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The Pontiac OHC Six

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  • #46
    Re: The Pontiac OHC Six

    Do you want me to send you a Chevy 250 crank? I'll have to dig it out of a motor but I was planning to tear one down anyway to take some measurements so it's no big deal for me. Just pay shipping and it's yours.

    Dan

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    • #47
      Re: The Pontiac OHC Six

      My experience shopping for pistons for my Chevy 250 (same bore/stroke) was that there were very few options (not alot of 307 SBC performance parts out there!) If you could fine something that truly had a flat top, they were VERY expensive. I saved money with stock (dished) pistons and decked the head and block. Roger your concerns about cam timing (although I am still pondering the mechanics of it...)
      1967 Chevelle 300 2 Door Post. No factory options. 250 ci inline six with lump-ported head, big valves, Offy intake and 500cfm Edelbrock carb.

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      • #48
        Re: The Pontiac OHC Six

        Very cool post from the HAMB with two great articles on the OHC Poncho.

        http://www.jalopyjournal.com/forum/s...d.php?t=499509
        1967 Chevelle 300 2 Door Post. No factory options. 250 ci inline six with lump-ported head, big valves, Offy intake and 500cfm Edelbrock carb.

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        • #49
          Re: The Pontiac OHC Six

          Can you explain how the belt is tensioned? I'm still thinking about timing...
          1967 Chevelle 300 2 Door Post. No factory options. 250 ci inline six with lump-ported head, big valves, Offy intake and 500cfm Edelbrock carb.

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          • #50
            Re: The Pontiac OHC Six

            The reason the Cam timing gets retarded with block and head milling is because the cam and crank sprockets get closer together and yet the number and spacing of the teeth on the belt remain the same, so as the two sprockets get closer, assuming the crank remains stationary, the cam has to rotate counter clockwise for the belt teeth to sit on the cam sprocket. The accessory drive sprocket is the tensioner. Look at the pictures (one is captioned pointing out the adjustment slots).

            I'm not trying to build compression so I'm not worried about a flat topped piston. The stock pistons already have valve relief's in them, but at 8.5:1 compression I'd like to mostly leave the compression alone, and just find a lighter rod/piston assembly to "reduce" the stress on the crank,etc. at higher rpm.

            Thanks for the offer Dan, but that won't be necessary.

            Scott, I'm fine with the 500-600 number. I haven't focused too much yet on the turbo end of it. Haven't started shopping on the turbo end of it until I know exactly what I have, what's available for durable parts and what I can do for a camshaft. I figure it wouldn't have to be too exotic as the displacement at the moment is ballparking the 3.8 Buick's. I do plan on having an intercooler as well.
            Escaped on a technicality.

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            • #51
              Re: The Pontiac OHC Six

              Originally posted by TheSilverBuick
              , and just find a lighter rod/piston assembly to "reduce" the stress on the crank,etc. at higher rpm.
              why, with 7 mains, how much stress you think there is?
              I'd think your cast pistons and rod bolts are the only issue here if the rods are not cast

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              • #52
                Re: The Pontiac OHC Six

                Because I want to make sure it's comfortable at high rpm. Peace of mind not re-using 40 year old rods, which I may use anyways.. or if I get a shorter piston I may need a longer rod for it anyways. I'm definately replacing the pistons, I'm on the fence about the rods (assuming they are forged).
                Escaped on a technicality.

                Comment

                • #53
                  Re: The Pontiac OHC Six

                  Originally posted by TheSilverBuick
                  The reason the Cam timing gets retarded with block and head milling is because the cam and crank sprockets get closer together and yet the number and spacing of the teeth on the belt remain the same, so as the two sprockets get closer, assuming the crank remains stationary, the cam has to rotate counter clockwise for the belt teeth to sit on the cam sprocket. The accessory drive sprocket is the tensioner. Look at the pictures (one is captioned pointing out the adjustment slots).
                  Why can't you assume the cam and crank sprockets remain stationary as the distance between the two gets smaller, and then the slack in the belt is removed by pushing down on the accessory gear which is essentially an idler and free to rotate?
                  Aha...in writing this I may have convinced myself of the answer... The idler of which I just spoke is driving the distributor! Now I can see see the mechanical linkage between milling the head and timing.
                  1967 Chevelle 300 2 Door Post. No factory options. 250 ci inline six with lump-ported head, big valves, Offy intake and 500cfm Edelbrock carb.

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                  • #54
                    Re: The Pontiac OHC Six

                    Originally posted by 67pete300
                    Originally posted by TheSilverBuick
                    The reason the Cam timing gets retarded with block and head milling is because the cam and crank sprockets get closer together and yet the number and spacing of the teeth on the belt remain the same, so as the two sprockets get closer, assuming the crank remains stationary, the cam has to rotate counter clockwise for the belt teeth to sit on the cam sprocket. The accessory drive sprocket is the tensioner. Look at the pictures (one is captioned pointing out the adjustment slots).
                    Why can't you assume the cam and crank sprockets remain stationary as the distance between the two gets smaller, and then the slack in the belt is removed by pushing down on the accessory gear which is essentially an idler and free to rotate?
                    Aha...in writing this I may have convinced myself of the answer... The idler of which I just spoke is driving the distributor! Now I can see see the mechanical linkage between milling the head and timing.
                    No, the ignition timing is independant of the cam timing. I can turn the distributor housing to correct any ignition timing issues. If there were a second belt adjuster on the other side then the timing could be adjusted by changing the "length" of the belt between the two sprockets.

                    Found this nifty picture. Look at the belt on the right side going directly between the crank and cam, the number of teeth and distance is fixed on the belt, so moving the two sprockets closer requires the cam to rotate to take up the slack in the belt.
                    Escaped on a technicality.

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                    • #55
                      Re: The Pontiac OHC Six

                      Originally posted by TheSilverBuick
                      Because I want to make sure it's comfortable at high rpm. Peace of mind not re-using 40 year old rods, which I may use anyways.. or if I get a shorter piston I may need a longer rod for it anyways. I'm definately replacing the pistons, I'm on the fence about the rods (assuming they are forged).
                      40y/o rods..
                      40y/o rods with 89k or 2008 rods with 250k..
                      what one to trust..don't matter mag it and it pass it good. a new out of box rod can fail and not pass a mag test..
                      I'd look at pistons, rodbolts(if forged) and with 7 mains thats it.. forged crank be nice but not needed..
                      i'd not worry about the piston hiegh or rod center to center if you are throw'n a turbo on it..
                      the head will be your limiting factor not the short block

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                      • #56
                        Re: The Pontiac OHC Six

                        notice the name on that blue print.... Delorean - interesting.

                        Is there any chance of adding another idler, or tensioner pulley to that timing belt set up?
                        There's always something new to learn.

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                        • #57
                          Re: The Pontiac OHC Six

                          A picture is worth a thousand words. :

                          I wasn't thinking about the fixed number of teeth on the BELT.
                          1967 Chevelle 300 2 Door Post. No factory options. 250 ci inline six with lump-ported head, big valves, Offy intake and 500cfm Edelbrock carb.

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                          • #58
                            Re: The Pontiac OHC Six

                            Originally posted by milner351
                            Is there any chance of adding another idler, or tensioner pulley to that timing belt set up?
                            Not with my current engineering and fabrication skills. Sending the sprocket off for modification to adjust cam timing is a better bet for me.
                            Escaped on a technicality.

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                            • #59
                              Re: The Pontiac OHC Six

                              Not instead of, in addition to.

                              I'm wondering about that long belt run between the cam and crank, seems like a ripe area for improvement, maybe a waste of time too...
                              There's always something new to learn.

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                              • #60
                                Re: The Pontiac OHC Six

                                Originally posted by TheSilverBuick
                                Originally posted by milner351
                                Is there any chance of adding another idler, or tensioner pulley to that timing belt set up?
                                Not with my current engineering and fabrication skills. Sending the sprocket off for modification to adjust cam timing is a better bet for me.
                                adjustable cam sprocket? can't be too hard to make one fit.
                                www.realtuners.com - catch the RealTuners Radio Podcast on Youtube, Facebook, iTunes, and anywhere else podcasts are distributed!

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