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E: Engine Valve Timing

vibrac

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VOC Member
Thinking about this the only problem with tapppet screw head method when you are doing all that opening and closing malarkey and want comparable figures with other engine data is that the action at the valve is a sliding action on a curved follower so the length is not equal I guess with the 4 degree method on both tappets it would not matter too much (not that I have ever tried it)
 

Bill Thomas

Well Known and Active Website User
VOC Member
I have been following this interesting thread. Mine is a 48 Rapide engine in a Wideline frame. Effectively laid up not quite finished in 1977. At the time I bought new cams from the Thresher down at Lymm. Later I head that there was a batch of cams about that were not properly hardened. I got mine tested; they were too soft so they were nitrided and retested so should now be good. I got out the dial gauge and the degree disc and checked with the following results. No 1 Inlet opens 50BTDC max lift 0.319" closes 92ABDC. lift at TDC is 0.083" and at BDC 0.190". No 2 Inlet opens 49BTDC max lift 0.325" closes 85ABDC. lift at TDC is 0.110" and 0.200" at BDC.
No 1 exhaust opens 76BBDC max lift 0.318" closes 45ATDC has lift 0.174"at BDC and 0.091" at TDC. I could not get the gauge on the valve stem of No 2 exhaust due to the proximity of the frame so put it onto the tappit screw head instead and obtained No 2 exhaust opens 66BBDC max lift 0.292" closes 21ATDC, lift 0.168" at BDC and 0.062" at TDC. I am surmising that if the rocker arm has equal lengths either side of the pivot the lifts should be the same as measured at the valve but the timing should still be correct whichever.
Now these cams are not standard as the lift is too much. The inlets are about the same and not too far away from the book numbers, No2 exhaust looks to have less lift that No 1 exhaust but that might be down to the rocker arm. Oh and I checked with the valves on full lift to see how much more I could lever things and the results were not a lot so I clearly have some work to do here. I look forward to the experts' opinions.
Have you shortened the top of the lower valve guides ?, I can't remember if it's only the inlet that needs doing
for Mk2 cams, And if you have captive rocker bolts, You may have to grind something off the top of the rocker arms, To get free rocking ?. Cheers Bill.
 

Gary Gittleson

Well Known and Active Website User
VOC Member
Thinking about this the only problem with tapppet screw head method when you are doing all that opening and closing malarkey and want comparable figures with other engine data is that the action at the valve is a sliding action on a curved follower so the length is not equal I guess with the 4 degree method on both tappets it would not matter too much (not that I have ever tried it)
In my project I just needed to confirm the settings before buttoning it all up. I had gone through all this two years ago and got things in order so that the factory timing marks were correct. This time, I didn't want to remove the UFM to gain direct access to the valves. So I first set the gears using the timing marks and then checked at 4 degrees BTDC on the overlap using the "pushrod" method. I passed the probe of the dial indicator through the rocker. I get exactly .050" on intake and exhaust, so I think I'm in good shape. At this position, the lift is sufficiently small that the arc of the rocker shouldn't make that much of a difference. I'm too lazy to calculate it.
 

timetraveller

Well Known and Active Website User
VOC Member
I am going to try to post a graph of MkII cam lifts so that you can compare them with your own. The maximum lifts on both your cams are not as high as MkIIs so you should not be running into clearance problems. If you only have, say, ten thou clearance then you do have a problem somewhere. The max lift on these MkII cams is .358 Ex and .368 In. Other cams might differ. On MkIIIs I have measured the equivalent figures are .35 Ex and .352 In so you can see that your values are not excessive.
If you copy and enlarge the graph then you can put your values for opening and closing times on there and then see what you think. 90 deg ATDC for the closing of the inlet does not look right and you might want to check your measurements.
For years I have tried to persuade people with unknown cams to produce lift curves like those here. It is tedious and time consuming. You only have to do it once but without this information you will struggle. If you look at this graph you will see that before the start of the inlet lift and after the end of the exhaust closing there was a fault. I have measured many cams with this sort of fault which is caused by the cam ramps not being correctly blended into the base circles. If you want to do this properly them please measure the lift every ten degrees of engine rotation and then plot an equivalent graph. Any spread sheet program will do. Once you have that detail then you might find, what I suspect, that there is a fault at the end of the lift on one, or both, of your cams and that is causing the strange values.

Incidentally you cannot just nitride any steel. It needs to be specified to be one which will respond correctly to nitriding e.g EN40b.
1585325567681.png
 

roy the mechanic

Well Known and Active Website User
VOC Member
For all practical considerations the only numbers that matter are the lift of inlet and exhaust at t d c . That's how all 6 cylinder Aston Martins are set-up, and they go quite well on it. From your numbers, I would be happy enough with no 2, the inlet is virtually as per the Aston numbers, and the exhaust is a bit less. No1 needs more inlet lift at tdc, which will bring the exhaust back a bit. As I said in my last post, I set-up a shadow last week using this system and it goes really well.
 

SteveO

Well Known and Active Website User
VOC Member
Well I've bitten the bullet and done the deed. I don't have access to a dial gauge although I do have a timing disk and the correct attachment to go in the end of the crank. I found 4 degrees BTDC on the exhaust stroke of the back pot and then used a variation of oexing's method. It's not as accurate, but I found that without the adjusters in I could "flick" the end of the pushrod and the cam would turn off the ramp (probably due to the needle rollers), so I could find the opening and closing positions on the valves. I marked these on the case with felt tip pen and also a mark on the camwheel. Counting teeth back from the marks enabled me to "split the difference". I repeated the marking procedure on the front pot. So the engine was returned to 4 degrees BTDC on the back pot and in went the large idler, then turned engine forward and put in the front cam. Turning the engine over a couple of times confirmed that the valves weren't hitting anything, so the engine went back onto the back pot, then marked everything up. The original marks had been set on the front pot so they were all wrong anyway. We'll see what this does.
 

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