A10 breather hole size

[limeyrob]

I'm working my way through the oil coming out the breather, reduced but not cured.  In the meanwhile I'm thinking of making up a catch can.  Does anyone know the hole size in the crankcase?

[richardrocket]

Hello limeyrob, the A10 breather hole is 5/16" diam. The problem you have with fitting a catch tank, is that as they come out of the factory that is what you get, just a a hole! To fit a catch tank or whatever, you need to attach a pipe. To these ends, a short length of 8mm micro-bore radiator pipe is ideal! It will either just slide in or just not slide in. If tight, reduce the diam in a lathe or a drill with some wet or dry until it just fits in for about 3/4", clean the hole of oil and secure with loctite.  Then fit your pipe 5/16" bore and take it to your catch tank. Or, why not use it to lubricate the rear chain?
Coming back to the original problem, two points to consider. The breathing system works on negative pressure as the pistons ascend. To work there must be no leaks in the system, so this means no extra breather pipes and also ensure that you have the crankshaft oil-seal fitted correctly! Best of luck!
   

[limeyrob]

Well since this post and in some frustration I fitted new rings.  The "old" rings were a bit of an unknown quantity as the bike arrived in bits with what appeared to be new rings.  This time I've fitted Hepolite and just done a couple of runs.  Its still blowing oil but nothing like as much and there's definitely more compression so I'm hoping that the oil out the breather will reduce as the rings bed in.

[limeyrob]

Update, new rings have upped the compression and cut the oil burning so that part is a success. But something is not right with the oil coming out the breather. I now suspect the oil pressure relief is pushing too much oil to the cam trough and timing gears. The threads are worn, when they are back in stock at SRM I'll get one of theirs as they say the threads are on the upper limit.
Yesterday I managed to tap the breather 1/8 BSP, as you say its about 5/16.  Waiting on a 1/8 BSP die to I can make up a tube. I'm hoping that the catch can set up will not be needed after a while, but who knows?

[limeyrob]

Further update.  I now have a breather tube and catch can. With hindsight and for anyone else planning this I would say the 8mm press fit is probably the best approach, the 1/8 BSP is a right faff.
Its done what I hoped:
1 Its proved the oil is coming out the breather. Before it was hard to tell.
2 Shown that the engine is not breathing heavily but dollops of oil are coming out after the engine has been running a while.
3 It does not appear to be RPM sensitive. Having a transparent catch bottle means I can now see what's going on.

I'm leaning more towards the oil pressure valve leaking and putting more oil into the cam trough and then timing gears but I won't know until I get hold of the new valve.  For now at least I can ride without fear of oil on the rear tyre.

[DAVE BRADY]

Hi,

More oil in the cam trough will not impact the breather as the breather drilling runs under and parallel to the cam though. The oil in the cam trough will get flung back on to the crank and back to the sump. The fit of the 'top hat' and breather corks are vital for proper breathing.
Your comment re "dollops of oil are coming out after the engine has been running a while" seems a bit odd.  Usually, any excess oil in the crankcase is blown out of the breather on start up and should clear after running a while, so why the 'dollops' of oil?

Dave.

[limeyrob]

Yes it is odd, I believe the cam trough drains to the timing gears.  When I took the timing cover off to fit a thicker cork there was a lot of oil in it, hence my looking at the oil paths.  The oil pressure relief valve is worn so its worth replacing anyway.
The engine is running well and has a lot of compression and is not smoking so I am puzzled.  The breather has a face with a hole so once its spinning it should allow air past and fling oil out so I'd like to see whether it only dumps oil when its at low revs.  It will create a small puddle at low revs or idle and there's oil on the rear wheel, but after a (say) 20 mile run the oil level has not dropped much, hence my suspicion that this is a low rpm problem.
The rings have only done couple of 100 miles so I'm hoping that this problem will reduce and I can take the catch can off in due course.