By David Porter, Customer and Technical Support at The Bonneville Shop

Function of Carburetor Mounting Orings

Frequently, confusion arises over which carburetor mounting Orings should be used when fitting Amal Concentric carburetors to Triumph twin-cylinder engines, or whether an O-ring should be used at all. We offer 3 different carburetor O-rings: the 244/1048, as used on the Amal Monobloc carburetors, and the 622/101 and 70-9711 O-rings, as used on the 626, 628, 928, 930, and 932 series Concentric carburetors. They are all meant to perform the same function, which is mainly to seal the inlet tract of the carburetor to the inlet manifold.

However, another important function is to minimize heat transfer from the cylinder head and manifolds to the carburetor. The use of the phenolic insulator blocks, part number 70-2968, is helpful in keeping the carburetors from getting too hot, particularly on the BSA A50 and A65 engines, where the carburetors are directly mounted to the cylinder head. Triumph fitted the phenolic spacers extensively on early unit-construction 500 and 650 models equipped with Monoblocs prior to 1968.

“…another important function is to minimize heat transfer from the cylinder head and manifolds to the carburetor.”

Thick 244/1048 Orings

The thick 244/1048 carburetor mounting Orings used on the Monoblocs easily fit into the circular grooves cut into the mounting flange on the carburetors and are usually squished down against the phenolic spacer to fill the entire groove on the mounting flange, making an airtight seal at the joint, with the phenolic insulating spacer acting as the heat sink.

The plain studs, washers, and nuts that were used in fitting the carburetors to the manifolds required a reasonable amount of tension to prevent the carburetors from loosening off the manifolds and inducing an air leak. (I’ve encountered thread-locking compound more than once on these studs). The temptation to overload the nuts on the carburetor mounting studs ultimately could and did result in the carburetor’s flange becoming warped to the point that the cylindrical mixing chamber of the carburetor became ovalized.

Once the flange is warped and the carburetor body becomes distorted, the movement of the throttle slide is affected. Throw in some heat transfer from the cylinder head, manifold, and the steel manifold studs, and the “pot metal” carburetor body doesn’t stand a chance.

Thin 622/101 Orings

The 1968 Triumph twin models used a similar arrangement with the introduction of the Concentric carburetors. As in previous model years, there was a NA43A (superseded to 71-1416) paper gasket against the manifold flange, then the 70-2968 phenolic insulating spacer, which formed an effective abutment for the thin 622/101 carburetor mounting Orings to make a seal against.

The use of plain flat washers and nuts still required a reasonably tight load on the nuts to prevent vibration from the engine to shake the nuts loose, and as there was no stated torque specification for these nuts, they were often tightened beyond the sensible 10-14 ft/lbs. of torque, resulting in, you guessed it, warped flanges and carburetor bodies. You had to pick your poison, which was to risk a vibration-induced air leak at the carburetor/manifold joint, resulting in poor performance and potentially excessive combustion chamber temperatures from the lean mixture, or overtighten the flange nuts and risk sticking throttle slides. Remember, the heat generated by the engine only exacerbated this condition.

Additional Hardware

Carburetor Mounting Orings: 622/101 Thin Vs 70-9711 Thick 16

By the 1969 model year, Triumph B range 650 twins saw a change in specification for the twin-carbureted T120R models. Although the thin 622/101 carburetor mounting Orings were still in place, new manifold studs were fitted, along with the 70-9554 insulating rings and 70-9555 cupped washers. Finished off with the 14-1301 thin Cleveloc nuts, a vibration-resistant system was now in place to obviate overtightening the mounting nuts, as they usually bottomed out on the unthreaded part of the studs after the rubber insulating rings were compressed to their limits.

This improved mounting system did not appear on the TR6 models until the forthcoming 1970 model year, which also saw the introduction of the thick 70-9711 carburetor flange O-ring, and the omission of the 70-2968 phenolic spacer on the T120R models. The thicker carburetor mounting Orings had an interesting characteristic in that they appeared to be oversized in the carburetor mounting flange groove and stood noticeably proud of the groove when pressed into place.

Measuring the Gap

When properly captured in the groove and compressed during the fitment of the carburetor to the manifold, a visible air gap of around .025-.030” is observed. The result of the air gap is a reduction of heat transfer, as well as a subtle “rubber mounting” characteristic, coupled with the revised rubber insulating rings and cupped washers. The ¼-28 Cleveloc nuts helped prevent the carburetor from migrating away from the manifold due to vibration and only required approximately 8-10 ft/lbs. of torque to properly secure the carburetors to the manifolds. The C range 500 models continued the use of the thin 622/101 carburetor mounting Orings and 70-2968 phenolic insulating blocks, although Cleveloc nuts were specified from the 1970 model year onward.

The improved carburetor mounting system continued into the oil in frame era, along with some intake manifold changes along the way, and the 70-9711 thick O-ring continued to be specified on the T140V models through the 1978 model year. The TR7 models employed the thick O-ring into the early 1980s. BSA twins used essentially the same system of a thin O-ring and phenolic spacer (although they called it “Tuffnol”) all the way to the end of their production.

As you can see, there was an evolution of how Amal carburetors were mounted, as the O-rings, gaskets, and hardware changed over the years. Nowadays, vintage Triumph owners have a vast aftermarket available to experiment with different configurations in an effort to improve the carburetor mounting system on their beloved motorcycles. The Bonneville Shop stocks both of the carburetor flange O-rings, as well as the phenolic insulating blocks in differing thicknesses. Another iteration of the venerable Amal Concentric carburetor has debuted featuring an aluminum alloy body, and we are proud to offer the new alloy Amal Premier Concentric carburetors.