Doing the cam timing: Twin-Cam

Crank, rods, sealing, pistons, block, flywheel etc
Guy Croft
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Joined: June 18th, 2006, 9:31 am
Location: Bedford, UK

Doing the cam timing: Twin-Cam

Post by Guy Croft » July 25th, 2006, 4:06 pm

The technique for setting up (dialling-in) the camshafts on a twin-cam engine, 8v or 16v is rather different than a single cam unit.

There is more tuning flexibility when you have two cams but setting up is more involved. The main thing you have to watch out for with 'inclined valves' - ie: not mounted vertically - especially on big valve or 8v twin-cam heads is 'valve clash' - where the valves physically hit each other. All twin-cam heads are done the way I describe below.

Never, ever turn either camshaft without looking to make sure the valves are clear of each other. Again, because of this risk, it is MOST unwise to time up a twin-cam engine on the block - always do it with the head OFF.

Tools needed are a large diameter protractor, magnetic base DTI (dial gauge) with enough range to cope with the cam lift, sharp tipped pointer. I stick the protractor to the pulley with 'Blu Tack', an invaluable substance with many uses in engine building. For big valve units like this 8v Fiat head you must make a rig that allows you to turn the cams over in-phase to check the true proximity valve to valve.

If the spec for the inlet cam is full lift at 100deg, this means that the inlet valves should be fully open at 100 crank degrees after tdc, so we need to find full lift on no1 inlet valve, mark it, and then turn the inlet cam counter direction of engine rotation by 50 cam degrees and mark that position too. This will be the position the cam is in at TDC. Conversely the ex valve - if FL 100 crank deg, will be 50 cam deg after full lift at tdc. I usually mark full lift and tdc positions, so we can do a final check and adjust (if we have adjustable cam pulleys) with the belt fitted). There is more on this at:

I am asked 'do I need adjustable camwheels?' Yes - always use them. This is a race engine website, if you're building a race engine, they are essential. Everyone who did not use them either got in a bit of a mess themselves or I inherited the problem of trying to do cam timing without them to my usual standard of accuracy. I just don't have the patience to explain any more what may happen if you don't use them!

It is not necessary to shim the head prior to doing the cam timing, but I usually get shimming out of the way first. At least that way - when I am measuring the valve lift fro my records, I know the valve really is closed when is appears to be shut, not held open by a tight shim. And, this does matter, if you have the lift at tdc data for the cams, you can cross check the lift done by cam timing method against this. If you find with your cam timing that the lift is way more than it should be at tdc, you have done something wrong.

This is a Fiat TC big valve head - for a Fiat Abarth 124 Rally 1800. It is fitted with race guides, fully flowed, 45/40 valves, GC 3D inlet cam and an original Abarth 068 ex cam. Same valve setup I used on our 'works' 200bhp NHRA engine in the 90s - and many other units.

Doing these big valve units is definitely NOT a DIY job, don't even think about it - far too difficult unless you have done them many times before. You have got to prep it in such a way that you have a pretty fair idea what will happen vis-a-vis shimming and valve proximity before you start work, otherwise you are going to end up with something you either cannot shim - or cannot turn over.
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Head can be turned over and blocked up safely so valves don't hit the bench for final tighten & shim check. The cambox gaskets settle about 2 thou" overnight.
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This is the closest valve proximity - if was running a 3D ex they would hit, I'd have to recess the exhaust seats way deeper and shorten the valve tips.
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Short belt and tensioner lets me turn cams in phase, I must be sure sure they start at their marked tdc position.
Note I turn from the pulley with the most belt 'wrap'. Imagine trying to do this without adjustable pulleys?
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Cams marked in my preferred way, red line on cam to white on the housing, these are the positions of the cam with cyls 1&4 at tdc
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I have done the inlet cam timing here, and this is the true full lift on the shimmed-up GC 3D spring-race cam, lift at tdc is around 5mm.
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Can't resist shots like this - inlet valve at full lift. This is where the power comes from..
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OE shim tool (which no-one ever wants when I have them in stock) in use for doing prelim shimming of the inlets at 16 thou". Tool holds bucket done and allows shim to be prised out. New shims are a MUST.
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Putting the buckets in, here I know roughly what thickness I need from my post-seat-grind valve setup height data. Using a bit of moybdenum grease instead of oil at this stage stops them falling out.
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Exhaust cam box bolts slackened off to retract valves so that I can do preliminary shim on the inlets without any valve clash.
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And here is the exhaust valve lift (shimmed at 18 thou") at tdc, the cam has been turned in direction of crank rotation 54 cam degrees. This gives ex FL 108 crank degrees before TDC.
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Protractor is taken off and stuck on again with Blu-Tack with needle at zero - with cam at full lift. I am now going to turn the ex cam in in direction of crank rotation (clockwise from front) 54 cam degrees.
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Scribed mark showing ex cam full lift position, later I'll paint the line on cam and housing white.
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True full lift is halfway between 0 and 47.5 degrees, 23.75 deg. You MUST not guess the mid-point of the cam full lift dwell phase: FL dwell can be several degrees and your cam timing will be miles out.
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The ex cam is now rotated clockwise thru full lift and down to 9.00mm lift on the other flank. Note reading 47.5 deg. It pays to have a system of anticlockwise then clockwise and use it every time.
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The cam is turned anticlockwise from approx full lift to 9.00mm lift on one flank and the protractor fitted & set on zero. You can use any flank height for this check provided it is 1mm or more below FL.
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Turn the cam until the dti reads full lift - there is a dwell phase, if you swing the cam it will hold FL for several degrees, you don't need to find the mid point, that is done by the method below. Bolt on the pointer.
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Fit the dti, close the ex valve and zero the dti accurately on the valve. Adhering to accuracy throughout allows you to record your full lift and lift at tdc without having to do everything twice.
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