Hi all,
Steve D here from Brisbane Australia, long time reader, first time writer.
I'm wondering if anyone can give me some pointers with an issue I'm having. I've recently rebuilt an A7 engine (2 bearing, oil jets, 30’ oversize) and it's letting too much oil past the rings. So much that it’s blowing a lot of blue smoke, fouling even the hottest of plugs, leaking out past sparkplug #2 and when I inspected the pots you can see the oil ponding on the top of the piston (see fig 1). Having the nickname of “old smokey” is not something I aspire to!
I used the split skirt pistons that had been in there before the rebuild as they seemed fine, nice fit and no nasty marks. The bore was honed and I installed a new set of rings, one oil at the skirt's base and another oil ring and two compression rings up top (see fig 2). Before the rebuild there was no excessive smoke, the rebuild was due to a bottom end bearing failure.
My theory is that oil is bypassing the bottom oil ring via the split in the skirt and to an extent the same for the upper oil ring as the blunting hole at the top of the split extends nearly to the top rail of the upper most oil ring (fig 3). The worst case scenario would be if the blunting hole aligned with the ring gap. In this case there is nothing to stop oil migrating from the inside of the piston to the 2nd compression ring.
So, if the pistons are the same before and after the rebuild why is there a problem now? My theory is that because of the rebuild a greater volume of oil is being delivered to the crank and as a by-product more is being splashed up on to the cylinder walls. What could cause a greater volume of oil to be delivered;
1. New oil pump internals;
2. Cleaned out 90 years of gunk from the oil galleries;
3. Replaced what I thought were damaged oil jets but upon reflection I think they might have been deliberately closed off; and;
4. Replaced the spring and ball oil pressure release mechanism.
The engine did not have oil baffles at the start and thus they were omitted for the rebuild.
So my first question to the group is, does the above theory hold any weight or am I heading down the wrong track?
If the above is true then is a good start to install a set of oil baffles between the block and crankcase?
Or maybe I should try to lower the oil pressure by backing off the adjusting screw? Cold starts shows 20psi, hot running is regularly 15psi and this is way in excess of the 1psi noted in Austin 7 companion.
Any suggestions would be most welcome.
Steve D here from Brisbane Australia, long time reader, first time writer.
I'm wondering if anyone can give me some pointers with an issue I'm having. I've recently rebuilt an A7 engine (2 bearing, oil jets, 30’ oversize) and it's letting too much oil past the rings. So much that it’s blowing a lot of blue smoke, fouling even the hottest of plugs, leaking out past sparkplug #2 and when I inspected the pots you can see the oil ponding on the top of the piston (see fig 1). Having the nickname of “old smokey” is not something I aspire to!
I used the split skirt pistons that had been in there before the rebuild as they seemed fine, nice fit and no nasty marks. The bore was honed and I installed a new set of rings, one oil at the skirt's base and another oil ring and two compression rings up top (see fig 2). Before the rebuild there was no excessive smoke, the rebuild was due to a bottom end bearing failure.
My theory is that oil is bypassing the bottom oil ring via the split in the skirt and to an extent the same for the upper oil ring as the blunting hole at the top of the split extends nearly to the top rail of the upper most oil ring (fig 3). The worst case scenario would be if the blunting hole aligned with the ring gap. In this case there is nothing to stop oil migrating from the inside of the piston to the 2nd compression ring.
So, if the pistons are the same before and after the rebuild why is there a problem now? My theory is that because of the rebuild a greater volume of oil is being delivered to the crank and as a by-product more is being splashed up on to the cylinder walls. What could cause a greater volume of oil to be delivered;
1. New oil pump internals;
2. Cleaned out 90 years of gunk from the oil galleries;
3. Replaced what I thought were damaged oil jets but upon reflection I think they might have been deliberately closed off; and;
4. Replaced the spring and ball oil pressure release mechanism.
The engine did not have oil baffles at the start and thus they were omitted for the rebuild.
So my first question to the group is, does the above theory hold any weight or am I heading down the wrong track?
If the above is true then is a good start to install a set of oil baffles between the block and crankcase?
Or maybe I should try to lower the oil pressure by backing off the adjusting screw? Cold starts shows 20psi, hot running is regularly 15psi and this is way in excess of the 1psi noted in Austin 7 companion.
Any suggestions would be most welcome.