abarth-gmr magazine > fiat twin camshaft timing

TC camshaft Timing by Guy Croft


Guy Croft:
Greetings GM and members, here is an article on how to time up camshafts, a subject many people are not clear about:

Cam timing and fitting data
****With a TC you should set the cams up with the head off****
If you do it on the engine you will bend a valve for sureGeneral
Nominal cam lift can be measured by putting cam between revolving centres or on V blocks, and rotating cam with dial gauge on cam profile. Subtract clearance to give true lift in running conditions.
The base circle diameter many competition cams is less than std so thicker shims / top-hat shims or cam tower machining may be needed, though when the valve seats are blueprinted (ie: opened out) the valves go deeper into the head and this takes up some of the clearance - without resorting to massive shims. Shims thinner than 3.25mm must not be used or the cam nose will hit the bucket.  Use new cam setting shims! Cam housings should be brush-honed at 180 grit silicon carbide or cleaned with fine Scotchbrite and then Jizer or similar & detergent with hot water. New seals should be used. If re-using old cams, polish the journals with fine Scotchbrite. If you are fitting head to a previously run block make sure there is no grit/muck from cleaning the block face in the 6 oil feeds in the block. Seal them off if scraping or rubbing down the block. Dirt in these galleries will go straight into the cam journals and seals on start up, causing scoring and seal damage. Head and block need to be flat in all directions to 3 thou or better. You can survey them with a good quality straight edge and a feeler gauge. After installing cams in their housing bolt the cam pulleys in place. Make sure the back of the pulleys does not foul the cambox or the seal, there is end float on the cams and the castings do vary in length. If the pulley is too close, get it machined on the back. Bolt each cambox to the head with new gasket and shim up close to the final setting. The gasket will settle 2 thou overnight, so when it has settled, slacken the cam box bolts fractionally and retorque before final shimming. You can do cam timing whether the head is shimmed up or not, but if the shims are tight, the cam may be hard to turn.

Never turn a TC cam unless you can see the valves – they may touch and bend. If in doubt remove or slacken off one cam box.

Step 1 – establishing Full Lift
There is a dwell phase around full lift of several degrees on most cams, to hold the valve open at full lift. A reground cam of same lift and duration as a billet cam will have a nose of much reduced volume and flow and a smaller dwell period.
To begin with a protractor and dial gauge (with scale big enough to accomodate cam full lift) is needed to establish the centreline of the full lift position, as this will be the datum from which the cams are timed. The cam does not need to be shimmed accurately up to do this.
To establish full lift turn the cam with dial gauge on the valve to approx visual full lift - then turn cam anti-clockwise till lift reads say, 9mm, or some other fixed point on the cam flank say 0.5mm lower than full lift. Make sure the cam does not rotate of its own accord. Then stick a large diameter protractor (you can get them from stationery shops) to the rim of the cam pulley with Blu-Tack, bolt a sharp-tipped pointer to the head and set the pointer to zero degrees. Then turn the cam the other way – clockwise, thru full lift and back down to the same point on the opposite cam flank and read the degrees. True full lift is halfway between 0 and this reading. Swing the cam to and fro and check that true full lift is exactly in the middle of the dwell phase.
Mark full lift on the rear cam journal relative to the housing, say with 2 scribed or pin-punch marks painted white – this gives you the FL reference point, which is useful if you want to check the cam timing later with the engine built and the belt on.
The same method as determining the centreline of FL is used to determine true tdc on the crank – using a protractor on flywheel, a fixed pointer and a dti on no1 or 4 piston.

Make sure you bolt a tdc pointer rigidly to the block and aimed at the flywheel or front pulley - before you fit the head. Once the head is on you will struggle to find true tdc with accuracy otherwise.

Step 2 - Cam timing – how it’s done
Cams rotate at ½ crank speed. The engine rotates clockwise viewed from the crank front pulley end. TC cams are always set up on No.1 cylinder, with the valves on the overlap phase (inlet opening, exhaust closing) – not the compression stroke, although when no1 is on overlap, no 4 will be on compression (both valves closed).
The cam degree setting at tdc, relative to cam full lift, will be ½ the crank degrees at which you want full lift.
So if you want cam full lift (see below) at 110 crank degrees the inlet cam will need to be set at 55 cam degrees before cam full lift (no 1 lobe) at tdc and the exhaust 55 cam degrees after cam full lift.

In the above 110 deg example –
For the inlet cam turn the pulley anti-clockwise 55 cam degrees from full lift to put it before its full lift posn.
The exhaust cam should be rotated clockwise 55 cam degrees from full lift to put it after its full lift posn.
Mark the tdc posn with a red mark on the cam relative to the white mark on the housing.
The values full lift, timed-in degrees and lift at tdc with the cam shimmed up should always be recorded for dyno reference. You can check and adjust the cam timing on the engine – but only with the belt final fitted and ONLY if you have adjustable camwheels.

Step 3 – Dry-build
With non-standard cams I always recommend dry building and measuring the clearances using modelling clay in the valve reliefs. It's not enough to say 'there's plenty of clearance' - you need to record what it is. This needs to be done by turning the engine over 2 revolutions by hand, with the cam belt fitted, before final bolting the head down. Plasticine is an ideal material, and it helps to spray the valves with silicon rubber lubricant to prevent them picking up the clay.
Note that the valve positions relative to the valves are not symmetrical on pistons 1 & 4 on the early (pre reversed port head) TC. The inlet valve will always be closer to the piston during its cycle than the inlet cam. The more that’s machined off block or head (or if the pistons are run proud of the block face), the less radial clearance you’ll get.
Ensure 80-100 thou” vertical piston-valve clearance (measured diagonally - in the plane of the valve axis), and 60 thou radially – from the valve circumference to the wall of the valve relief. How much additional clearance beyond valve full lift you allow in the vertical plane between valve and piston depends on your confidence in the valve springs. Radial clearance is affected by piston - bore clearances and guide wear.
Some comp cams have as much as 5.5mm lift at tdc (on the overlap) but cams with 3.5 mm or less lift at tdc and can often be fitted without the need for deeper valve reliefs.
BUT - don’t forget that in the first 20 or so deg after tdc on the inlet stroke the inlet valve tries to overtake the piston, so to be on the ‘safe side’ don’t base your estimate of valve relief depth purely on the lift at tdc – the valve does move closer to the relief. It's no good just dropping the valve thru the guide onto the piston at tdc and saying 'OK, I've got 4mm vertical clearance', for one thing it's going to be alot closer in the true cycle, and for another it doesn't tell you the radial clearance. As many engines bend or break their valves from low radial clearance as any other cause. Generally closest proximity is around 15-18 deg. If you are contemplating swinging the cam timing on the dyno it is imperative that you ‘map’ the valve lift around tdc, eg clearance from piston at 110/108/106/104/102 deg etc.

It's worth mentioning that if you think that the piston valve reliefs (cutouts) might need machining to complete the build, and BEFORE you build the head up, use an old valve stem ground to a sharp point as a punch to put down thru the valve guide to mark the valve centres on the pistons. The pistons must be held in the centre of the bores by packing them with thin card, otherwise the slop between the piston and bore will totally upset the valve relief machining. This can later be used for flycutting the valve reliefs to make them deeper or wider. Also make sure, that when you offer up the head to so this marking-out, that the gasket and gasket locating dowels are in place.

Lubrication and running
Oil the cam and cam tower (or box) bearing housings and rear thrust faces – front and back - at the build stage. The biggest cause of cam lobe damage is dry-scuff or low-speed running after start up. Damage can be caused to the cam journals especially at the front end if the oilways in the block or head are contaminated, this will cause scoring to the housing and cam - and may cause even a new cam seal to leak.
Old cams re-used:
Fill the cam boxes with engine oil just before cranking so that the cam lobes are not spun dry. Moly grease is also a good lubricant to smear on the cam lobes but don’t use on deep-drilled or cross-drilled cams or if you are going to leave the engine standing for months – it dries out.
New cams:
You must use a good quality special cam lubricant (most cam manufacturers sell it) and pour it over the cam lobes just prior to cranking and start (cam lube will typically be a zinc dithiophosphate compound)
All cams:
Fill all the oil system accessories, filter, lines, cooler etc and crank up oil pressure on the starter with the plugs out with the cam box covers off and make sure oil is pumping into the valve train before starting the engine. It should prime up in 3 bursts of 7-10 sec crnaking and in not more than 20 seconds overall. More than that and it's never going to get oil pressure. If the engine has been standing for a long time (months) you may have to prime the oil pump by removing the filter housing and injecting oil down to the pump.

With new cams it is imperative to run the engine at 2000-2500 rpm for 5-7 minutes to allow the cams to bed in - before allowing the engine to return to idle (750-850 rpm). Obviously timing and fuel system have to be very well set statically to permit this without overheating or stalling/flooding the engine.
Remember that the cam boxes on the TC will empty completely if the car is driven up a ramp onto a truck or trailed on the way to a race, so they will start bone-dry and wipe out the cams, whether they are cast iron or steel. Not good. Refill them!

Cam timing figures – what they mean
Typical competition cam example:
Quoted timing 40/80 80/40.  These degree figures are crank degrees and mean the following:
Inlet opens 40 crank deg before tdc and closes 80 after bdc
Ex opens 80 crank deg before bdc and closes 40 after tdc
Overlap is sum of 40 + 40 = 80 crank degrees
Duration of both cams is 40 + 80 + 180 = 300 crank deg
Full lift (FL) is at (300/2) – 40 = 110 crank deg.
Thus inlet FL is 110 deg after tdc, ex is full lift 110 deg before tdc.

All cams I know of for the TC are symetrical (same shape of open and closure flanks) and this maths can be used on any of them.

Common TC timing and full lift posn
131 1600 & 2 liter 5/53 53/5 (114 deg)
105 TC 10/48 53/5 (109/114 deg)
130TC 7/51  51/8 (112/111.5 deg)
Lancia Beta 2 liter 13/45 49/9 (106/110 deg)
Lancia Vx 13/39 37/3 (103/107 deg)
(Incidentally the true durations of the TC cams are longer, these figures are quoted with an industry standard wide running clearance which assumes no flow at very low lifts).

Don't blame cams from a reputable supplier if you cannot get the power you want
The performance (area under the torque curve) of an engine, ignoring mechanical losses, depends how well the whole spec is integrated. The camshaft is just an opening device. Here are some of the main parameters that determine power:
-   Exhaust port and manifold length, diameter and manifold configuration (4-1 or 4-2-1)
-   Inlet port and manifold length, diameter and rampipe length
-   Head airflow inlet and exhaust – encompassing valve size and discharge coefficient, and port shape and size, combustion chamber shrouding
-   Cam profile (lift by degrees) and camshaft timing (ie: FL posn)
-   Bore and stroke
-   Compression ratio
-   Ex silencer back pressure

As a general rule you can set FL of inlet and exhaust cams anywhere between 100 and 113 degrees. Inlet and ex FL do not need to be the same. The cam timing will define the torque characteristic and altering it from any stated spec is not necessarily be a bad thing, but dyno development is needed to optimise it.
It is not necessary to run like-for-like inlet and ex cams, indeed with supercharged or turbo engines it’s a bad thing. Blown engines should always have less lift and duration on the ex than inlet. Sometimes you get a superior torque/driveability characteristic from having more lift and duration and lift around tdc on the inlet cam than on the exhaust.


If in doubt - e mail me on [nonactive] or talk to my good friend Guy Moerenhout.

Guy Croft:
well, dear readers 300+ readers and not a single view and deadly slience! Is it 300 people reading it or one reader 300 times?

Was it helpful? I should really like to know (before I post again)!! I don't want to bore everyone to death on this wonderful and highly respected website!

Is there perhaps any other tech topic requested?


Hello Guy Croft,
I doubt that 300 ":silences" is an indicator. I was so appreciative of your article I downloaded and copied it to place with my copy of your  great book "FIAT/LANCIA TWINCAM ENGINES" to absorb in detail in a month or so, when I rebuild my 1995cc as part of a current restoration of a 69 Fiat AC coupe.  I would only encourage you towards further articles as available competancy in this particular field of older engines is diminishing.
Many thanks already     Trevor

Nick /////:
Hi Guy

Remember me, Nick Bailey /////
I've only just arrived here and there is quite a lot of real good info to plough through.
Your article is well recieved and appreciated.
Like many Fiat/Lancia twin cam owners, due to the bullet proof reliability of these engines, apart from a standard  service cam belt replacement we have never had to time and engine from scratch. 
Some of us are aware of the 'interesting problems' like the auxilary shaft / fuel pump timing problems, (which you have not mentioned :-) ) but ANY and all correct information that we can all provide is so important to maintaining the species.
Here is a minor plea to you and others.   
I think/believe the majority of Abarth owners wish to keep their cars in standard/factory configuration and so the basic tuning and timing and general  information is of great value for reference purposes.  Hopefully most of us will not need to experience a back to basics, nut and bolt rebuild and timing nightmare.  However the information you have provided is well received, extremely valuable and I hope others will provide similar important and valuable information.

Nick /////


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