cam timing/Ignition timing

[haggis]

Hi everyone, just need to ask a few questions about Cam timing, I have installed a rebuilt short engine and gearbox over the weekend to my GT6 Mk1. its a late HC vitessse engine with standard cam, and i have a DTI, stand and cam timing disc all ready to go.

 

Unfortunately the cylinder head is still in the shop and wanting to make progress.

 

Can I still do the cam timing without the cylinder head installed? is it easier with the head on?

 

The cam sprocket is duplex and was originally fitted to a Kent Cam (the engine wrecker and reason for rebuild) but it only has 2 holes!, the other posts on here talk about original sprockets having 4 holes and 1/4 tooth adjustments, will this make it more difficult to get it accurate?

 

I think I need some cam data? to get it accurate, does anyone know what this might be, and how it is used?

 

so many questions, so little time, else I'll never make it to Santa Pod.

 

Help always appreciated.

 

Thanks

Hag

 

 

 

[Pete Lewis]

its easier with the head off as you can make a piston stop,  bolt it to the block and accurately mark the stop angles btdc and atdc then halve it to get exact  crank TDC

 

same applies to the cam if you have a dti then measure the height somewhere approaching lobe btdc measure the dti lift   note the angle,

 repeat atdc  same  exact amount of lift  measure angle ,  halve the result and you have cam lobe TDC

 

how you set where this is in relation to the crank is in the hands of the cam spec

 

hope thats clear    Hmmm !!!   you cant write this stuff to make any sense 

 

Pete

[haggis]

Hi Pete, as clear as mud! Your right difficult to explain in writing, I'm sure in reality it's simple in reality.

I'm assuming the 2 holed sprocket makes no difference?

How do I now if I have the cam that needs retarding by 2 degrees?

Thanks, hag

Ps gearbox went in nice, thanks for all your help.

[JohnD]

Haggis,

Thanks to Triumph's amazing foresight, for designing an engine without an overhead camshaft but one that runs inside the block, it is not only possible but best to time the cam without the head.

 

First find Crank TDC,  You can use a dial gauge, but far better, and easier, is to use a piston stop (aka dead-stop)

 

 

Attach the stop, turn the crank until it meets the stop.

Mark your degreeing wheel (on the crank).

Turn the crank the other way, until it is stopped.

Mark again.

TDC is EXACTLY halfway between the two.

This is much, much easier than trying to judge the precise position of TDC with a dial gauge.

 

There are three ways to set the cam shaft:

1/ Use the marks of the OE Triumph sprocket - but you don't have one so forget it.

2/ Set the cam to max lift (TDC very useful) on No.1 inlet cam, and turn the cam through a 'magic' number of degrees that the am grinder tells you.

I don't suppose you have that number either?

3/ Use the "Equal lift on Over Lap" method!

Because the front  and rear cylinders of either four or six cylinder engines are exactly 360 degrees out of phase (or, say, are mirror images of each other - they're not but the image may help), they are both at TDC at the same time, but one is firing and the other is at the end of one four stroke cycle and the beginning of the next.    At that point the exhaust valve is closing and the inlet opening, a the same time, and they will have the same lift just at TDC.    Because the valves are overlapping, this is Equal Lift on Over Lap.

 

Set up your dial gauge to measure the cam lift.  You can have the cam follower in the block if you wish, but a short length of dowelling is useful as a dummy push rod, Wrap it in insulating tape so that it is a sliding fit in the follower bore. Two dial gauges can simplify the next bit.

Turn the camshaft until the two cams are at equal lift.

 

Now, fit the timing chain and you are done!

 

 

AS you don't have the equally (!) cunning (!!) four hole Triumph sprocket, you will not be able to get nearer than one tooth to the optimum point.    That's 8.6 degrees on the 42 tooth sprocket, quite a lot.    You might do best to get hold of an original duplex sprocket, 2.5 L style. the two pairs of holes, offset to each other, allow you to adjust down to quarter tooth, +/- 2 degrees.   Vernier sprockets are available, but as you can't swing the cam after the engine is built and the timing cover fitted, unlike most belt-driven OHC engines, there's not much point.     If you must choose between +/- 2 degrees, advance the cam.

 

Good luck!

John

 

PS posted after |Pete's;  hope it's clearer than mud!

[haggis]

Thanks John, it does sound so complicated, its just my small brain and the fear of doing it wrong. 

 

Regarding the 'Dead Stop' I see from your picture its a piece of metal screwed into the cylinder bolt holes, the screw in the middle is set down into the chamber, I'm assuming it doesn't matter how long this is? I always thought that just getting the piston to very top was TDC?

 

I have no idea what a 'am grinder' is, am i completely stupid? any idea of the magic number?I guess max lift is the peak of the cam lobe?

 

I do have both original sprockets, I guess I could just put them on and then change for the duplex in the best position? but how accurate is this? i guess it depends on the factory setting it correctly in the first place.

 

ideally get a four holed duplex sprocket?

 

I will go out there tomorrow night and see if i can suss it out.

 

Thanks Hag

[Pete Lewis]

The whole idea is that the crank can rotate a good few degrees each way whith the pistion apparently at tdc

making use of a stop you can get exactly like turning the crank think clock from 5 to 12 and 5 past 12 , halve it and you have exact 12

not a few degrees either way

 

the cam is the same theres a lot of rotation with the lobe seemingly at its highest but you can turn the cam and the lift doesnt change, turn it a fixed amount by measuring and determining a known dti lift point

 

back to the think clock turn to achieve like 0.020" from max lift either side of max and note the protractor degrees

 

Halve it and you have the centre of the max lift

 

Mud mud glorious mud

 

pete

[JohnD]

haggsi,

the 'am' grinder is the Cam grinder, I forgot the C! Sorry!  They are the person who grinds the shape of the cams on the cam shaft.    S/he would tell you to advance the cam by XXX degrees after TDC. The number depends on the precise design of cam.    It's a Dark Art, hence , magic number, but you do not need it for a Triumph cam.  Read the rest.

 

Max lift is the peak of the cam - Good!

 

I didn't elaborate on the four holes in an OE cam, you can find the detail in your workshop manual, but checking with a four hole and then using your two hole sprocket wouldn't help.   You would still be limited to +/- 8degrees.  Your Kent cam sprocket is duplex and I presume the crank sprocket is duplex too?  If it isn't then you have two choices.     All Vitesses were built Singlex, only 2.5 engines had Duplex, and they seem to survive with not much wear.

   So, 1/ buy a four hole, Singlex Vitesse/GT6/2000 cam sprocket to match the singlex crank  sprocket

OR

2/ Buy a duplex crank AND  Cam sprockets set.

 

Oh, and the stop.  Think of of it this way.   You want to find the middle house of the street, TDC.   Its very long, the houses are terraced and look identical, you don't know how long it is and it's a crescent so you can;'t see to the end.   Measuring or pacing it out will take ages.

So you go to the first house and ask.   Where does Mrs. Middlehouse live?  And they tell you, so many houses down.

And because it's a crescent it's quicker to walk straight to the other end than all the way along, so at the other end they tell you the same, so many houses, and its the same!

Easy-peasy now, you count the houses and find the middle house, TDC.

 

John

John

[haggis]

Thanks Pete & JohnD for all your time trying to explain to a simpleton, I have both full set ups, Singlex & Duplex, I think I'll set it up with the original Singlex and then order a new 4 holed duplex sprocket for the Cam (eventually) and hopefully it will be a straight swap! hurray, brain ache gone.

 

no doubt there will be more questions, but for now thanks for your help.

 

Hag

[Pete Lewis]

Brilliant John, Well ive been up and down the road , there was a nice young lady leaning on the side gate

I never went any further

 

Pete

[haggis]

Hi All, 'I had all the gear and no idea', felt mechanically dyslexic the more I read on this matter, cheated slightly in that having a full set of both duplex & singlex I carefully marked the original timing marks on my duplex sprockets and hey presto. it all seemed to match up.

Thanks for all your help, fingers crossed for start up,   with some luck it might even make it to SEM.

 

Hag.

[JohnD]

Peter,

Ah!  That deceiver minx, Doris (Teresa) Carr.  She'll lead anyone astray.

 

Glad you've got it right now, Haggis.

John

[Pete Lewis]

Have had a few Minx in my life  my first car  was bright red washers under the valve springs  Talbot mainfold and it would pull 7000

used to tear off crownwheel bolts or shear the haft shafts     ............... but never had one called Doris !!

 

Pete

[JohnD]

Haggis,

Just noticed that the TR Register shop has a sale on and is selling four-hole, duplex can sprockets for £29.50 (from £44) and Vernier sprockets for £89.50(from £120). No idea if they will extend the discount to non-members, but why not try?

Ask info@trshop.co.UK

 

John