Oil.
When I get the bit between my teeth I do a lot of research and it seems that whilst the A65 engine is generally considered to be a good engine people feel that the plain bearing on the timing side is a weak area and is usually the cause of engine failures when it gets blocked and then starves the other side of the engine. Personally I don't think that's a fair assessment as its actually going to be dirty oil causing that to happen which is a) a consequence of the designers/bean counters not including an oil filter (but then that's 'of its time') and b) bad maintenance.
As such Im going to be thoroughly cleaning the system and adding a modern spin on filter kit.

My mate had cautioned me, and Id read, that the oil tank can get lots of sludge so I decided to go ahead and drain the oil and then clean it out.
Why oh WHY did they put the oil tank on the RHS of the bike if the side stand is on the LHS? It would make draining the oil MUCH easier if you could just lean it that way and then drain...
And also why place the plug in such a position so that it just goes EVERYWHERE, even when you think you've got a funnel and a tray in a good place!! 
If you look at my avatar picture you can see how much sludge was in there. Id hazard a guess it was 3-4mm deep!!!
If you look at the picture of the oil tank you can see the sludge 'twinkling' at the bottom.
Used petrol to flush it through, forgot to capture the first flush which went straight into my waste oil container but the ones after I put into my drain tray.
As you can see that's a LOT of large particles left after the petrol had evaporated.

Next was the oil filter kit that I bought from Paul Goff. It seems like a nice design and should fit nicely behind the gearbox and be accessible for changes. Will let you know how the refitting of the rear mudguard goes though.. 
Not Pauls fault but when I examined the filter head you could see a load of casting flash on the inside where the oil is entering the head.
Out came the dremel to remove this and to do a bit of porting so that there is less resistance to the flow entering the head, I want the scavenge pump to be pushing oil rather than creating pressure!
Decided to use petrol, as I had it around, to clean the swarf up and inadvertently removed all the paint on the inside which upon reflection is no bad thing...

Is there any indication where the leak is at all? Does it leak more when running or is it the same running and standing? If you can't see any obvious leak then clean the bottom of the engine as best you can and then try taping suitable pieces of kitchen paper towel to the various areas (along chain case, bottom of gear box, bottom of timing case) to see which one gets the oil drips first. That should give you a better indication of the problem area.

is it always a matter of a new gasket? Well, it can certainly be that. But sometimes it is a scored mating surface or excess engine pressure and a number of other possibilities. Fitting gaskets is not a hard task. It might take a bit of "mental mechanical orientation" but it is technically not difficult. It helps to revise the procedure with the factory service manual. It will tell you the steps you need to take to do any particular task. Gasket changes are usually quick and simple, at least on the chain case side. If the mating surface is damaged or heavily scored then some more remedial action might be needed. The problem can often be solved with some silicon or other sealant made for the job.

If not the gaskets then some of the oil seals might need replacing. That is more demanding but quite within the remit of the amateur mechanic. The factory manual and YouTube will help at lot. if you do have to disassemble anything for more than a short while then somewhere to do it and to layout the disassembled bits is an important consideration. It can help to take photos as you go.

But the first step is to locate the place(s) the oil is leaking from. Good luck! :0)

One of the first jobs was a very leaky fork leg. The oil that was left in the legs was relatively new but I was quite surprised as how much 'shiny' was in them..
As I'm finding with a 61 year old vehicle lots of nuts, screws and bolts are either worn, wrong or BOTH! 
In this case the screws were old and knacked and the fibre washers were actually too large so that they were being 'dished' before they even contacted the sealing surfaces.
I bought new screws and correct washers and then carefully 'flattened' the sealing surface at the bottom of the leg so that the washer had some good area to seal against.
Im confident that it will be leak free as only one of the legs was leaking in the first place so hopefully one job 'done'....

Hi All,
I joined the forum a short while ago and have already posted a quick 'Hello'.
The bike I joined with has been named Rosy by the family and she's a 1964 BSA A65 Thunderbolt. One of the PO's got some details from the factory/register(?) that show that this is a replacement engine.
As I understand it the most recent PO had started/done(?) a restoration but didn't fully complete it so my mate purchased it on my behalf.
I live in London and Im tired of the tube for my commute so want a bike again. The issue is that theft is so endemic round here that any modern bike that Id like would get stolen in mere moments!!
Hopefully an old bike that is a) kick start, so they wont be able to get it going b) heavy, so they cant use their mates scooter to give them a 'pushy' and c) foot controls on the wrong side, so even if they do try they'd crash and (hopefully) hurt themselves badly!! 

Aims are to go through all the stuff that might have been missed e.g. loose bolts etc. and also to modernise for reliability and to try to make as leak/drip proof as possible!

Cheers,
Marc

Quote from: Dibble on 04 July, 2025, 14:09:59Probably sounds like a daft question but im ready to renovate A65 barrels and they will need boring and rear head bolt thread upgrade (to 3/8) so visit to machine shop. Would you wise people first blast and stove enamal or do that bit last?

Is stove enamelling even available now?
Personally I'd degrease and/or blast prior, then when all the renovation work is done spray (not brush) with hi-temp engine paint.

Quote from: neil1964 on 05 July, 2025, 09:10:06I presume that you know it is not wet sumping and the breather disc is moving freely?
Oil from the breather sounds like excess crankcase pressure being part of the issue?

Definitely not wet sumping as I also took sump plate off to clean filter all ok. Yes breather is moving freely after I cleaned it. I have no idea why there seems to be excess crankcase pressure. Bike was going fine until I took magneto off to get a bearing changed. Put it together, set timing, starts 1st kick every time . I ended putting granville clear sealant around gasket etc and will test ride again tomorrow but concerned that there definitely seems to be excess crankcase pressure.
Thanks
Brian

I presume that you know it is not wet sumping and the breather disc is moving freely?
Oil from the breather sounds like excess crankcase pressure being part of the issue?

The engine prefix A65D indicates it is from a 1965 USA specification Lightning Rocket (UK market Lightnings were prefix A65DC). The original frame for this engine would have a prefix A50B.

The frame is from a 1965 USA specification A65 Thunderbolt Rocket. So the bike has had an engine change at some point during its life.

The Lightning Rocket and Thunderbolt Rocket frames are different patterns as are the forks and wheels.

Below is a 1965 USA catalogue illustration of a 1965 Thunderbolt Rocket.

Sorry I can't help directly (know virtually nothing about unit BSAs) but I suggest to post some photographs (or links to your pics on a free hosting site like IMGUR).
That way experts will be able to tell you what you have vs what you should have. Some helpful and knowledgeable people here

Quote from: DAVE BRADY on 04 July, 2025, 14:22:17Hi,

Sorry! it should have been this.

https://www.youtube.com/watch?v=nIm-aMPRlI8#:~:text=How%20To%20Shorten%20or%20Repair%20Throttle%20Cable%2C%20Brake%20or%20Clutch%20Cables%20with%20Solder%20%2D%20Motorcycle%20|%20Part%2018

Dave.

Thanks Dave, that's much more helpful than the other video!