Adjusting Twin Cam Valve Clearances.

Adjusting Twin Cam Valve Shims & Clearances And Checking Cam Shaft Timing.

If you are a DIY twin cam service man, then there’s a good chance that at some stage you’ve had a go at adjusting the valve clearances on the twin cam engine.

It is not an easy job, and working on the twin cam top end is vastly different than working on, say, an MGA or MGB engine where adjustment is carried out with a feeler gauge, ring spanner & screw driver to adjust the tappets or rockers to the required clearance. The BMC job is dead simple and can generally be completed in a short time

To the uninitiated twin cam owner, no such simple adjustment exists on the TC, instead, there is a process to follow. But first, the cam cover nuts need to be undone and the cam cover can then be removed, but that is just the beginning.

With the cover removed, you are presented with what looks to be a complex twin cam arrangement which is driven from the crankshaft by a single row roller chain, each camshaft is secured to the head with several alloy blocks which hold small bearing shells, the alloy cam blocks are secured with longish studs and nyloc nuts, which are torqued to a certain tension.

Under each cam lobe sits a  cast iron or steel bucket or cam follower, inside each cam bucket sits a steel shim of a given thickness located in a recess in the valve spring retainer, it is the thickness of the shim which controls the clearance between the cam lobe and the bucket or cam follower, if the clearance closes up, the valve seats will begin to burn, if the clearances are too wide, they will rattle, and the cam timing will be slightly affected.

When adjusting the shims, each camshaft is removed to gain access to the shims inside of the cam follower. The job can be carried out quicker if the thickness of the shims are known, then it is a simple matter to check and note on paper the clearances for each valve. After the clearance checks are measured and written down, the cam shafts can then be removed and shims of the correct thickness can be installed.

Take a sheet of A4 paper, pen & micrometer, these are the simple tools to use during the shim adjustment process, maybe add a calculator (Smart phone app) also. The paper is used to draw a table that represents the valve layout of the twin cam head, clearances are written down in the table that corresponds to each valve, the micrometer is used to measure the shim, the calculator quickens the figuring if you hate addition or subtracting like I do.

Shims of many thicknesses are required. But if you possess a lathe, they can be easily made from 4140  5/8″ round bar stock, hardening of the metal can be done with an Oxy torch and a container of water or quenching oil, the metal will be brittle when quenched, so they will need a slight drawing of temper afterwards.

Adjusting the valve shims can be a long drawn out job, don’t rush take your time to get it right with the first attempt, and don’t invite old mate around who likes to talk none stop and offer you free beer while you work, it could cost you dearly.

Most twin cam builders will tell you that the twin cam must be fitted with an endless chain, IE, no joining link, which is normal for a race prepared engine, the timing chain must be of the endless chain type, it is not recommended that a joining link be used, but for a road engine to my way of thinking it is OK.

To employ a joining link in the timing chain will be howled down by the twink gurus out there, but take a minute to read the following reason. (it is accepted practice now)

I have owned a lotus elan for nearly thirty years, in all of those thirty years the cam chain in the twin cam has been operated by a chain which has a joining link, yes, a joining link. The BIG argument against having a joining link in the chain is that the clip holding the split link together could fail, or be slipped off the pegs on the link if the engine briefly runs in reverse as they sometimes do when switched off.

The strange thing about this argument is that if one takes a close look at the joining link, fish-plate and holding clip, there is no chance of the U shaped spring keeper clip being accidentally dislodged, as it is hard enough to remove even with pointy nose pliers, as an extra precaution, the link and clip should be replaced every twenty thousands KMs, and the “U” shape of the clip should face the same direction as the chain rotation.

I say this with confidence, because after covering nearly 60,000 miles, not once has the timing chain with joining link created any dramas, now that’s not to say that I have not worried about it happening, but the joining link and clip is examined every 3000 KMs, just to make sure.

The point I will make now is that a timing chain with a joining link makes the operation of removing the cams to adjust valve shims much, much, easier than if an endless chain is employed. One big advantage is that if done carefully you wont lose the ignition timing either.

The Process:
But first things first. If you are not familiar with the twin cam engine you are working on, then the process of establishing true top dead centre should be carried out. Yes, I can here you saying that there are already timing marks on the crankshaft pulley, well you might be surprised to find that they are not always correct, and if the the crankshaft pulley is out a few degrees, then the cam timing will be out of whack also.

How to Find True Top Dead Centre?

Tools Required:

• Magnetic Base Dial Gauge Holder.
• Dial indicator gauge.
• Extended probe for dial gauge.
• One L  Shaped Metal Bracket.
• Red Marker Paint.

Plus the usual spanners and workshop tools.

Remove all spark plugs so that the engine is easier to turn over, engage third or fourth gear and roll car forward, watch cams, stop when on compression stroke. Place L shaped plate onto rear cam cover studs (Inlet side), secure with washers and plain 5/16″ UNF nuts, tighten with spanner. Next, set up magnetic base, dial indicator, attach long probe to dial gauge, carefully place assembly onto upright of mag base, and then slide the probe down  through spark plug hole, roll engine over with spanner placed on crank pulley bolt.

There will be a bit of slight forward and backwards movement until the gauge will indicate that true top dead centre has been achieved.(Lobes on Ex & In cam pointing outwards & cam lobes on No 4  just rocking).  When number one is on TDC, there is a movement of about 1- 2 degrees where the dial does not move, that is true TDC.

Now take a look at the crankshaft pulley, if the engine is clean, the mark on the rim of the pulley should be visible. If the mark on the pulley and the TDC mark on the timing chest align correctly, you are in luck, you have proven that the marks are correct & true.

Look closely at the photo above, the mark on the pulley is slightly incorrect, but the paint mark indicates where true TDC is.

Once the TDC is found, the next job is to find the joining link in the chain, and that is not as difficult as it may sound. Just do the following, select third gear, and with the cam cover removed, slowly roll the car forward and watch the cams rotate, at the end of each compression stroke on number one cyl, stop, and walk to the front and look closely at the chain, with a bit of luck, the joining link will appear at the top between the cam sprockets when the crank is in the TDC position, if that is not the case, try once more, snick it out of gear, roll the car back, select third gear again, and roll the car forward once more… the link will appear at the correct position at some stage.

(If the chain is of the endless type then the procedure is going to be different, both cam shaft and sprockets will still need to be marked with paint and then removed from the cam shaft, the exhaust cam side sprocket is etched with a large EX on the sprocket, do not be interchange them).

But lets continue with the joining link chain procedure.

Don’t crank the engine on the starter, as the engine will turn too fast to see what is going on. When everything is in the correct position, take a rag soaked in petrol or de-greaser, and clean the sprocket and chain on both cams to remove the film of oil, then take a small brush and place marker paint on the sprocket timing marks with red or white paint (Quick dry Auto paint).

Before the chain joining link is unclipped, take a photo of the cams showing timing marks in relation to the top of the timing chest as described in the lotus workshop manual, it is quite easy to get the timing “one tooth out” which will throw the cam timing out, and possibly bend the valves, yes, you’d be surprised at how close the valves run together. The timing marks on the sprockets never seem to line up perfectly and it’s acceptable to have one or the other slightly up or down, what ever the case may be.

Most race engine builders now use vernier adjustment sprockets which do away with the old way of offset dowel pegs to arrive at correct timing for the  Inlet or Exhaust cam timing

As an extra precaution before attempting to remove the joining link clip and link from the chain is to insert rags at the rear, front and around the cam sprockets to close off the timing chest openings, making sure that nothing can fall into the timing chest and into the sump, don’t tempt fate by not doing this.

If a brand new engine is being built then setting up the cam timing will be much easier with the engine mounted on the engine stand, the addition of a degree plate which is attached to the front of the crankshaft will allow dead accurate cam timing in relation to the crankshaft, it ‘s an involved process and we wont cover that here, as miles Wilkins has thoroughly covered the subject in his book, but below are some additional tools that will be required.

The dial indicator fitted with extension probe are essential tools, they were shown above when finding true Top Dead Centre, the indicator is set up on a magnetic base holder, which in turn is placed on an “L” shaped steel bracket which is fastened to the rear cam cover studs (inlet side). The extesnion probe shown above was custom machined from 3/8″ round aluminium bar stock, the small threaded end that screws into the indicator rod was a test of thread cutting patience on the lathe. Long probes can be purchased, but they ask silly money for them.

There’s no need for a commercial here. Moroso is not the only company to produce timing degree plates, they are essential tools if you want to achieve accurate valve/cam timing.

No matter what type of engine that is being worked on, you will need a special threaded stud or spigot in which to secure and hold the degree plate. They can be bought, but they are easy to machine to suit the application if you posses a lathe and turning skills. The spigot is made to the same thread as the crankshaft pulley holding bolt, the head of the stud is drilled and tapped to hold a smaller cap screw or hex headed stud which is used to clamp the plate to the crank. If you want more info on cam timing find a copy of Miles Wilkins book Building The Twin Cam Engine, or search the net for info.

But if you don’t feel confident in adjusting valve clearances or setting/adjusting the cam timing then seek professional advice and help.