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Learning Rocker Geometry
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SkrapMetal
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PostPosted: Wed May 06, 2009 5:15 pm    Post subject: Learning Rocker Geometry Reply with quote

n00b here when it comes to this stuff, but I've been reading all I can, and I'm ready to get this thing back together finally. For the record, 1915, engle 120, Scat 1.25 rockers, chromoly pushrods.

Ok first off, I had a head stud break on me, so I sent my heads out to Jeff Denham to go through them and check them out for me. He had to flycut them, but he said that I might still be able to use the same pushrods, but I want to be 100% sure.

I know at half lift, the adjuster should be parallel to the pushrod, but how do I know when I'm at half lift?

I have a dial indicator, but how do I read it? I'm pretty sure I have it set up correctly... parallel with the valve from all angles.

Zero lift
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Full lift
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How do I read this little bugger?
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Mongo63
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PostPosted: Wed May 06, 2009 5:33 pm    Post subject: Reply with quote

One full circle of the needle is 100 thousandths of an inch. Start at 0 lift and rotate face of dial indicator so pointer is at 0. Rotate engine and count the number of revolutions of the indicator needle as the valve opens, the point when the needle reverses direction of travel is just past full lift. For example if the needle makes exactly 5 full revolutions that's .500 lift. Take the number you found to be full lift and divide by 2, that's half lift that you use to set rocker geometry. The other option you have is to take your cam card and multiply the specified lift by your rocker arm ratio, in your case that's 1.25 then divide that by 2. Hope it helps.
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Sigurd
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PostPosted: Wed May 06, 2009 5:46 pm    Post subject: Reply with quote

I just dealt with this and I too have a 120 with Scat 1.25s. Here's a hint...they aren't 1.25. More like 1.31. I got around 0.520" lift at the valve. Don't go by cards and paper specs; actually do it. When you find full lift, divide by 2...that's half lift. Then rotate the engine so that the pointer reads that half lift number. Once you are at half lift, you can check the pushrod/rocker alignment. Also have to check the wipe pattern on the lash caps with this type of rocker. Hope that helps.
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Mongo63
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PostPosted: Wed May 06, 2009 5:50 pm    Post subject: Reply with quote

Sigurd wrote:
I just dealt with this and I too have a 120 with Scat 1.25s. Here's a hint...they aren't 1.25. More like 1.31.


That's why it's better to see full lift istead of just mathing it out.
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Sigurd
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PostPosted: Wed May 06, 2009 6:45 pm    Post subject: Reply with quote

If I had assumed the advertised 0.496", there would have been a problem. To further complicate things, the exhausts were about 0.508". I had 4 different pushrod lengths; intake and exhaust for each side.
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Hophead
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PostPosted: Wed May 06, 2009 10:21 pm    Post subject: Reply with quote

Read the Bob Hoover Blog about dialing in your cam. It was also in VwTrends three or four years ago. Did you find TDC first? Is your degree wheel dialed into TDC. On my 2110 the keyway on the crank for the fan pulley and cam gear was cut 1 degree retarded. You need to read where half lift is directly from the cam/lifter for best accuracy. If you can find a way to read the lift from the push rod side of the rocker that would be better but no where near as accurate as if it was read back when the motor was a shortblock and it could have been determined directly from the lifter. My FK-8 had .001 more lift on the 2-4 exhaust lobe.
It needed to be advanced 2 degrees. It was no big deal to find half lift at the lifter and mark the actual point on the degree wheel instead of trying to sort through it at the head.
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SkrapMetal
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PostPosted: Thu May 07, 2009 4:13 pm    Post subject: Reply with quote

Alright, after measuring, I came up with 0.5089 at full lift, hard to tell from the picture, but I'm hoping I did that right? The needle passed 0, 5 times.
Image may have been reduced in size. Click image to view fullscreen.



Then, I took .5089/2 and came up with 0.25445, so again.. the needle passed by 0, 2 times, and ended up here, correct?
Image may have been reduced in size. Click image to view fullscreen.



Here is the angle of the adjuster to the angle of the pushrod
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And this shows the wiper pad to the lash cap
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How is this looking?
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PostPosted: Thu May 07, 2009 5:07 pm    Post subject: Reply with quote

Sigurd wrote:
If I had assumed the advertised 0.496", there would have been a problem. To further complicate things, the exhausts were about 0.508". I had 4 different pushrod lengths; intake and exhaust for each side.

That would be where I'd send that cam back and get a good one.

Max
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chrisflstf
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PostPosted: Thu May 07, 2009 6:35 pm    Post subject: Reply with quote

I dont think having 4 different P/R lengths would be good for valve train balance either. Better to fix that problem at the source.
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PostPosted: Thu May 07, 2009 7:38 pm    Post subject: Reply with quote

SkrapMetal wrote:
Alright, after measuring, I came up with 0.5089 at full lift, hard to tell from the picture, but I'm hoping I did that right? The needle passed 0, 5 times.
Image may have been reduced in size. Click image to view fullscreen.




I dont mean to be a stickler but the guage reads 091. So that would be 0.591. Sorry, I am used to having to set up machines to +/- 0.0002"
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SkrapMetal
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PostPosted: Thu May 07, 2009 8:02 pm    Post subject: Reply with quote

sturgeongeneral wrote:
I dont mean to be a stickler but the guage reads 091. So that would be 0.591. Sorry, I am used to having to set up machines to +/- 0.0002"


As I was turning it over, the needle was rotating counter clockwise, and I was reading the dial in reverse also, so that's how I came up with 0.509. Is this wrong? Confused
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PostPosted: Thu May 07, 2009 8:20 pm    Post subject: Reply with quote

SkrapMetal wrote:


As I was turning it over, the needle was rotating counter clockwise, and I was reading the dial in reverse also, so that's how I came up with 0.509. Is this wrong? Confused


If the needle traveled counterclockwise you are right on the money! I wanted to be sure, otherwise you would have been off quite a bit. Good job!
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Sigurd
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PostPosted: Fri May 08, 2009 10:12 am    Post subject: Reply with quote

Max Welton wrote:
Sigurd wrote:
If I had assumed the advertised 0.496", there would have been a problem. To further complicate things, the exhausts were about 0.508". I had 4 different pushrod lengths; intake and exhaust for each side.

That would be where I'd send that cam back and get a good one.

Max


Too late...already cut the pushrods. And I should clarify; the two sides are different because it was decked funny. I compensated by trimming respective cylinders. Now, both heads seal and all deck heights are within a thou or two. And by four different lengths, I mean 10.66", 10.67", 10.68" and 10.69". I'm not losing any sleep. Very Happy

Actually, while we're on the topic, one of my 10.66s is actually 10.63 Confused . Is that a problem?
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SkrapMetal
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PostPosted: Fri May 08, 2009 2:31 pm    Post subject: Reply with quote

Not sure what to look at now. I know I'm supposed to go through and check all the valves. What's next?
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PostPosted: Fri May 08, 2009 4:05 pm    Post subject: Reply with quote

Check them all for pushrod to adjuster alignment and also for wipe pattern on the lash cap. Add shims/shave bases. Check again. Then measure the pushrod. Cut new pushrods to match.
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