The Long Cam Bolt.

In days gone by, a broken cam shaft on the twin cam lotus engine was almost never spoken about, but for some reason, we are now hearing more about this unpleasant, expensive breakage occurring on the Lotus Twin Cam engine.

But the question should be asked as to why it happens, and truth be known, the problem could be on more than one twin cam owners mind.

Could your camshaft be the next victim on the list?

There could be several reasons why breakages occur.

Here’s four that come to mind; (1) very high lift cam lobes, (2) double strength valve springs, (3) much higher Rev limits, (4) Timing chain adjusted too tight. 

These are four things combined that could potentially place an increased mechanical load on the standard cast iron cam shafts.

The Breaking Point.
Usually occurs where the circular oil gallery groove and hole are machined in the first bearing of the cam shaft, while necessary for the oil feed lubrication to the valve train, in practical engineering terms, that particular point is a potential stress riser, or weak point in each cam shaft, but in the past, there has generally been no cam breakage problems experienced with a mildly tuned road engine.

And speaking from personal experience, the writer has covered more than eighty thousand miles with mild rally cams installed, and never had any issues with cam shaft breakages, plus the timing chain is also of the split link type, which has also never been an issue, despite the fact that the twink gospel preachers out there stipulate the use of an endless chain.  

How many lotus twin cam engines could be classified as being totally standard today? 

In reality, the original cast iron cam shafts are not what you would classify as heavy duty items. They measure 22.47mm Dia (0.884″) in fact, with today’s valve train trends, IE, bigger cam lobes, big valves, extra strong valve springs… chasing the higher horsepower figures, is it any wonder that the phenomenon of the broken camshaft is not more common.

The problem seems to rear its ugly head mostly with the exhaust camshaft, although the inlet cam has been known to fail as well. Apparently, a sudden downwards pull (Caused by a missed gear change or other peak rev cause) on the cam chain, exhaust side can put a momentary increased load on the cam shaft, cast iron is rigid and wont flex like steel, so it will snap, and that’s when you’ll hear that sickening metallic cracking sound, and then everything grinds to a halt!

The Mechanical Fix.

As luck would have it, the long term historic racers and engine builders (Mostly Australian and USA) have come up with a great fix which seems to work quite well.

The fix is carried out by installing a much longer 3″ X 7/16″ UNF- HT bolt deeper into the cam shaft, past the first cam bearing.

But the process entails careful and precise lathe work, deep drilling, and hand taping of a much longer thread into the cam shaft bore.

AND PLEASE NOTE: THIS IS NOT A JOB THAT CAN BE DONE IN SITU.

The camshafts must be removed from the engine in order to carry out the machining, drilling, threading, cleaning, and re-install work.

There is no need to remove the buckets or shims, leave them where they are when removing the cams.



Cam shaft set up and ready for deep drilling operation

Mount the cam shaft in the engine lathe or bench lathe. Use copper sheet strip wrapped around the end bearing to protect against the lathe chuck jaws. Use the tail stock and centre to hold the cam on centre while setting up and adjusting stationary steady onto the cam bearing, apply a squirt of oil, turn by hand to make sure that all is secure before switching the lathe on to begin work.

Set the spindle speed on low, and take care with your work. We are not providing lathe turning lessons here, the writer assumes that the reader is familiar with this type of basic machine shop practice.

The black mark on the shank of the bolt (Above) indicates how much distance is taken up with the cam sprocket, the steel retaining washer, and the spring washer – that is the allowance you need to factor in when estimating where to machine the groove and the oil gallery hole in the bolt.

Measure the components fitted to your cams – They may be of different dimensions – Do not take it for granted that things on your engine are standard. 

Don’t be concerned if the cross drilled hole in the bolt does not line up perfectly with the cross drilling in the cam shaft, the oil will travel around the machined groove in the bolt and find the drilled oil hole in the bolt – But make sure to test that fluid will pass through it by assembling the cam, sprocket, steel washer, and spring washer, and then force some fluid through the cam shaft from the other end by undoing the brass plug. REPLACE and secure the plug after the test is completed – There should be copious amount of fluid flowing through and out of the cam shaft. When cleaning, use air pressure to blow all oil feed holes out, because the waste cast iron from machining and taping of the thread will clog every hole it can find.

Another view of the long bolt. It is otherwise a standard grade eight (six markings) HT 3″ long x 7/16″ UNF threaded bolt, but it requires some modifications via machining and drilling before it can be used in the given application.

The machining work does require lathe turning for accuracy.                 
Twin cam lotus Long bolt and 7/16UNF Tap – Note that this is a plug or finishing tap – Do not use a tapered tap. The bolt is centre drilled, and the gallery oil hole cross drilled (One side only). Measure carefully to locate/machine the oil groove opposite the groove and gallery hole in the cam shaft when the bolt is torqued up tight – DOUBLE CHECK TO MAKE SURE.
The bolt shown above has been waist machined on the shank to allow some give or take clearance on the internal thread in the cam shaft bore. 

Some people may be tempted to hand drill the cam shaft before taping the thread – think twice about doing that, you don’t want to drill off centre, or at an angle, otherwise you could end up with two cam shafts that are only good for the scrap bin.

Measurements must be accurate and make sure that threaded bore in the cam shaft is correct, trial fit all parts to make sure everything fits correctly before the final installation.

The dowel pins will need to knocked out of the cam boss, otherwise they will foul on the tap holding tool during the process.

If you have been wondering about the long cam bolt modification, then this will give you an overview of what its all about. And the writer can also read your thoughts about not having a lathe to carry out the work.

Look around your circle of car ans or engine building friends, someone is sure to have a lathe tucked away in the corner of their workshop, or they might know of someone else who has a well equipped workshop.

Cant do the machining yourself? And you don’t know anybody who could machine the special bolts for you?

Elantrikbits can now offer the custom modified cam shaft retaining bolts – Place Your Order Here. 
And reduce the work required.
Once this modification has been completed, you can pretty much say good bye to any future broken camshaft problems.

Good luck with your project.

Elantrikbits – Croucher engineering – Sharing tips and knowledge with other twin cam lotus owners – Helping to Keep The Lotus Marque Alive.