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.
Following recent re-builds, both my engine & my Uncle's engine have had similar problems with oiling, both have improved now they have some miles under their belt, my Uncle's was worse than mine, but he will keep fiddling with the spark plugs to check and they are only hand-tight, which I am sure doesn't help, but he won't be told.
The piston ring gaps should all be staggered and they need to be correctly gapped, too wide and they will not be as effective as they should be, too tight and they could break. I would be inclined to put it all back together and give it some decent road use and see how it is after about 500 miles, mine is improving (although I have not had it out of the garage since before Christmas), let the new piston rings bed in and see how it goes.
I don't have oil baffles, lots of engines don't, in some ways, the extra oil up the bores could be regarded as a good thing, but if the rings are doing their job it should not make any difference.
(07-02-2018, 11:57 PM)GK5268 Wrote: Following recent re-builds, both my engine & my Uncle's engine have had similar problems with oiling, both have improved now they have some miles under their belt, my Uncle's was worse than mine, but he will keep fiddling with the spark plugs to check and they are only hand-tight, which I am sure doesn't help, but he won't be told.
The piston ring gaps should all be staggered and they need to be correctly gapped, too wide and they will not be as effective as they should be, too tight and they could break. I would be inclined to put it all back together and give it some decent road use and see how it is after about 500 miles, mine is improving (although I have not had it out of the garage since before Christmas), let the new piston rings bed in and see how it goes.
I don't have oil baffles, lots of engines don't, in some ways, the extra oil up the bores could be regarded as a good thing, but if the rings are doing their job it should not make any difference.
Hi Gary,
Thanks for the rapid response.
I should have noted that prior to me pulling this piston out I had done around 400km since the rebuild with no appreciable improvement. After 200km I droped in a can of that "stop oil smoke" stuff as I thought that might help things take up as the rings bedded in, but no change was evident. I gapped all the rings to 0.007" and while the big red book tells us to arrange at 120 degrees I thought that must have been for the 3 ring versions. I put mine with the compression ring gaps aligned with the crank centerline (one facing forward, the other back) and the oil ring gaps perpendicalur to compression rings.
But like you said it might just need a few more miles.
HI Steve,
When you say the bores have been honed was this done with a parallel or deglazing hone
the bores after honing should have a distinctive Crosshatch Pattern this can only be obtained by slow rotation and vertical movement .
At the moment what you have are glazed bores.
they will need to be done again using a parallel hone.
if you do not have the gear take it to your nearest Engine Shop they should be able to do this with out dismantling the Engine
You make a good point but I'm not sure it's that. The hoing was done by a shop that specialises in vintage engines, he also made my white metal bearings. I'll have a look at the bores tonight and make a call. Thanks for the rapid reply.
From your photos your pistons don't look very old ?
I'm not a fan of the old fashioned split skirt 4 ring pistons and for the cost fitted a more modern design.High silicone pistons expand very little and don't need the split skirts.
Have you asked your reconditioner for advice on the oil burning issue ? I would think if its burning oil to that degree it will never be very good.
If I were using those pistons I would fit the oil baffles.The oil pressure relief valve isn't adjustable and in a splash feed engine will only open when the oil is cold and engine revving.
I would be concerned if I was getting 15 psi hot,perhaps a blocked jet?
Have you tried taking the oil filler cap off when running to see how much back pressure/blow by there is ? Maybe the crankcase pressure is pushing oil up the bores.
(08-02-2018, 10:48 AM)Austin in the Shed Wrote: From your photos your pistons don't look very old ?
I'm not a fan of the old fashioned split skirt 4 ring pistons and for the cost fitted a more modern design.High silicone pistons expand very little and don't need the split skirts.
Have you asked your reconditioner for advice on the oil burning issue ? I would think if its burning oil to that degree it will never be very good.
If I were using those pistons I would fit the oil baffles.The oil pressure relief valve isn't adjustable and in a splash feed engine will only open when the oil is cold and engine revving.
I would be concerned if I was getting 15 psi hot,perhaps a blocked jet?
Have you tried taking the oil filler cap off when running to see how much back pressure/blow by there is ? Maybe the crankcase pressure is pushing oil up the bores.
Also worth checking what oil you are using.
Crap 20/50, mono grade or running in oil I hope. Nothing sophisticated
Some have a correct orientation - a dot marking the 'upper' side.
The rings I use are very hard and require about 2000-3000 miles before being run in properly.
The rings came from our club's spare parts department, we've got a meeting tonight so I'll ask the question there about bedding issues. I'm pretty happy that I got the orientation correct but stranger things have happened, will pop them off for a quick check. Thanks for the response, all good points
(08-02-2018, 01:58 PM)Charles P Wrote:
(08-02-2018, 10:48 AM)Austin in the Shed Wrote: From your photos your pistons don't look very old ?
I'm not a fan of the old fashioned split skirt 4 ring pistons and for the cost fitted a more modern design.High silicone pistons expand very little and don't need the split skirts.
Have you asked your reconditioner for advice on the oil burning issue ? I would think if its burning oil to that degree it will never be very good.
If I were using those pistons I would fit the oil baffles.The oil pressure relief valve isn't adjustable and in a splash feed engine will only open when the oil is cold and engine revving.
I would be concerned if I was getting 15 psi hot,perhaps a blocked jet?
Have you tried taking the oil filler cap off when running to see how much back pressure/blow by there is ? Maybe the crankcase pressure is pushing oil up the bores.
Also worth checking what oil you are using.
Crap 20/50, mono grade or running in oil I hope. Nothing sophisticated
Charles
Hi Charles,
Your idea to remove the filler cap and have a look inside while running is very logical (wish I'd thought of it before I dropped the piston out). With respect to the oil pressure relief valve I didn't think it was adjustable but when I checked the big red book (Austin Seven Manual by Woodrow) they nominate that external plug as an adjusting screw. I think if I had my time over again I'd just put in solid skirts. As for oil she's on the same Penrite HPR30 20W/60 that the previous engine used without fault.
First of all, I'm no expert, but I reckon my eyesight is OK...
Looking at your photo of the piston and rings I can see that the top compression ring appears to have a tapered profile as there is no apparent contact wear on the lower portion of the ring face. The next ring down appears to have a shamfer on both upper and lower edges. Could these two rings be mis placed?
Furthermore, the two oil control rings have two contact faces, one of which is slightly wider than the other, although each ring is fitted upside down relative to its sibling. The fact that you've got the gap on the upper oil control ring coincident with the hole in the piston skirt would suggest that it's not helping much with oil control.
Just my observations, I might be talking bollocks. (it's usually the case!)
