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Post by dave1800 on Feb 6, 2022 5:23:53 GMT
The original LOCA (Australia) website had an interesting "readers tips" section still available here. It may be worthwhile reproducing something similar here where readers can pass on useful tips that may help others that may not be covered elsewhere on the forum or possibly buried in long threads. What are your thoughts? As a bit of light fun in the cold UK evenings maybe readers could come up with questions to get the brain cells functioning (those from warmer climes are also included!) To kick off here is a question - hint the answer is more complex than you may at first imagine. What are the functions of the O ring on the distributor shaft (and does yours still have one?)
David |
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Post by jeff on Feb 7, 2022 19:26:17 GMT
Distributor O ring.
I thought it might be there to prevent oil from getting inside the distributor???
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Post by dave1800 on Feb 11, 2022 2:15:06 GMT
No one think this is worthwhile? The original LOCA (Australia) website had an interesting "readers tips" section still available here. It may be worthwhile reproducing something similar here where readers can pass on useful tips that may help others that may not be covered elsewhere on the forum or possibly buried in long threads. What are your thoughts? David |
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Post by dave1800 on Feb 11, 2022 2:22:55 GMT
Thanks Jeff The answers seems quite complicated. It appears BMC/BL stopped using an O ring and deleted the groove from the distributor shaft for a while with the 25D versions as used on Mk1/Mk11 crabs but it seems to have been reinstated and from what I have been able to determine is present on 45D versions as used on most Mk111 crabs. I will elaborate on my findings later to see if anyone else has ideas. Just as a taster the "experts" range of opinions range from it being of no use to being a key component to ensure good engine life. David Distributor O ring. I thought it might be there to prevent oil from getting inside the distributor??? |
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Post by Penguin45 on Feb 11, 2022 18:29:04 GMT
I'd always assumed it was there as an oil seal. Given the propensity of the "B" series to mark it's territory from every possible orifice, I made sure that my (25D) got a new O ring when I rebuilt it.
C.
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Post by dave1800 on Feb 12, 2022 4:43:46 GMT
Well done Chris you have identified what is described in Stephen Strange's publication on Tuning the MGB as a less significant purpose - but your garage floor may not agree.  The lengthy extract below from his publication below goes into the consequences of not fitting the O-ring including the potential for premature engine wear and damage. It raises the importance of ensuring everything connected to the the crankcase needs to be airtight for the vacuum to be maintained. On the crab this not only includes the rocker, tappet and timing gear covers but also the transmission. I would add the O-ring may possibly make it less likely for the distributor shaft to get stuck in place through corrosion although many could well have disintegrated over the years if not serviced. MGB forums I have followed suggest the O-ring makes no difference but it is often being considered by readers concerned with performance modifications where long engine life is not the priority. The article below makes reference to MGB engines but there is a direct read across to crab engines with the MGB 18V being similar to the 18H fitted to Mk111 cars. MGBs did away with the PCV valves a couple of years or so before the crab changes.
"The primary purpose of the O-ring (BMC Part # 12H 6547) on the shank of the body of a Lucas distributor is to maintain the partially-sealed state of atmospheric conditions inside of the engine so that the fuel induction system can create a partial vacuum inside of the crankcase. Without this O-ring, the purpose of the vented oil filler cap (BMC Part # 12A 402) of either the 18G, 18GA, 18GB, 18GD, 18GF, 18GG, and 18GH engines or the restrictor tube in the rocker arm cover of the 18GJ, 18GK, and 18V engines would be compromised, the -2 PSI vacuum inside of the engine being decreased by leakage through the engine block at the base of the distributor.
Its secondary purpose is to assist in establishing the proper alignment of the rotational axis of the distributor action shaft with that of the distributor action shaft prior to tightening it down, thus both prolonging the lifespan of the distributor action shaft support bushing and minimizing “timing wobble”. Note that the O-ring should never be mistakenly installed onto the driving dog of the distributor. In that position, the rotation of the driving dog would quickly destroy it. The O-ring also has a second, although less significant, purpose: to keep oil mist inside of the engine instead of allowing the pulsating atmospheric pressure within the engine to force oil to ooze out around the base of the distributor and drip onto your garage floor. Oddly, most people seem to illogically believe that this secondary purpose is its sole purpose.
Properly maintained, crankcase gases are drawn into the combustion chambers of the engine by the vacuum created by the fuel induction system, either through the intake manifold as in the 18GB, 18GD, and 18GF engines, or through the carburetors as in the later engines. This permits the crankcase to function in a partial vacuum of about -2 PSI which not only reduces power loss due to the pistons, connecting rods, and crankshaft forcing the atmosphere inside of the crankcase to move about (a condition that is technically termed either “Windage Loss” or “Pumping Loss”), it also causes the oil mist inside of the crankcase to condense more rapidly while being drawn upwards towards the camshaft and tappets. Because the oil mist becomes more quickly and more highly condensed in the partial vacuum, more of it tends to fall into the oil sump rather than remaining in suspension as a fine mist and being drawn into the fuel induction system. An oil separator that works on the turbulence principle is incorporated into the design of the front cover of the tappet chest in order to assist in preventing this. The mesh inside of the oil separator is designed to catch the oil mist in the air that travels from the crankcase to the carburetors.
If it is in good condition, then the oil returns to the sump in liquid form and only air & fumes go into the carburetors. On the other hand, if the oil separator is choked with carbon deposits, then the oil travels up the mesh because of the reduced availability of air passages as well as at a faster air speed. Consequently, gravity does not get a chance to return the condensed oil to the sump. Instead the flow of air will carry it onward into the combustion chambers where it will be burned, turning it into a fine coating of carbon atop the piston crowns, as well as on both the roof of the combustion chambers and the heads of the valves. Due to the fact that the effectiveness of the system is dependent upon vacuum, all connections between the fuel tank, the vapor separator, the adsorption canister, the rocker arm cover and its oil filler cap, the dipstick, the distributor, as well as between the oil separator of the front tappet chest cover, and the fuel induction system, must be well-sealed in order to maintain the optimum state of vacuum. Should you attempt to run the engine without the oil filler cap in place, you will find that because the airflow into the fuel induction system lacks restriction, so much air will be drawn into the fuel induction system through the intake manifold and thus bypassing the carburetors that the engine will not run.
In addition, without the partial vacuum induced by this system, the pressurized gases inside of the crankcase would cause oil on the cylinder walls to be blown past the piston rings into the combustion chambers, leading to carbon buildup on the roofs of the combustion chambers as well as on the piston crowns, with consequent preignition problems. The carbon can also collect inside of the groove provided for the compression ring, causing the compression ring to seize. In addition, an excess of these pressurized gases and oil mist would also be vented partially through the breather tube of the rocker arm cover, and resulting in an oily film inside of the engine compartment of engines equipped with a vented oil filler cap (BMC Part # 12H 1836, Moss Motors Part # 460-102) of the 18GA, 18GB, 18GD, and 18GH engines......." David I'd always assumed it was there as an oil seal. Given the propensity of the "B" series to mark it's territory from every possible orifice, I made sure that my (25D) got a new O ring when I rebuilt it. C. |
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Post by Penguin45 on Feb 12, 2022 20:34:51 GMT
OK, so why is there a hole in the spigot below the "O" ring?  C. |
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Post by dave1800 on Feb 13, 2022 3:32:06 GMT
Not sure where you mean? David OK, so why is there a hole in the spigot below the "O" ring? C. |
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Post by Penguin45 on Feb 13, 2022 11:15:34 GMT
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Post by dave1800 on Feb 13, 2022 12:45:59 GMT
That's interesting. The Mk11 distributor 25D4 I have here doesn't have a hole but it does have an "O" ring!
Clearly it is important to be able to regulate the amount of air and fumes that are drawn from the crankcase so as not to unduly affect the air fuel mixture and try to avoid sucking too much of the oil mist into the combustion chambers. This appears to be achieved by a combination of a restriction in the tappet cover pipe, the PCV valve (or constant depression connection on later carbs) and the orifice in the vented oil filler caps. Looking at the amount of oily deposits on the back of the inlet valves should give some idea of the effectiveness of the system and the general engine health.
Now it seems we may have to consider the oiling hole in the distributor shaft although I guess if it is full of oil I assume it may not provide a ready path for air to be pulled through the crankcase.
The whole issue of crankcase ventilation is quite interesting. There needs to be sufficient clean air drawn in to be able to purge the damaging fumes and water vapour arising primarily from blow-by and burn them in the combustion chambers without upsetting the air fuel mixture. This suggests that the SU needles may have been calibrated to take this additional air into account. Any unintentional sources of air being drawn in (eg by a missing distributor "O" ring) will upset the calculations.
If the crankcase ventilation becomes blocked or restricted by, for example, a dirty filter in the tappet cover this may lead to even more oil leaks as the crankcase pressure increases forcing oil out of seals and gaskets and the engine potentially running too rich.
David
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