Some years of answering the same questions finally makes one realise that there is some confusion over the various head gasket and push-rod tube seal variations over the years of production of our favourite motorcycle.
All the following information is, in general, about the Trident, but all applies to the Rocket Three for the same production years.
The very early production, i.e. late 1968, can be ignored, basically because I have no experience!! So we come to 1969. This year used a metal “Corgasyl” head gasket, with two 70-4752 rubbers and a 70-4746 bottom cup specified for each push-rod tube.
From 1970 through to 1974, a fibre head gasket was specified, still with the two 70-4752 rubbers and a 70-4746 bottom cup.
The 1974 N.V.T. production used a copper head gasket, 71-3941, still with 70-4752 rubbers and 70-4746 bottom cups. However, due to the increased thickness of this version of gasket, a fibre washer 71-1190 was fitted under the bottom cup. Also in this N.V.T. production, a new push-rod tube 71-4000 was fitted, in place of 70-6955. The 71-4000 push-rod tube has a longer engagement spigot than the previous 70-6955, to make engagement into the rockerboxes simpler during assembly.
The T160 continued to use a copper head gasket but due to a change in the cylinder head bolt/pillar stud configuration the hole size in the head gasket had to be changed on the outer four fixings and the part No. was changed to 71-4250. This gasket can be used as a universal copper head gasket for the Triple. The T160 also continued to use 70-4752 rubbers, 70-4746 cups and 71-1190 gaskets. The push-rod tube was again changed to 71-4378. The push-rod tube required more room in it, above the cam follower, to accommodate the extra space required by the cam follower ball end and the cup on the push-rod. The previous feature of the long top spigot was continued from the T150 for the T160.
So, and now to the problems and suggested cures. The “Corgasyl” head gasket was not successful and is not worth using. The fibre gasket was successful in sealing around the drain dowels but occasionally failed to seal against combustion. The original fibre gasket was reinforced with wire and sometimes a wire strand would protrude into the combustion chamber and, glowing red-hot, would either cause pre-ignition leading to a holed piston; or burn out, leaving a passage to the outside world. Pattern fibre head gaskets did/do exist but they are not properly reinforced and fail very quickly.
The copper head gasket cured the compression sealing problems but was not so successful at sealing around the drain dowels.
There have been two attempts at producing a new style of fibre head gasket, eyeletted around the cylinder bores. The first attempt was not successful. The cylinder bore eyelets were not concentric with the cylinder bores, and the fibre material was too soft and not reinforced. This gasket is identified by the copper colour of its eyeletting rings. The later version has aluminium/steel coloured eyeletting rings and the fibre material is re-inforced with a wire weave. This gasket is very successful, sealing against both compression and oil with little fuss. (Additional Feb 1996: This gasket has now been changed again, and the wire weave has been replaced by an aluminium plate. I expect continued success for this new variation) NOTE: If this type of head gasket is used then it is necessary to check to head bolt torque frequently, until there is no more movement. My suggested regime for this retorque is (after thoroughly warming the engine and allowing it to cool) 0 miles, 5 miles, 50 miles and 500 miles
The most common cause of push-rod tube leaks is over compression of the rubber seals. If any material has been removed from the cylinder block or cylinder head, the amount of material removed will be equal to extra compression of these rubbers. Also, if 71-1190 is used, this will also add to the compression of the rubbers. The fix is to ensure only reasonable compression of the rubbers. There is a thinner rubber available under part No. 70-3547. Often I have found that use of one of these, at the top of the push-rod tube, will ensure a perfect amount of rubber compression. If necessary, mixing and matching of components will often allow fine adjustments.
Another problem I frequently see in this area involves the fitting of later, extended spigot tubes into early rockerboxes. Although the position of the sealing face of the rubber into the rockerbox was not changed, the amount of aluminium cleaned away for spigot clearance was obviously much less on pre-N.V.T. rockerboxes, originally machined for the shorter spigot. The fix is, of course, to ensure the push-rod tube is an easy fit into the rockerbox with no trace of any foul condition between the two parts. Aluminium can be removed from inside early rockerboxes to achieve the required clearance.
I hope the above information and suggestions will be useful to owners and that some persistent oil leaks can be cured for ever.
Copyright © 2000 Philip Pick
Version 0.20 Date January 2004