedway engines is the "Greenwood" manual.
So - I popped a search into ebay, and one came up! Score! It turned up today, and as I was reading it (it's only about 8 pages), I was thinking.... this is all very familiar.....

Checked the books/manuals in the workshop and ....

ffs.
Dad had a photocopy of the damn thing.

The interior of the timing chest:

Note shonky paper gasket and crap.
See if you can spot the wtf...


Inlet cam roller


Exhaust cam roller



WTAF?


No - I could NOT figure out what that was.... took me an hour.

This is the inside of the timing chest cover - with the exhaust valve lifter gubbins:






This is the cam-shaft in place in the timing chest cover


Close up of the cams:





I was looking at the valve-lifter assembly above, and comparing it the position of the valves... and I couldn't for the life of me figure out how on earth it was meant to do anything. The lifter assembly is the same "thickness" as the cam-shaft gear, and the exhaust roller is tucked away behind that gear when the chest is assembled ... so ... what did it "act" on??


I eventually worked it out when I got to pulling out the rollers :



That little stub of metal "was" a pin that had been driven through the exhaust roller arm, and which would then stick past the cam-shaft-gear and the valve lifter could act on that pin to lift the exhaust valve. But: the pin has obviously snapped at some point in the past: no wonder the valve lifter was utterly useless!!
TBH I'm probably going to just strip the lifter mechanism out completely, rather than try to get a new pin driven into the roller. It's only 9:1....

Inlet, and exhaust rollers:


However... I think a new inlet roller may be in order, not sure, but I think they come as a pre-assembled part. :





EVENTUALLY.. after quite a bit of twisting, looking, perusing, swearing and general fuckery....


IT'S OUT!


And that was about all I really had time for. Next jobs are to continue stripping down the cases to check the crank bearings are still in good shape. Ideally I'd split the crank.... but I'm REALLY sure not sure I'm brave enough for that....


This frame really has been bodged about.



This is the hole we were talking about... all the way back on page 1...

Keep up the great work, pictures and description. I really love posts like this.
Thanks @Rockburner

Buckaroo wrote: Thu Apr 03, 2025 11:22 pm Keep up the great work, pictures and description. I really love posts like this.
Thanks @Rockburner

To be honest, it's just as useful for me as an aide memoire for when I do it all again next time!

You'll be the UK's top consultant on Jap engines after all this.

Skub wrote: Fri Apr 04, 2025 9:47 pm You'll be the UK's top consultant on Jap engines after all this.

Hardly. There's still some old guys campaigning them in vintage speedway races! That's why old parts are hard to find.

Hadn't noticed this nick on the conrod before.


I'm thinking this conrod is likely to be second hand, so the nick is likely old too. I hope...


Some carbon on the piston.








From the pile of tools I brought back from Dad's workshop I eventually unearthed this, among the 5 or 6 other "pullers". Put a new bolt in it, for some totally unknown reason he had a carriage bolt in it.. .



I had to file it down a touch, but it worked perfectly.




One of the other pullers to remove the crank sprocket from the timing side.


This plate needs to come off, but I've no idea what it's been such on with, or what's behind it that I don't want to damage while removing the plate.


Timing side of the crankshaft


Piston is cleaning up nicely, but I'll keep at it.

Bit more progress on the JAPton strip down yesterday:

Stripped off the oil seal plate on the crank output shaft :




I wonder if that's a modern update. I don't remember seeing anything about it on any of the diagrams or books, and the thing is held on with metric cap-screws... (well, I didn't check the threads, but they take a 3mm metric Allen key, and none of my Imperial Allen keys would fit....)


While I was fiddling with the output side.... I heard a "clonk" from the other side of the cases....



The oil drain box cover fell off.

I'd already removed the screws last time, but it had appeared to have been solidly glued down with silicon sealant... Apparently not so solidly!





Kellogg's still make the best gasket paper.


The gunk inside the oil drain chest.





This is the underside of the oil drain chest - it's a one-way valve of some sort - I'm going to clean up the mating edges for the outer covers, but otherwise leave this alone.



Next...

I used to work at Light Metal Forgings in Oldbury. We supplied Omega with piston blanks. Sorry, neither I nor them are there any more.

Piston broke in Bristol were good....again if they are still there...

David wrote: Wed Apr 30, 2025 6:37 pm I used to work at Light Metal Forgings in Oldbury. We supplied Omega with piston blanks. Sorry, neither I nor them are there any more.

David wrote: Wed Apr 30, 2025 7:19 pm Piston broke in Bristol were good....again if they are still there...

Luckily i think the piston itself is ok.

The scary part!!




Plonk! (tinkle tinkle tinkle..... feck where did the pins go!)


There's 10 of the little buggers each side.


Output shaft:



Magnet to pull the pins out when they refuse to fall out by themselves.



Simple engines are great.



Timing side: ALL the pins fell straight out!


Timing side bearing liner is ok.



Output side not so much....

That patch feels rough as hell. New liner time.


I'm currently mentally debating the wisdom of breaking the crank down... looks simple enough... it's the reassembly that scares me!





Insides of the cases are epoxied to buggery. I don't know if this is my Dad's doing... but it's likely.









Another view of that liner.



These "thimbles" screw into the cases and provide the base for the screw-down head rods


They're a git to get loose...


Paper gaskets infused with oil take heat well....


I'm reliably informed that the thimbles come out a lot easier if they're assembled with coppa-slip. Luckily, I've got plenty!

The thimbles have these little pucks underneath to stop any galvanic reactions with the cases (or that's what I've been told - it could be just to provide a flat step for the thimble to wedge down onto).



Epoxy:


I'm glad to see that the drain plug hasn't been epoxied over....


Blue hylomar... my old friend!

You can sort of see the "lip" that JAP used to create a labyrinth seal on the cases here.

As said - I'm just pondering the wisdom of splitting the crank up to check the big-end bearings - they "feel" ok - but while I'm here it might be worth doing. I'm just being cautious about that because bolting it back up "straight" is going to be paramount.

All that only took me about 90 minutes, and I managed to finally get down to the local "Classic vehicles" club night and join the club - they do a monthly meet only about 3 miles from me so it's dumb really that I've not joined before now. Already hopefully got leads on a couple of hopefully useful contacts.


Just in case anyone is wondering: I'm using this thread mostly for myself to provide my thoughts and notes as I go, for reference to when I put it back together, or do it again! Pretty sure there's no-one else on the forum with a JAP speedway engine to dismantle.. (happy to be proven wrong though! )

Being deserving of a dedicated thread the JAPton marathon rebuild project will be removed here.

Tada! I've probably dragged some stuff across that belongs back in with the Puch and left some things there that should be here. Let me know RB and I'll tweak.

Rockburner wrote: Wed Apr 30, 2025 2:27 pm SplitTheCases.jpg

Fred Gassit roolz

A great read as always RB. Keep the thoughts coming.

mangocrazy wrote: Fri May 02, 2025 10:20 pm

Rockburner wrote: Wed Apr 30, 2025 2:27 pm SplitTheCases.jpg

Fred Gassit roolz

Thank you, I was racking my brain for the character's name.

Crank stuff is child's play RB.