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#894647 11/02/22 6:33 pm
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Getting to the bottom of all the boxes of this A65 project. I see nowhere in the boxes, and my older model parts books the breather lines and connections. The timed breather that runs off the cam is identical to the one on Triumphs. The breather connection on the oil tank looks identical to Triumphs. These 2 orifices are connected with a tee and route over the back fender on Triumphs. Which by the way, drools an occasional drip of oil when back from a ride on my T120. Would a more up to date parts book have it shown? Shouldn't the BSA have similar to avoid the mess it can create? Thanks, Gary


67 BSA A65L, 67T120R, 71 Hodaka 100B, 72 HD Aermacchi 350SS "cafe racer" 60 HD Duoglide, 77 HD FLH, 79 HD FXEF, 12 HD FLHTC
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in 1967 , I think , the cam-breather hole in the primary side case
Just dumped into the chain tunnel on top of the sprocket .
... dont think they added a brass stub or hose or anything ? ... but these would be thoughtful custom extras
That can change the location of leakage

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Yes, just dumps out on the floor.

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Thanks for the response. Since the cases are apart, I'm thinking about installing a tube and duplicate what my Triumph has. Thanks, Gary


67 BSA A65L, 67T120R, 71 Hodaka 100B, 72 HD Aermacchi 350SS "cafe racer" 60 HD Duoglide, 77 HD FLH, 79 HD FXEF, 12 HD FLHTC
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The A65 never had a tee connecting the timed breather to the oil tank breather. The timed breather is here:
https://www.britbike.com/forums/ubb...k-case-breather-vent-location#Post837190
Getting a hose out from around the chain likely means routing it down under the motor.
You could plug that hole then drill through to the primary and add a breather there. Be sure to then plug the chain oiler from the rear most primary cover screw.

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No surprise that all the racing BSAs I've seen use a larger breather out the top of the primary case, just like later Triumphs. One assumes that they left out the crank oil seal in order to make it actually work.
My '71 A65 had been modified to breathe through the primary. Don't ask me why they put the outlet at the front of the primary case, nor why they left the DS crank seal in place. crazy
I removed the crank seal and installed a Mike's XS reed valve at the end of the tube routed to the front of the air box. Blanked off the chain oiler and the original breather, no oil puddle under the rear of the engine. laugh


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Originally Posted by do it twice Gary
Thanks for the response. Since the cases are apart, I'm thinking about installing a tube and duplicate what my Triumph has. Thanks, Gary


Not sure what type of breather modification your thinking of doing but some food for thought.

Im currently rebuilding a motor for a friend (who bought a bike with a motor that had aparently seem much milage since build). the DPO had fitted a fitting with a hose going to a catch can...

Sounds good yes?.... NO, the tube had become blocked becasue when oil vapours pass through the camshaft they remain as vapour until they start to condense again when it meets a cooler temperature. because of the A65's design, there is no way of coming off the breather and continuing a drop. So, oil collects, the pressure from the engine isn't enough to blow it out and it eventually congeals. Then the acid atmosphere from moisture and oil vapour inside the engine cannot clear and you end up causing more damage to the motor.

Flip side is, if you build it properly, seat the rings properly too then you shouldn't have excess CC pressure and under normal riding conditions you should find that no oil comes from the engine oil breather. You'll get a bit if your widing it up to 7K but it won't be much.

You could fit a breather off the rocker box, you could also add a PCV valve which would not be a bad thing either.. but please don't stick anything off the timed breather exit.

If you want to block up the timed breather and use a breather off the rocker box or primary case etc, the hole through the cam is roughly 3/16", it can be tapped to 1/4" UNF to accept a grub screw. a drop of blue locktite on the grubscrew and your done. It can also be removed in the future if you want to put it back to standard.


Life is stressful enough without getting upset over the little things...

Now lets all have a beer!

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (now rebuilt)
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I’m not against better breathing, but have never needed it. Not Racing, not generating enough internal “pressure” to need/require the extra relief. IMHO, there’s other places to focus attention, and the breathing mod to the top can be done later….. after it’s actually back on the road. 🥸


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Related to the topic, have others here got an opinion on the A65 “bottom Breather” design dripping oil? Is that a common thing? In my case, the oil that hits the garage floor comes from other places, NOT the breather. It would be rare that I was just lucky in this respect. On the other hand, the ‘70 TR6C with the hose along the fender coats the License plate and collects grime, and parked eventually drips oil on the floor. (Along with the chain guard from the chain oiler) This is due to the long hose which allows the vapors enough cooling and surface area to condense into liquid. IMHO, that design is another example of “it looks good on the drawing board,… should work great!”


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My opinion, the stock A65 timed breather is barely adequate,and drips, IMO the primary chaincase "oiler" was fitted to give dealers an excuse for the peristent drips in this area. If the bike is ridden sedately very little comes out, if the revs are kept high quite a bit comes out.

The breather passage is long and convuluted, the camshaft centre hole is only 3/16" diameter, as revs increase the time delay between case compression and gases reaching the disc valve means that the timing gets out of phase. The later triumph twin breather , through the drive side main bearing into the primary case then out a generous hose , is a far better solution.


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I stand with KC on this, the only time that the timed breather has not been sufficient was when I had fitted a crank pinion which was made incorrectly and timed 20 degrees advanced. The timed breather was then out of phase and if an aditional breather wasnt fitted then it would blow gaskets etc. With the cam timing set correctly (different crank pinion) I have never found an issue.

That said, mineral oils have a higher volatility than synthetic oils, I run synthetic which is probably why the problem is vastly reduced on mine.

Last edited by Allan G; 11/04/22 4:51 pm.

Life is stressful enough without getting upset over the little things...

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Hmmm, i seem to remember Allan, you fitted a breather above the cam pinion.
Plus used a japper reed valve thing on another motor of yours?

With hard use the breather will loose oil, i like the primary open as it means i
don't need to check oil level in there, if the vent is routed well and the motor
is in reasonable condition it doesn't blow any oil out. I doubt the oil type has
any effect on the exhausted breathed matter.
If you plod around as 90% of blokes do on these old crates, the timed breather
will cope although if the bike is left for a few days and it is a 'wet-sumper' then
the evidence will be apparent.

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If you read Rex's blog or even buy the e-book on engine breathing you will see when the Uin of Aukland actually tested the timed breathers all of them eventually worked backwards because air has mass and thus inertia so eventually the movement of the gasses and the opening of the ports are out of phase
They as you rightfully noted work best at idle ( showroom speeds ) so the new bikes appeared to be oil tight till the first time you open up the throttle .


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It looks like my Bitsa breather doesn't work because lots of oil get thrown out from valve cover when riding.. I expect the channel is clogged and don't remember if I checked it out a year ago when starting engine assembly. Instead of taking engine apart, I'd like to make additional breather. I remember Allan used valve cover to install it in, I also vaguely remember a picture of engine where front bolt covering a hole to check timing was used for an elbow for a hose going to some reed valve on a top of the frame.
What solution do you find better and why?
I don't intend to drill the cases for a big diameter hose or make it breathe into primary Triumph style, I like it with separate oil for a clutch and engine.

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"I also vaguely remember a picture of engine where front bolt covering a hole to check timing was used for an elbow for a hose going to some reed valve on a top of the frame."

That was me , it didnt work, loads of oil emerged.Over a litre in twenty miles.


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Thanks for clearing it up Gavin.
So valve cover it is.
How it is executed, just an elbow and a hose or some valve in between?

NickL #894876 11/07/22 12:27 pm
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Originally Posted by NickL
Hmmm, i seem to remember Allan, you fitted a breather above the cam pinion.
Plus used a japper reed valve thing on another motor of yours?

With hard use the breather will loose oil, i like the primary open as it means i
don't need to check oil level in there, if the vent is routed well and the motor
is in reasonable condition it doesn't blow any oil out. I doubt the oil type has
any effect on the exhausted breathed matter.
If you plod around as 90% of blokes do on these old crates, the timed breather
will cope although if the bike is left for a few days and it is a 'wet-sumper' then
the evidence will be apparent.


I did, I have tried rocker box breathers and one (descrete) screwed into the bulbous part outside the camshaft, (never above, not on a road bike, but it is a decent enough location).

The jap reed valve still needs to be fitted, just need to do a small mod to it or try something better size/shape, thats for the 823 thunderbolt... I thought that many CC would be too much for the timed breather.

But the most my lightning has ever had has been an open breather. After plugging the end up I didn't see any advantage over the timed breather..... Least not with the cam shaft timed as standard. Change it a couple of degrees and this will change.


Life is stressful enough without getting upset over the little things...

Now lets all have a beer!

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (now rebuilt)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

NickL #894877 11/07/22 12:34 pm
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Originally Posted by NickL
I doubt the oil type has any effect on the exhausted breathed matter.

I wouldn't say that. Oils are tested for volatility because of this reason, older spec oils have wider tollerances than newer specifications.

If your breather was positioned in an area of "high fling" like where gavin placed the breather in front of the crank, then fair enough... But I don't think the cam location or rather the centre of it is in a high fling area, outboard the cam pinion maybe, leaving the only option of escape to be from vapour which condenses as it passes..

A nice reverse scroll down the centre of the cam might be a nice idea, but probably more costly than a reed valve.


Life is stressful enough without getting upset over the little things...

Now lets all have a beer!

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (now rebuilt)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

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Originally Posted by KC in S.B.
Related to the topic, have others here got an opinion on the A65 “bottom Breather” design dripping oil? Is that a common thing?
My A65 always had a spot of oil under the stock breather, even after I blanked off the chain oiler. This even with the extra breather from the primary.
FWIW I've never seen any oil from the breather pipe on my T120, and I don't see any signs of oil in the air filter on my T150


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The only time the timed breather on either of my A65s spewed oil is when the engine wet-sumped.


Mark Z

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Originally Posted by Mark Z
The only time the timed breather on either of my A65s spewed oil is when the engine wet-sumped.
Which was virtually every day with mine until I finally located a cast iron oil pump.


"Gosh, it's not a 1/4 20, must be metric."

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Originally Posted by DavidP
Originally Posted by Mark Z
The only time the timed breather on either of my A65s spewed oil is when the engine wet-sumped.
Which was virtually every day with mine until I finally located a cast iron oil pump.

Ditto.


Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.
2007 Triumph Bonneville Black

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