Friday, December 4, 2009

Repairing a cracked pre-unit Triumph head

Just about every pre-unit Triumph with an alloy head will have cracks that run to the valve seats. An old-timer that I know told me "we used to run them 'till you could hear them whistling through the head bolts, then we would fix 'em". Triumph fixed this issue when they went to the 9-bolt head on the unit bikes for 1963.

Here we have a single-carb, pre-unit head that is going to get a basic valve job. While the head is off, we might as well fix the cracks. You can see each crack runing from the seat to the head bolt hole. The bottom of the head has been bead-blasted and washed to clean out the cracks:


Now we carefully, grind V-shaped notches prior to starting the welding:


Pat goes to work:



Welded up and ready for clean up and the rest of the valve job:

Here we are after grinding down the welds and truing the mating surface. Afterwards, we fit new valve guides, cut the seats, and install new valves and springs. Ready to go for another 50 years!

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