Cam Caps-DIY Replacements.

During the manufacturing period of the twin cam engine, lotus Engrg perfected the art of machining accuracy which enabled the production of a multitude of identical parts.

The alloy pressure die cast cam caps of the twin cam the cylinder head were produced in high numbers, then assembled onto the head and line bored on each cylinder head, the caps have reference numbers stamped on the cap and the top of the cyl head for easy reference during reassembly, but surprisingly, due to accurate machining, individual caps can actually be moved to a different location, (If required) without creating serious alignment problems, but of course, best engine building practice is place number three cam cap exactly where number three belongs, and so on.

But a dilemma could arise where the purchase of a totally bare cylinder head is made that is sans camshafts and cam caps. Can you install a set of cam caps from another cylinder head onto the bare head you have purchased? It seems the answer is yes, to a point, but very serious consideration regarding the running clearances and cam/cap alignment must be given.

The worst case scenario of course, is to carry out another line or tunnel boring operation.

The writer acquired a bare stromberg head for conversion to twin weber carbs, the word was put out to other Aussie lotus owners in order to source a set of cam caps, and as luck happened, a set of eight caps were obtained, (There are ten cam caps per set) and that raised a question as to whether we should simply purchase two new cam caps, or attempt to re-manufacture machine new cam caps from alloy billet.

The writer enjoys a technical challenge, so you can see where this is heading, cant you.

The Boring Jig.
After spending a few hours producing a special turning/boring jig, the stage was set to begin the marking out, drilling and machining, the process was surprisingly easy to do, regardless of what some people may think, but of course, great care is required at all stages of manufacture, only two cam caps are required in this instance and we were hoping that there will not be any need for line boring to be done, so the special jig had to be spot on, the machining and tolerances had to be spot on, the turning/boring work was carried out with the jig being centralised (Dialled in) using a four jaw chuck with a dial indicator to less than .001 run-out.

The jig components were accurately machined to size and fit, then checked in-situ to ensure they were correct, then the cap holding nuts are tightened, and the steel mandrel and steel jig holding piece are welded together in the bare head shown above. (Arc welded in this instance) the jig is removed after cooling down from the welding process.

To build an accurate jig, it has to be built and welded together in the head where the camshaft operates, all of the caps are removed, the bearing shells are removed, and the following procedure is best done on a totally dry, bare, and de-greased head.

When the decision is made to re-manufacture an engine component, there is always the requirement to produce special tooling and a jig to enable the parts to be made, this is where it is easy to come unstuck, simply through the extra time and effort required to build the jig, but if you enjoy an engineering challenge, the project can provide immense personal rewards in achieving a goal through simple means and for incredibly low cost, IE, your own DIY time.

The photograph below illustrates the configuration of the boring jig, an arbor is machined to be an exact fit into the cam bearing journal as this is required to be clamped tightly during the welding process.

The fixed main holding block welded to the mandrel is simply a replica of an original cam cap complete with inserted locating dowels. The alloy billet is fastened to the steel holding block with 1.75″ X 3/16″ cap screws, the other half of the jig is employed as a drilling jig.

Because the boring jig is manufactured to the exact same dimensions as the original cam caps complete with locating dowels, new cam caps can be machined with a fair degree of accuracy, this project has been approached and completed the old school way of marking out with a sharp scriber and engineers blue, and then employing careful drilling, machining and boring operations. (Remember your old tech or trade school days). Sure this work could all be done with CNC mill, but the end goal was to produce two new cam caps, so hardly worth the effort to spend the time drawing a CAD file.

The cam cap boring jig is mounted and centred with a dial test gauge in the independent four jaw lathe chuck, run out is kept to less than .001 of an inch.
The alloy spigot protruding from the tool is screwed into the mandrel and locates on a taper, this is used to rest the point for the dial test gauge, then it is unscrewed and removed before boring commences. The solid billet of alloy is attached and underneath the steel holding block, the hole has been pre-bored to with in 1/16″, careful and accurate measuring is required during the boring operation.

The biggest expenditure on the project was workshop time, some would say that is an inefficient way to acquire parts, but the material cost was only a few dollars worth of high quality aluminium bar stock, the parts were completed within two days, which is much quicker than waiting for parts to arrive from the USA or UK.

This is one more “must do” job that can now be ticked off the Big Twink 1760cc Engine project list.

So after having done all of the painstaking work, the burning question on your mind is: Did They Work?

Well surprise surprise, we are happy to say the answer is yes, the new cam caps were installed with a set of old cam bearing shells and a pair of old camshafts. They weren’t perfect but close, there was a slight bit of binding felt with the cam during rotation, but some adjustment with 3 thou shims under the cam caps corrected that problem, but what we did not know at that time was that the cyl head had been cooked at some stage of it’s life and it was warped to the tune of 23 thou, so try as you may, nothing will ever beat a true line/tunnel boring operation to create a perfect alignment on a set of cam bearing caps, or a crankshaft mains.

The head war-page caused the camshafts to bind, so now it looks like the line or tunnel boring will have to be done in the future anyway. But to be honest, I think we have done extremely well. Two new cam caps for next to nothing.