Valve train questions

[ricardo]

Yesterday I rebuilt my short engine. Today I set the cam timing and tried to take some measurements of the cam (SW5-07). I'm trying to know my engine/parts the best I can. This is something I have never done (actually this is my first engine rebuild as some of you know) so I may have done something wrong although I think I didn't.

With the cam timed at 105.75º (the chain will stretch yet) and cylinder head fitted, I took the following measurements with a DTI.



Since my values were very inconsistent, I double checked them (a few were corrected) having previously checked again valve clearances (15 thou). The protractor wheel pointer was pointing exactly to TDC at TDC and BDC at BDC, that I'm sure was correct.

Although Swiftune doesn't list the official settings for this cam, there are some 'trustable' measurements on the web:

Checking height: 0.016" @ lobe
Timing: (Intake): 15/47 (Exhaust): 52/10
Nominal lobe lift: (Intake): 0.286" (Exhaust): 0.286"
Duration (Intake): 242 (Exhaust): 242
Lobe centre angle (Intake): 106 (Exhaust): 111
Lobe separation angle: 108.5
Lift on overlap: 0.0385"
Accuracy: Exceptionally Good

Question 1 regarding time/point of events:
Are my readings normal? Is it common to have these manufacturing tolerances?
If yes: should I adjust my valve clearances, within reason, to make sure all valves open at the same point/crank angle?
If no: what could have caused this? I double checked valve settings and DTI readings. Cam has no evident wear, followers and pushrods are new, both checked. Observing my results, the opening points don't seem to be too far off, it's the duration (and the corresponding clue of max lift on inlets) that seem too far off. Can this be in fact a rockers problem?

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Rockers:
To pick a set of rockers for my engine, and curiosity as it seems I have no life , I measured some on a small bore last week (12 thou valve clearance). The ratios I got after measuring 2 rockers of each type (almost no wear on them) and average them:
- Pressed steel small bore rockers (round pad): 1.19:1
- Pressed steel 1300 rockers (oval pad) - 1.24:1
- Early (?) S forged rockers - AEG426 - 1.19:1
- Late (?) S forged rockers - 12G1221 - 1.21:1

Formula I used to calculate ratio (valve clearances perhaps should be discarded, but I didn't thought of that at the time): full lift/(cam lift-clearance)

Question 2:
- Official ratios are higher than mine. Have anyone tried to measure rockers? What are your findings?

Question 3:
- I'm finding rocker spacing a bit hard. I tried to swap rockers to suit, but it just doesn't work and some rockers are really pressing valve tips on the side instead of the middle. Only solution would be thinning down rocker posts. Is this normal? How can I improve it? It isn't bad... but could be better!

[R&R]

My 2p,

Try checking at 0 clearance, and plotting the lift every five degrees. Graph the results (lift v duration), and take off the cam duration from the graph at the checking height. Rocker ratios, valve tip heights, followers and rocker pad shape can affect rates of lift, the duration is difficult to check accurately without the proper equipment, but the above method is much more accurate. Try using the same rocker for each lobe check, then use different rockers on one of the end exhausts, you will be amazed at the difference in the rockers, but should find the cam to be pretty much spot on, but only if you know what the cam should look like in the first place.

Paul