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Neil Cocksedge is build­ing a Tri­ton us­ing a 1953 Tri­umph Thun­der­bird bot­tom end and a late 650cc bar­rel and head. He has a se­lec­tion of tap­pet guide blocks and pushrod tubes, but is un­sure which com­bi­na­tion of these to use. As these are a premier source of oil leaks, I de­cided to ask Arthur at Tri­umph spe­cial­ist Rocker­box for his opin­ion.

Arthur says the best sys­tem is the 1970on type with two O-rings and a flat seal. The bot­tom of the pushrod tube is rolled to pro­vide a hous­ing for an or­ange O-ring – it’s softer than the black O-ring which is more heat re­sis­tant and fits the cylin­der head end – don’t mud­dle them up.

Arthur lightly as­sem­bles the head to the bar­rel with a head gas­ket (but no seals) and mea­sures the up-and-down move­ment of the pushrod tube. He then mea­sures up the seals and looks for a 30% crush, so there’s enough squash on the seals for them to seal prop­erly, but not so much that they squeeze out of place or split.

Prob­lems arise when the cylin­der head doesn’t match the tubes – heads that use O-ring seals have a deeper coun­ter­bore (or re­cess) – or if the head has been skimmed some­time, so you may need thicker or thin­ner seals. There are a cou­ple of sizes avail­able, al­though for skimmed heads a thicker gas­ket is also avail­able. Arthur says you can deepen the coun­ter­bores in the head, but be­ware as they’re ma­chined slightly off-ver­ti­cal to match the an­gle of the tubes.

Fi­nally, bear in mind that sil­i­cone is a lu­bri­cant and sealant can cause the seals to squeeze out. Per­son­ally, I stick one side of the seal down at a time, leav­ing the sealant to set overnight. Then it will stay put when you tighten the head down.

Get­ting pushrod seals right cuts down on oil leaks

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