Filling in the Holes

There are a number of different ways in which the now-superfluous holes in the timing-side crankcase, primarily the 2 large holes previously used for the camshaft bushes, could be filled. As this part of the structure needs to be strong, a method that retains structural rigidity whilst being machinable yet not too invasive on the crankcase itself is required.

Epoxy fillers, such as Devcon and JB Weld, are not really sufficiently strong for this job, although JB Weld is used later in the project for filling in the threaded tappet block holes.

Welding, on the other hand, is certainly strong enough but the amount of heat that would be needed to fill these relative large holes and the danger of distortion and warpage to the overall structure that could result makes this a potentially hazardous option.

In the past, I have had good success using Lumiweld for filling holes in gearboxes with ruined threads and crankcase damage on other engines and therefore opted to use it here.

The “weld” part of “lumiweld” is misleading; it is not welding, it is a brazing process, Lumiweld melts at 390^oC whereas aluminium, typically, melts at more than 250 ^oC higher temperature. The strength of lumiweld is of the same order as aluminium. For the record, literature values for the UTS (Ultimate Tensile Strength) of lumiweld and 6061T6 (a medium to high strength aluminium alloy) are 60 kpsi and 45 kpsi respectively.

The process of applying lumiweld is, however, quite different from traditional welding, brazing or soldering. To get lumiweld to adhere to the aluminium surface, the surface must be scraped vigorously with a stainless steel rod during heating; this is of paramount importance. After cleaning the surface of grease and roughing with coarse emery cloth, the overall process that I have evolved for filling holes is as follows:

     1)    Block one end of the hole to be filled with a piece of stainless steel plate using a g-clamp or some kind of improvised jig. This is important to stop the molten lumiweld from simply running out of the bottom of the hole and it will not adhere to stainless steel allowing the plate to be removed later.

      2)    The structure can either be pre-heated in an oven or use oxy-acetylene to warm it up initially. As the structure comes up to a high temperature concentrate the heat in the area that is to be filled.

      3)    Melt a small piece of the lumiweld into the hole in the flame and then stir it around the hole vigorously with a stainless steel rod, keeping the heat on the structure. Keep the stainless steel rod out of the flame as it will simply melt it!  Eventually, the surface is seen to  “wet” and the lumiweld starts to adhere to the surface around the bottom of the hole.

      4)    Add more lumiweld into the existing puddle and continue stirring, working up the hole in the stirring/surface wetting process.

      5)    Eventually, when the top of the hole is reached, add some lumiweld around the outside of the hole (this ensures that the edge of the hole is properly covered) and build up an excess (ie a little “mound”) immediately above the hole (or rather, where the hole was). On cooling and solidifying this mound will contract and this ensures that there is enough material to machine back to a flush surface.

The picture below shows the side of the crankcase after hole-filling that was blocked off with a piece of stainless steel plate.

And the next picture shows the inside of the crankcase from where the hole was filled.

And finally, the crankcase is machined to clean-up the excess lumiweld and to form a recess, centred on where the drive for the overhead cam (which consists of a gear driven by the crankshaft and 2 sprockets for the overhead cam and magneto drives respectively – identical to the K7), into which an aluminium housing will be inserted to support the OHC drive itself. 

Here, the smaller holes in the crankcase will be retained but the larger holes around the outside (those with threads) that previously supported the tappets and a breather will later be filled with JB Weld.