Valve Timing - stupid Question...???

[Dilkris]

Because this rebuild has become "somewhat obsessive" (anyone else become like this???  ), I want to check the valve timing (by first principals), before I take the engine off the engine stand and finally mate her with the transaxle. (Note: I am running "vernier" cam sprockets).
My engine specs show Inlet Opens 26 BTDC/Closes 66 ABDC - Exhaust Opens 66 BBDC/Closes 26 ATDC. I am armed with my timing disc, DTI, probe extension and have fabricated a carrier for the magnetic base. 
Now the stupid question:-  Are the above figures applicable to No.1 or No.4 cylinder.....  

[Pfreen]

Yes, but you usually set the valve timing by mop, which is 110 degrees for the stock cam.  The Wilkins book covers the procedure thoroughly.
Number 1 and 4 have the same timing but are 360 crank degrees apart.

[MRN I J]

What cams / engine are you running as I have some TC cam specs converted to valve lift at TDC specs

[MRN I J]

Whatever you do it will seem complex, always adjust cams very carefully at TDC. Always turn the engine carefully clockwise.
I always thought TCs cams were on keys, but its 35 years since I rebuilt my last TC.
So as a starter set the inlet cam lift at 0.075" open at TDC & the exhaust at 0.072" open at TDC then check the Max Opening Point & see how far out from 110 degrees max opening you are, its easier if as you are coming up to TDC record the lift on the clock at (say) 107 then keep turning clockwise to the same clock measurement on the way down the lift at say 115, subtract the 107 which would be 8 then divide by 2 = 4 + 107 + 111 so you would need to add 0.004/0.005" (thats about a degree of movement) to the lift at TDC.

[BDA]

After you have finished and have the cams where you want them, put witness marks on the cam and the head (when at TDC) so you can put them back where they were without going through this again.

[jbcollier]

First find true TDC.  You can use a piston stop, but a dial gauge is quicker.  Find the "high point in the piston's travel and reset the gauge to zero.  Go one way until you have moved "X" distance and not the reading on the timing disk.  Now go the other past TDC to the same "X" reading.  Note the disk reading. Now turn the timing disk so it will read the same degrees of travel past TDC at both points and you have found true TDC.  Mark it!


Now find out what your cam specs mean.  Usually it means the valve clearances are at "Y" and your opening and closing points are at 1mm of lift.  turn it over through the full two revolutions and record the various intake and exhaust openings and closing points.  Adjust your venier sprockets to suit.

Make sure you tighten the sprockets before turning again!

Make sure you tighten the verniers properly!

In Citroén SMs, they got the cam tightening wrong and the engines used to go "woof" flat out.  At $100 a pop for valves IN THE 70s!, that was a big whoopsie.  They issued a bulletin to use a BFG to tighten the cam nut until it stopped turning.  No more problems.

[MRN I J]

perhaps these 2 diagrams will make a difference, vizualising the concept of cam timing over 2 revolutions 720degrees

for further reading
https://www.tildentechnologies.com/Cams/CamBasics.html

[Dilkris]

Thankyou everyone, (and especially MRN 1J who has been patiently accommodating PM's from me), all your input is very much appreciated.

I just couldn't get my head around this MOP (Maximum opening point) and failed to see the connection with the cam specs of Inlet Opens 26 BTDC/Closes 66 ABDC - Exhaust Opens 66 BBDC/Closes 26 ATDC. In my ignorance I thought you were actually  measuring these points on the degree wheel resulting from the DTI movement, in reality of course you can't actually do this because of the effect of dwell angles.

After much web browsing on the subject of valve timing with a DTI and degree wheel I found a comment which for me caused the "penny to drop."

Quote:- MOP is an English term. In the USA, the term is "Lobe Centers"

For me, this brought all your comments and those of Miles Wilkins together and it all suddenly made sense.

It was one of those Eureka moments - could be a 6 beer night. 



 

   

[jbcollier]

Just remember that not all cam grinds are symmetrical.  Check your specs!

My Ducati Desmo single has a race grind and it is wildly asymmetrical.  No matter, it’s not hard to dial it in with a timing disc and a dial gauge.