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Thread Like Summary
Allan G, gavin eisler, Rich B, S-NJ-W, Stuart Kirk
Total Likes: 7
Original Post (Thread Starter)
by Adam M.
Adam M.
I have a story to tell.
My friend in Poland just disassembled his 71 A65 Thunderbolt after using it for 5 years and covering 120 000 km on it. The reason for disassembly
was low oil pressure, but a culprit wasn't infamous timing side bush ( this is still intact together with his crank ) but his original rods got oval.
He just bought a pair of MAP steel rods and installed them.
I have to add he has an iron body oil pump and extra spacer covering a bush from outside A70 style to tame oil leaks from this area that's all.
Liked Replies
by Andy Higham
Andy Higham
Originally Posted by Ignoramus
A65s actually had 2 oil filters ex factory , 1 in the oil tank 1 in the sump the BSA engineers seemed to think that was adequate , subject of course to regular oil changes i recon it is as well.

Those are not filters, at best they could be called strainers. The only thing they will stop are half housebricks, stray dogs and children.
BSA chose not to fit adequate filtration because it was cheaper not to
2 members like this
by NickL
If your mate got 120.000kms out of his motor he can't complain, it's due for a rebuild.
If he has ridden it for a long while with poor oil pressure, that will have ovaled the rods.
The A70 bush with the thrust does little to improve oil pressure, the thrust only works
slightly at one end of the bush and still has clearance. The advantage is setting end float.
The width of the bush affects overall life of the bush and allows the crank to flex more
without 'hour-glassing' the bush so much. The bush setup is fine if oil is clean and the
owner accepts the service intervals for replacement/sludge trap cleaning.
The balltype relief will not stick if any debris goes through but the drillings around that
area are variable, the later cases being better.
120,000 k's is about 75,000 miles so that equates to around 2 or 3 engine stripdown
rebuilds if you go by the book. These are 60's engines remember.
A new or reconditioned oil pump would be due by now as well.
If you do that kind of mileage and don't want to strip it every 25-30k, have it end fed with
a needle race, fit steel rods and a nicasil barrel. Magnaflux and nitride the crank.
A proper oil filter is a must on any engine really, i don't know why people object to them,
for the sake of a few bucks they do a stalwart job. and are easy to fit.

This is of course, just my 2c worth.
I don't expect any more than 30k out of my beezer engines before, but then i
tend to thrash the old crate
2 members like this
by DMadigan
This gives you a better idea of the OPRV problem:
[Linked Image from]
The drilling at the top is from the oil pump (blanked off by a screw on the bottom). The hole to the left is the drain to the case. The early engines dumped it into the sump. Later the drilling goes to the drilling for the pickup tube with a ball in the pickup tube to prevent the oil from going back into the sump.
Notice the 2-1/2 - 3 threads between the base of the hole and the drain hole. Lots of potential for leakage there. When installing the OPRV you should clay the bottom of the hole to see how far the OPRV sits to the end of the threads and adjust the thickness of the sealing washer. Or you can run the OPRV all the way in an use feeler gauges to check the minimum thickness of the sealing washer.
A non-hardening sealer such as Hylomar can be used. It has to be put in the case threads between the drain hole and the bottom. You have to experiment with how much to use so you do not have a wad sitting in front of the OPRV. If it gets pushed into the piston it will probably stick it.
With a different OPRV the drain hole can be blocked and the relief oil joined with the return line or a hole can be made in the lump to the left of the OPRV hole (cross drilling for the return to pump) and route the oil that way.
I have the filters and mounts available. Unlikely will make more of the Norton mounts though.
The '71/'72 OIFs had the oil feed pipe out the side of the backbone which puts it in the way of the Charles filter. Later frames put the outlet tube in the sump plate.
2 members like this
by gavin eisler
gavin eisler
Over the history of the BSA twin engines the TS bush got shorter, A10 longest , A65 a wee bit shorter, A70 shorter still.
Devimead became SRM, based on fixing this properly. with a non "lucky bag " real bearing which wont starve the big ends when it wears a bit.

The sludge trap is a good idea, a real filter plus the sludge trap is more better, the strainers are just to stop instant catastrophe, the strainers do not prevent suspended particles doing the long term damage.
Regular oil changes help.
1 member likes this
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