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#863106 11/13/21 8:27 pm
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A65 twin carb. I have only covered a small mileage on the bike over the last 2 years, it’s ran ok apart from a bit of smoke from LH pot. I recently had a top-end overhaul with new valves, springs, guides and the seats re-cut. The bike ran fine, plugs good colour, cylinder vacuum balance spot on, never took it over 4k rpm. After 200m or so, noticed oil seeping to top of L.H. head studs, then intermittent misfires started which worsened and found to be L.H. plug oiling up, and smoking from the same pot when opened up after overrun.

I removed head, barrel and pistons for inspection. As I lifted the head, I noticed the copper gasket was covered with oil and pistons wet.

My torque wrench (afterwards) was found to be in error by around 3-4 lb/ft (this has now been corrected) so the studs were probably under torqued.

I annealed the solid copper gasket to cherry red. I intended to re-torque at 500m but perhaps should have been earlier.

With the top piston rings set 10mm below top of bores and at the bottom of bore, the LH gaps are 18 thou top and bottom, RH are 19 thou top / 15 bottom.

Pistons show some scuff areas in center portion, also there’s a 10g weight difference in them (about same as a pound coin). They are 0.5mm oversize, marked BSA and stamped 8.11.72.

I have lapped the head/barrel mating surfaces as they were slightly out of true.

Any advice/observations welcome.

[Linked Image from i.imgur.com][Linked Image from i.imgur.com][Linked Image from i.imgur.com][Linked Image from i.imgur.com][Linked Image from i.imgur.com]
Any advice/observations welcome.

Last edited by semprini; 11/13/21 8:33 pm.

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Looking at those pistons, it seems like the bores were not scrubbed before assembly or
the oil has plenty of crap in it.
It has not run in as honing marks are still very evident, what oil did you use when you built it?
Ring gaps of that size will not cause excessive oiling but unsealed ones will.

A new set of rings and a hone will sort it but run some mineral cheap 20-50 oil when running in
an SG20/50 is all you want and give it some stick don't baby it.
After honing scrub the bores with soapy water for 1/2 an hour, lightly oil them and assemble.

My 2c.

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I've run it on 10/40 synthetic since getting the bike, I would never run it on old oil, but who knows what the previous owner did. It came from Germany 4 years ago and was a part re-build.


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If you really want to use synthetic oil, run it in on either a mineral 20/50SG or some dedicated
running in oil. Avoid synthetic or friction modified oil while running in.

10/40 is a bit light anyway 15/40 would be better. These are old air cooled crates with iron bores and rings.
Not nicasil water cooled steel ringed things. They need to be honed quite coarse and bedded in, super lubes
tend to prevent that.

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Agree with NickL, looks like the rings haven't seated probably due to using synthetic oil during running in.

I would get the bores re-honed using a coarse grade (around 180-220) and then try using a new set of rings, Hastings sell rings for the A65 which include a 3 piece oil control ring which is better at oil control than the single piece iron type, so maybe worth trying.

For oil I would try a classic car 20W50 mineral oil, Halfords stock one which works OK.


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I've took on board the advice given.

I've just noticed that the LH small end bush is protruding about 2mm out of the con-rod, which is a bit alarming! I made a tool and pressed it back in, although the rod had to be quite warm for it to move. Luckily, I have a 2ft precision rod (taken from a scrapped professional photo copier) which is just a few thou less than the gudgeon pin so I was able to check the bush alignment, which appears ok.
So again, any ideas as to why it should have moved, presuming that it was installed right in the first instance?[Linked Image from i.imgur.com]


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Nothing to worry about, it's not that uncommon any slight mis-alignment of the barrel
will bias them one way or the other and if they got a bit hotter than normal as the rings
were not sealing well they just moved a bit. As long as they are tight they will be ok.
The late steel capped rods didn't use small end bushes just ran on the alley same as
t140's (well it was the same rod forging anyway)

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Originally Posted by semprini
never took it over 4k rpm. .
You don't do these engines any favors by keeping the revs low. You must wring its neck a bit to seat the rings..


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A couple of other points, have you got the rings the right way round? You should see marks on the top edge of the rings showing which side is top.

If you fit them upside down they will pump oil into the combustion chamber instead of out, hence the excess oil.

With the oil on the gasket, maybe its leaking from the drain holes, or elsewhere, I would coat the gasket with spray on Permatex Copper High Temp sealant, see This Link Should stop any further leaks and improve heat transfer.


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Welseal is the goto on headgaskets for me. (old fashioned i suppose....)
You can use a couple of small o rings on the front olilways if you wish, that
is also a well recognised mod.

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NICKL- That's a relief! I've selected a couple of suitable O rings and will fit them

GUNNER- first time the barrell's been off since I bought the bike. There's only one ring out of the 4 comp. rings that's marked. One RH ring has a T mark topside, the LH rings are unmarked, I've had the magnifier on them and can't see anything, so possible that one is upside down, and the LH has always had the oil problem.

On further close inspection, all the top 2 rings appear square edged, except the LH second ring which has a slight bevel on the inside top-face.

DAVID P Yes, when I've rebuilt and confidence builds I'll push the revs higher, but with premium fuel at aroun £7/ uk gal, it doesn't help!

Last edited by semprini; 11/19/21 2:57 pm.

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