This week's assignment: SHP Big Block Valve Lash - NCRS Discussion Boards

This week's assignment: SHP Big Block Valve Lash

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  • John H.
    Beyond Control Poster
    • December 1, 1997
    • 16513

    #31
    Re: the specs i have for the #535 L-88 cam

    Just to confuse things further, 3904366 isn't a casting - it's the finished '67-'68 L-71 camshaft.

    Comment

    • Michael H.
      Expired
      • January 29, 2008
      • 7477

      #32
      Re:I'm beginning to think that's correct

      John,

      Of all the info that's coming in here now and everything that I've questioned on these cams over the years, the only thing that actually makes sense is the fact that GM used the 3904633 starting in 67 instead of the 3863143. The parts book always listed the 143 as the replacement for all 65-71 but if that's the case, why was there a 3904633? The part number is about right, numerically, for the 67 model year. Almost everything released for 67 was 390xxxx and that's exactly when the cam rear journal had to change to the "ungrooved" style. Has this whole thing just been a mixup in the Corvette parts book all these years? There's absolutely no mention of a 3904366 in the Corvette parts book but I'm told the 3904366 is in the Chevelle book.

      I know this has been a long thread but we still have no definite answers to the original question. Duke sent the origial specs for the 143 and hopefully, I'll be able to compare these to the original cam and then to the 3904366 cam.

      Comment

      • Duke W.
        Beyond Control Poster
        • January 1, 1993
        • 15610

        #33
        Re: the specs i have for the #535 L-88 cam

        Well, that's what the note says. It was added as Change #18 6/14/66.

        I think the note means that the 3863144 can be made from the 3904366 finished cam by machining the groove IAW drawing 3856354. (As I said previously drawing number 3863144 just shows the lobe geometry and refers to drawing number 3856354 for other camshaft dimensions. Drawing 3863144 does include a drawing of the complete cam with some notes and the rear journal groove IS PRESENT, but no dimensions.) Add the dowel pin and you have the 3863143 assembly. The note should not have included the word "casting".

        I think this is further evidence that the '67-up SHP BB cam is the same as prior years with the exception of not having the rear journal groove. In other words, the lobe geometry and lobe phasing are identical!

        Duke

        Comment

        • Matt Purdy

          #34
          Re: Enhanced typing skills

          Hey Mike/Duke,

          New to the NCRS forum, I have been following you guys (I think) on this discussion. I was able to pull up the prints you are speaking of here at work, because they are all now scanned into the online database. I agree with what you are saying, Duke:

          Drawing 3863143: Is the assembly of cam# 3863144 and dowel pin# 3836903
          Drawing 3863144: Is the L72 cam with grooved rear journal. Date 10/16/64
          Drawing 3904366: Is the same cam without the grooved rear journal. The cam specs on this drawing are identical to the specs listed on the 144 drawing. Both show the lash as .024/.028. The date on this drawing is 3/21/66

          Can you clear up exactly what we are trying to verify? Is it whether these two cam lobes are the same, or are we trying to answer the question as to why the lash would be different between the 30-30 and the L72?

          Is there any other drawings we are interested in for this discussion? If so, I can do some digging here at work to help out....

          Matt #14693

          Comment

          • Duke W.
            Beyond Control Poster
            • January 1, 1993
            • 15610

            #35
            Re: Enhanced typing skills

            My original "challenge" to the group was to determine the proper range of operating lash and what the lash should be set to based on the lobe data I extracted from the drawing and my analysis of dynamic properties that is included in my lead in post, but the "differences" between the early and late BB SHP cams also cam up, which has always been a gray area.

            It looks like you've settled the cam difference issue by looking at the other drawings, but the "correct" lash is still unresolved.

            I don't think the .024"/.028" lash is ideal because it "assumes" that the rocker ratio is constant 1.7:1. It's not. I've measured the SB rockers and they start at about 1.37:1 at the lash point and achieve 1.44.1 at max lift with an approximate 0.3" lobe. This led to my recommendation for different than OE lash specs for the SB cams, and this documented by John Hinckley and me in a Word file on valve adjustment that we pass out to those who are interested.

            I think a couple of guys might be working on measuring the BB rocker arm ratio behavior, so maybe we'll be able to select better setting clearances. It's clear from the lobe data that the top of the ramp is about .012" above the base circle. This times rocker ratio at the lash point should be the MAXIMUM operating clerance. The setting clearance should be this plus any necessary compensation for expansion. I have measured SB clearance "hot" (fully warmed up idling engine) versus cold and found no signficant difference. Mike Hanson reports differences on cast iron block/head BBs. The other issue is exhaust valve clearance during hard running when the exhaust valve gets much hotter than at idle. Clearance probably tightens up a few thou, but that's why the ramps are fairly tall, and my conclusion is that, at least for a SB, the lash should be set at the top of the clearance ramp dimension times actual rocker ratio at the lash point and then rounded down or reduced by about one thou.

            Duke

            Comment

            • Michael H.
              Expired
              • January 29, 2008
              • 7477

              #36
              Re: Thanks Matt....

              Thanks for the info Matt. It looks like one of the mysteries has been solved. The .024/.028 lash listed in the original engineering drawings for the 3863143 cam means the 65 and 66 owners manual and shop manual were most likely incorrect. We may never know why these publications listed .020/.024 but from the engineering prints and everything that was published since, the .024/.028 was the original intended setting and still is today.

              One of the things that was confusing was the fact that the 3904366 was never listed in the Corvette parts book as the correct replacement for 67-69. It listed only the grooved rear journal 3863143 for all sp.h/perf 396-427 in 65-69. I suppose it was an unintended omission but it sure caused confusion. Thanks again,

              Michael

              Comment

              • Duke W.
                Beyond Control Poster
                • January 1, 1993
                • 15610

                #37
                Re: Thanks Matt....

                The original clearance and timing on the drawing is probably as listed in some references. Using clearances of .020"/.024" the timing numbers would be different.

                The first seven revisions are lost because the drawing was "redrawn" with change #8. Change #14 is "timing diagram revised", which could be when the clearance/timing was changed since timing is a function of clearance. Whenever you look at an old drawing, look at the change record to see what changes occured since the original release. Same applies to AIM sheets.

                Based on the .012" height of the clearance ramp above the lobe base circle. I think that .020"/.024" is a better cold clearance spec.

                Duke

                Comment

                • Michael H.
                  Expired
                  • January 29, 2008
                  • 7477

                  #38
                  Re: Thanks Matt....

                  Duke,

                  I have an original 143 coming and will chart a few lobes to see if the various different lash settings give us something close to the original spec's shown in early publications. Should be interesting and I'll keep you posted. I'll also run it through a stock steel rocker arm to see just how far off the 1.7 ratio number is.

                  I agree, the .020/.024 is a much safer lash, especially with the clearance ramps that your map showed but the HP sure dropped with those settings.

                  Comment

                  • Duke W.
                    Beyond Control Poster
                    • January 1, 1993
                    • 15610

                    #39
                    Re: Thanks Matt....

                    I don't see how tighter lash could cost top end power. Tighter lash is a trick that's been used over the years to increase top end power because it lengthens effective duration. That's why GM tightened the inlet lash spec on '63 engines with the Duntov cam.

                    If anything, tighter lash could cost some low end torque, but not top end power.

                    Duke

                    Comment

                    • Michael H.
                      Expired
                      • January 29, 2008
                      • 7477

                      #40
                      Re: Thanks Matt....

                      Tightening lash...a little...will increase top end power, but there comes a point where the whole band is off the scale and at that point, the bottom and mid range is nearly helpless. If one or both are a bit tighter than specs, especially when drag racing, some improvement may be gained but with closed exhaust on the street, I see no benefit.

                      Comment

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