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Projects Old school cool 248 Gmc

Discussion in 'The Hokey Ass Message Board' started by Neb Hillbilly, Dec 26, 2019.

  1. jimmy six
    Joined: Mar 21, 2006
    Posts: 9,978

    jimmy six
    ALLIANCE MEMBER

    Call Isky, They have never let me down.
    I use an ATI super on my GMC race engine. It's the only one that keeps the flywheel bolts tight.
    The one in the photo is a 292 shortened by Tom Langdon. They are readily available and inexpensive. I bought his because he had one done. There is a 3/8" plate behind the 228 water pump which has a 3/8" vee-belt to line it up.
    If you use any of the Chev V8 ones you can press off the water pump pully and use a flat one from a hardware store and put pulley on the front of the damper. You need extra space behind the radiator for this. The avitar photo has gilmer belt pulley system for the external oil pump, vacuum pump, and alternator all bought off e-bay cheap when guys went to cog belts.
     
    Robert J. Palmer and Torkwrench like this.
  2. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 336

    Neb Hillbilly
    Member

    is this the ATI damper number you guys are using? 918425
     
  3. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 336

    Neb Hillbilly
    Member

    is there a place to buy nos rod bolts?
     
  4. Terry Buffum
    Joined: Mar 20, 2008
    Posts: 292

    Terry Buffum
    ALLIANCE MEMBER
    from Oregon

    The LDB cam is to a spec by the late Kenny Bigelow who was a lakes racer in the 1940-50s. Since this is the L or Long Duration Bigelow, I assume there was also an S or short duration. We had an LDB+10 (ten degrees additional duration) in the mid 1950s and I think you will be happy with your cam.
     
  5. jimmy six
    Joined: Mar 21, 2006
    Posts: 9,978

    jimmy six
    ALLIANCE MEMBER

    My ATI is a 7 or 7-1/4” standard SBC. NHRA and NASCAR approved. I do not know of any NOS rod bolts but occasionally rod sets in cosmolene from the military come up. Mine probably had 100,000 miles on the before I started racing them. Also there were some NOS 302 complete short blocks out there too. I wouldn’t over think these GMC’s they are hell for stout and manufactured to run for hours on end without a full flow oil filter. I only hurt one in 45 years of racing and that was because I used the original castle rod nut and it broke. Ruined the block but did not hurt the crank. Went to big block nuts and continued to use the rod bolts to this day. 3EDD2EC4-76BF-4BB1-97A5-D414EC013593.jpeg
     
    Last edited: Dec 29, 2019
  6. saltracer219
    Joined: Sep 23, 2006
    Posts: 828

    saltracer219
    Member

    This from my 1950 Isky catalog reprint in the Rodders Journal catalog set. 225-270 GMC - Track L.D.B., Int timing 26-65 , Exh timing 65-26, Lift .280 on both, valve lash, Int .012, Exh .015. Characteristics same as Ford #1007 grind: Recommended for roadsters and stock cars on short tracks. Excellent acceleration coming off turns and down straight-a-way. I hope this helps, G....
     
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  7. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 336

    Neb Hillbilly
    Member

    if my current bolts weren't rust damaged I would leave them alone. The block appears to be drilled for full pressure now. I thought the 53-55 Chevy 235 Vette balancer might be a good fit. Says 8 weeks delivery though.
    it helps a lot. 270 adv duration. I wish it had more than than .392 lift but we are talking 1950.
     
    Last edited: Dec 29, 2019
  8. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 336

    Neb Hillbilly
    Member

    Has anyone looked at a 390 caddy rod bolt? the head looks very similar and the diameter the same. This is what the HAMB listed as the dimensions. 2.4688 in length & .3906 Dia. I will try and measure them tomorrow
     
  9. saltracer219
    Joined: Sep 23, 2006
    Posts: 828

    saltracer219
    Member

    Have you tried calling ARP tech with your bolt dimensions?
     
    6-bangertim likes this.
  10. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 336

    Neb Hillbilly
    Member

    not yet, watching the kid at home today. Try and get down to the machine shop this afternoon.
     
  11. Gofannon
    Joined: Feb 8, 2007
    Posts: 712

    Gofannon
    Member

    I was told Big Block Chevy rod bolts fit.
     
  12. saltracer219
    Joined: Sep 23, 2006
    Posts: 828

    saltracer219
    Member

    ARP Tech line 800-826-3045. They always treated us real good and have great tech support.
     
  13. jimmy six
    Joined: Mar 21, 2006
    Posts: 9,978

    jimmy six
    ALLIANCE MEMBER

    on another thread..there's a guy running 460 Ford rod bolts in a 261 Chev rods that looking to use GMC rods. Might be the right ones or close enough. Before call ARP take one out and have every demension written down on a drawing right in front of you. I guarentee that will help....or just sent the tech one.
     
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  14. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 336

    Neb Hillbilly
    Member

    GMC rod bolts are .375 diam and 2.32 long
     

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  15. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 336

    Neb Hillbilly
    Member

    a 460 rod bolt is close but has a different head. I will call tomorrow, disassembled rods to day, going to Sonic check the block and disassemble head tomorrow. Worried about the block thickness
     
    Last edited: Dec 30, 2019
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  16. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 336

    Neb Hillbilly
    Member

    Sonic check shows all cylinders .114 plus except a thin spot of about the size of a quarter in the middle thrust side of cylinder 6 showing .085. Probably going to do a short fill
     
    Last edited: Dec 31, 2019
  17. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 336

    Neb Hillbilly
    Member

    tried a 460 rod bolt today. Minimal grinding and it works. Valves are a little thin so we are going to replace them. springs look like stock outers an white painted inners. Shaking and baking the block, heads and tin. Stuff will slow now, time to go back to my real life job of reliability manager on the 2nd. I will keep things going on the weekends.
     

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    Last edited: Dec 31, 2019
  18. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 336

    Neb Hillbilly
    Member

    I do have to say, after this engine is done, I so want to build a hot 302 and find a 49-54 coupe to build a gasser. This seems completely stupid with the fact that I have a 9 second Camaro in the garage.
     
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  19. jimmy six
    Joined: Mar 21, 2006
    Posts: 9,978

    jimmy six
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    Not stupid.... you will enjoy both..:D
     
  20. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 336

    Neb Hillbilly
    Member

    Couple of pictures of the block after "shake and bake".
     

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  21. LAROKE
    Joined: Sep 5, 2007
    Posts: 1,950

    LAROKE
    Member

    I'm stupid too. My daily driver is a '17 Caddy ATS-V two-door, three-pedal fast mover that has more horsepower than my two inline sixes combined, yet, I yearn to get the old rides rebuilt and back on the road because I enjoy driving them more.
     
  22. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 336

    Neb Hillbilly
    Member

    Well the cam turns out a bit smaller than the old isky catalog suggested. May have to my stock steel 248 GMC cam reground Suggestions?
     

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  23. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 336

    Neb Hillbilly
    Member

    Well like most things, this is a little 1 step forward and 2 steps back. I stopped by the machine shop to see if my buddy wanted to hurt his back Sunday trying to get the other 248 out of my shed He showed me the block. The 2 sleeves were put into the block without steps so they were pinned. We don't feel this is a great idea to use this block now. Soooo, the current choices now are 1) try and bore my extra 248 block .235 over, 2) find a 270 short block and bore it to match the Jahns pistons. The taller deck and 4" stroke should allow use of the same rods and pistons. Or 3) pull the military 302 I just bought out of the shed and use what I can to build the 302 I intended to build from the start. I have my machinest asking a local yard with lots of old trucks if they have any 270s. We will see what next week brings
     

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    Last edited: Jan 3, 2020
  24. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 336

    Neb Hillbilly
    Member

    I guess option 4 is sleeve my original 302 that needs at least 2 sleeves, with 6 sleeves to reduce the bore My buddy quoted me $720 to do that, seems dumb to do that only to reuse a set of 65 year old cast pistons
     
  25. jimmy six
    Joined: Mar 21, 2006
    Posts: 9,978

    jimmy six
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    For 720, Ross, CP etc will make you custom Pistons with pins, locks and rings....
     
  26. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 336

    Neb Hillbilly
    Member

    Yep that is why I really don't consider that an option
     
  27. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 336

    Neb Hillbilly
    Member

    Do you guys have any cam grinder suggestions? Probably going to go overboard on the cam figuring it will never be quick so it should sound quick.
     
  28. M A S
    Joined: Jan 3, 2020
    Posts: 1

    M A S

    Let me know if you want to make those flanges. I/we can even use a rotary table and an unreasonable amount of time so it's period correct if you want.

    It's the least I could do since I let that T5 slip through my fingers...
     
  29. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 336

    Neb Hillbilly
    Member

    Stop worrying about that tranny.

    I might have already bought a large port nicson for this and won't need flanges for this build.
     
    M A S likes this.
  30. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 336

    Neb Hillbilly
    Member

    The local yard had a 270 of unknown condition for $300. I told them to get it out and coming to the machine shop. Since I am planning a big overbore, my only hope is no split cylinders. Pictures to come.
     

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