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MG MGA - SU Damping Oil

I know this has been covered over and over in the past, but things change. There lots of suggestions in the archives for alternatives to expensive "SU Damping Oil", but I can't get single viscosity 20W oil today (30W yes), and there isn't one automatic transmission fluid - there's a dozen. Anyone have any suggestions on what they are using today, with today's oil chemistry?
F. J. Bruns

I have been using ATF for over 20 years and find that it allows the carbs to work as they should. The lighter fluid allows the piston to react to the throttle faster than a heavy oil. This will give a richer mixture and results in better performance. The difference between the various ATF is not of any consequence.

George
George Raham

ATF is common, and you can use multi-grade as well. Whatever you're running in your engine should work in the carbs also, such as 20W-50. With a factory stock setup, 20W is about right but these days every engine is different. No harm in experimenting, it can't hurt anything. You can even run without oil at all and it will drive down the road just fine!

Steve Simmons

I've used ATF in my 62 A carbs for over 35 years. The type of ATF was whatever I was using for the daily driver at the time; never an issue.
Nick Kopernik

The lighter fluid does allow the piston to react to the throttle faster than a heavy oil, but faster piston rise makes for leaner mixture on acceleration. Purpose of the damper is to enrich fuel mixture on acceleration, and thicker oil make slower piston rise and richer mixture. You can use same oil as you use in the engine for all the same reasons.
Barney Gaylord

The height to which the piston is lifted is controlled by the amount of air passing beneath it. When the piston is at the bottom of its stroke, with the engine idling, opening the butterfly allows the manifold depression to be communicated to the main volume of the body and then through a cross drilling in the lower part of the piston into the suction chamber above the piston. This depression immediately lifts the piston, allowing a mixture of air and fuel to pass the lower side of the piston and relieve the depression. The piston height is therefore stabilized at a depression controlled by the weight of the piston, the load of the piston spring, and: the area .of the large and small diameters of the piston. It will be noted that the underside of the large diameter of the piston is open to atmosphere. The air/fuel ratio is controlled by the diameter of the needle in the fuel jet. The optimum dimensions of this needle are normally found by experiment on an engine dynamometer, and by road testing of the vehicle. Once determined, a profile cam is made to give the desired needle dimensions, and production needles can then be very accurately reproduced to the desired shape.

From this it will be seen that once the correct needle has been specified, the jet has to be set to a datum dimension on the needle to ensure that the desired mixture is obtained throughout the range of the engine operating conditions. This position is set when the carburetter is tuned for idling.

Most S.U. carburetters incorporate a piston damper, the function of which is to restrict the speed of lift of the piston on snap throttle openings, and to allow the piston to fall at its normal speed on throttle closure. This one way damping is obtained by means of a non-return valve situated at the base of the damper.

When the speed of piston lift is retarded an additional air depression is put on the fuel in the jet resulting in an increase in the quantity of fuel discharged. A richer mixture is thus obtained until the piston resumes its position of equilibrium. This enrichment is necessary to provide satisfactory pick-up. The piston damper also improves cold starting and driveability from cold.
dominic clancy

Dominic, did you just swallow a book?
Neil MG

Motorcycle shocker oil SAE20 does it.
Dave H
Dave Hill

In Australia, at least, Penrite make a special dashpot oil -- and not expensive. I always have a supply.
Barry.
Barry Gannon

F.J. - Go to your local motorcycle shop and get 20W Fork shock (damper) oil. Its usually about $4/qt. Also can be used in the suspension dampers without issue.
... CR
C.R. Tyrell

When putting the ATF in the piston, I fill it to a level where I can feel resistance when the piston is raised around 1/4 inch. When the throttle is opened, this will raise the needle in the jet slightly, thereby allowing a greater volume of fuel thru the jet to accommodate the greater volume of air required by the throttle butterfly. I have always considered this to be a type of injector pump to improve acceleration. I would ask Barney Gaylord and/or Dominic Clancy or any others, to comment on my conception. I am always searching for understanding.

George
George Raham

Factory specification for oil height in piston tube is approximately 3/8" from the top. In case there is anyone new to the procedure, please note this is the steel tube on the piston, NOT the threaded hole in the dashpot cover. I've seen people fill the dashpot and it's a big mess. Best to remove the dashpot dome and fill the piston tube, then refit the cover. Easier, cleaner and arguably faster than doing it with the cover in place. And make sure the pistons don't stick after the cover is reinstalled!
Steve Simmons

This thread was discussed between 01/02/2016 and 04/02/2016

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.