The early MGTC carburettor was equipped with brass pistons. These were
later replaced by aluminum or pot metal pistons, with a cast iron or
steel ring on the outside of the piston. Pot metal or aluminum being
lighter that brass, the ring was added for more weight.
Unless the brass and non-brass pistons weigh exactlly the same, they
will not operate the same.
According to the S.U. Reference Catalog, MG TB & TC's were equipped with
a AUC 4587# spring. The Y types did not have a spring. The TD's also had
the AUC 4587# spring. The competetion TD's, (1 1/2" carbs, MK II's) had
AUC 4587 springs. The "#" indicated "where fitted". The TF 1250 and 1500
also received the AUC 4587 springs. From this information, it can be
concluded that if the pistons were brass (TB's, TC's, and early TD's),
that there was no spring installed. The TF 1250 and 1500 definitely had
pot metal or aluminum pistons, hence they should also have springs.
As to dashpot tops... Round Brass Caps, AUC 8103, with knurled edges,
and vent holes, were used on TB, TC, Y Saloon, TD. Hexagon Brass Caps,
AUC 8102, with vent holes, were used on the TF 1250's and 1500's. These
cars were equipped with 1 1/2" S.U. type H4, AUC 728 carbs.
John Seim
Irvine, CA
Scott Allen wrote:
>My '52 came to me with one piston that was pot metal, and one that was
>aluminum. Both had springs, and both had lead weights in them.
>
>>From a balance standpoint I figured both pistons should at least be
>made out of the same materiel, (and weight about the same) so I picked
>up another pot metal piston and cover combo to replace the aluminum
>one.
>
>When I sent them off to be rebuilt by a guy that's been doing it for
>40 years they came back to me with the springs removed.
>
>When I sent the car off to have the engine and tranny to be rebuilt by
>a guy that's been working on T's for 50 years he asked, "where's the
>springs?"
>
>Maybe there's someone on the list that can give a definative answer?
>
>Scott Allen
>
>52 TD
>74 1/2 BGT
>
John Quilter wrote:
>For over 25 years I have been running stock SU carbs on my TD but the PO sold
>me the car with one carb with a brass (?) weighted dash pot piston and one
>with a pot metal or aluminum piston with a spring to force it down. It
>finally dawned on me this could not be correct, though the car ran OK all
>these years. I have now located and installed a matching pot metal piston
>with spring and the car seems to run about as well as before. My question:
>did all TDs with HS1 1.25 inch carbs have the brass weighted pistons or did
>some have the pistons with springs. Is there any problem running the spring
>type? My 1960 Morris Minor runs one HS1 1.25 inch SU with the spring. Also,
>were the dashpot caps the hex type or the knurled type?
>
>John Quilter
>Brisbane, California
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