Cam lobe to crank throw clearance & cam lobe to rod

Discussion in 'Small Block Tech' started by Mart, Oct 5, 2018.

  1. Jim Blackwood

    Jim Blackwood Well-Known Member

    I think if you check you will find that the cam lift is increased by grinding the base circle deeper, meaning that as far as lift is concerned the lobes are all the same distance from the cam centerline. Only the base circle and duration changes.

    .040" is adequate clearance but more doesn't hurt. Some combinations will just need to be clearanced, simple as that. Take a little off the shoulder of the rod and maybe chamfer the end of the rod bolt on that side. The metal removed should be minimal. You'll probably have 2 rods that need clearanced and will want to do them all for balance. Fatter rods with bigger bolts will stick out more. I suspect that once the rods are clearanced for the two that have to be trimmed the degreeing of the cam may not make any difference, but check it out. It's been awhile since I did this and I don't really remember.

    The stock 340/350 stroke can cause interference with aftermarket rods so you'd think extra stroke would just tend to make it worse. However, the rods used with an offset grind may clear just because the rod journal is smaller. The extra stroke comes off the inside.

  2. sean Buick 76

    sean Buick 76 Buick Nut Staff Member

    I agree Mart I am not using a roller cam at this time so I have no clearance issues but yes that will be another issue when or if I move from the flat tappet cams to a roller. because I am boosting I am just using small custom flat tappet cams but each to their own. There is only so much room so something has to give to get the stroke if you want to go that way, especially with the roller cams.
  3. 300sbb_overkill

    300sbb_overkill WWG1WGA. MAGA

    Yeah exactly, squawking about stroking a sbb 350 because of cam to rod clearance, LOL, the Molnar rods need clearancing with the factory stroke for crying out loud!

    Stroking other brand engines has the same hurdles as the sbb, take a sbc for instance;

    A sbc 383(3.750") needs block clearancing AND a small base circle cam for cams with lobes bigger than .333"(would be .500" lift with a 1.5:1 rocker ratio), with a 4.00" stroke crank the sbc needs block clearancing, a small base circle cam AND clearanced rods that can be bought already clearanced. The sbc rods that can be bought pre-clearanced are cut into the bolt just like Ivan had to do and are still rated for 700 HP!

    Anyone one planning to do a stroker OR using the Molnar rods should probably consider using a reduced circle cam right out of the gate so the rods won't need as much or no clearancing depending on cam grind.
  4. 300sbb_overkill

    300sbb_overkill WWG1WGA. MAGA

    The lobe sits on top of the circle, when the circle is smaller the same lobe will be lower while still giving the same lift.

    With a smaller base circle say like .200" smaller will give an extra .100" clearance per side with a lobe that has the same lift as one on the larger base circle. So essentially the lift can be increased another .075" and still have .025" more clearance with a .200" reduced base circle.

    2.00" circle plus a .350" lobe = 2.350" measured from the base circle to the top of the lobe.

    1.800" circle plus a .350" lobe = 2.150" measured from the base circle to the top of the lobe which is .200" less that would be .100" per side lower giving an extra .100" clearance for the same lobe.
  5. Mark Demko

    Mark Demko Well-Known Member

    My machinist said the same thing about the SBC 383 stroker, lots of issues to overcome to do it.
    I was surprised it wasn't plug-n-play for the SBC, LOL.
    He said its no big deal having to clearance for different rods in the SBB with a roller cam, any time you deviate from stock there may be issues no matter the engine make.
    300sbb_overkill likes this.
  6. Mart

    Mart '71 350 GS, almost stock

    Well rods are finally getting some room! Stroking definately has its drawbacks with roller cams, especially big duration grinds. I have 14 degrees on degree wheel of same clearance closeness do to the fat nose lobes. Not a good scenario for the average car guy in my opinion. Lots of close tolerance fitting, measuring, etc., over and beyond the average build.
    20181106_190735.jpg 20181106_190805.jpg 20181106_190904.jpg Studio_20181106_203515.jpg
    Last edited: Nov 6, 2018
    Julian and Mark Demko like this.
  7. gsjohnny1

    gsjohnny1 Well-Known Member

    hopefully not chitty-chitty bang-bang
  8. Mark Demko

    Mark Demko Well-Known Member

    Looks good Mart!
  9. Mart

    Mart '71 350 GS, almost stock

  10. 300sbb_overkill

    300sbb_overkill WWG1WGA. MAGA

    If the ''average car guy'' were to do a sbb 350 stroker even with a roller cam, he could order the reduced base circle cam from the start just like the guy with the factory stroke that uses the Molnar rods should do to avoid having to clearance the rods.:rolleyes::D

    Using a reduced base circle cam would also gain clearance with the crank's counter weights to cam as well.

    Also have to make sure that the rods selected should have .787" or larger wrist pin bores for a larger selection of wrist pins to choose from.

    Other than that, its not that different than any other performance build, I don't see a problem. To be fair though, some of the issues Mart had to deal with were a bit out of the ordinary because of the rods he sourced and the roller cam he already had that he didn't want to have to pay for regrinding it.

    As far as the amount that was ground off of his rods, Eagle sells sbc 6.00" H-beam rods for a 4.00" stroke that are already clearanced close to what Mart had to do and those are still rated for 700 HP! A sbc with a 4.00" stroke needs clearanced rods AND a reduced base circle cam for everything to get along in there. Not to mention the hours of block grinding that needs to be done for the rods to clear the pan rail! Plus it needs a special oil pan to clear the 4.00" stroke!:eek:

    So as far as stroker engines go, the sbb 350 isn't that bad to do IMHO compared to some of the other brand engines. Now all you sbb 350 guys, get out there and get stroked!:D:D:D
    Julian and Mark Demko like this.
  11. Mark Demko

    Mark Demko Well-Known Member

    That's pretty much what my machinist said about the 383 Chevy stroker, theres many issues to address in doing one of those, he wasn't surprised either about having to grind away on the Molnar rods for the Buick 350.
    Heck, even bone stock, if you get the cam/crank outta phase, the crank/rods hit the stock cam, and you cant get the cam in or out:eek:
    Its just butthole puckering being this is all new stuff for the Buick 350.
    Is Mart going to dyno it/track test it?
  12. Mark Demko

    Mark Demko Well-Known Member

    Mart what pistons/comp ratio you going with?
  13. Jim Blackwood

    Jim Blackwood Well-Known Member

    My cam grinder told me that Buick already uses a reduced base circle cam. Well that is what he said and I am not expert enough to argue the point. My understanding is that they start with a blank where the lobes come pretty danged close to the diameter of the bearings and then any lift you get comes off the inside. More lift just means there's less meat in the center. So what is the base circle? It sure can't be the root diameter because that changes with the lift. Must be the max lobe diameter, it's the only thing that makes sense in this scenario. The question has to be asked, do you know a cam grinder that sells a "reduced base circle" cam for the SBB? Because I don't. I went around and around with this when I was building the long rod 340 and in the end the solution that made sense was to trim the shoulders of the rods.

  14. Mart

    Mart '71 350 GS, almost stock

    1. Reduced base cam = will still hit aftermarket rods slightly, more if stroked, period. Crank throws?, yes, that should make it clear.:)

    2. This is not a chevy, so nothing pertains, just as the chevy bearing clearances don't either.:D

    3. Again, this is not a chevy. :rolleyes: You are not looking at what I am seeing. Stroke at your own risk.:eek: Blueprinting needed. Yes it can be done. Necessary? Maybe not.

    4. Autotecs Mark. 10.58 with iron heads & .042 gaskets, 11.10 with alum heads and steel .020's. when the heads are available. :cool:
    Track t&t first, if it runs good numbers , I'll be happy!:)
    Mark Demko likes this.
  15. gsjohnny1

    gsjohnny1 Well-Known Member

    let's take a collection up for pampers.....:D:D:D:D:D
  16. Mart

    Mart '71 350 GS, almost stock

    Now John, you know I hugged this block just like you told me too....:D:):D:)
    Last edited: Nov 7, 2018
  17. 300sbb_overkill

    300sbb_overkill WWG1WGA. MAGA

    The sbc was mentioned just to point out that the sbb 350 isn't the only engine with obstacles to add stroke.:rolleyes: Seriously, just because a sbc is even mentioned doesn't mean its not relevant to stroking an engine. I have dealt with both of them and yes they are 2 different engines but they DO have some of the same issues to over come! :p

    Both are 90* V8 push rod style engines. And like I wrote about the sbc with 6.00" H-beam rods with a 4.00" stroke, "the rods need to be clearanced AND it need the reduced base circle cam."(sounds VERY similar to what a sbb 350 stroker would need with a huge roller cam:confused:)

    I didn't write that a reduced base circle cam would be the cure all end all for a sbb 350 stroker, I wrote there would be less need for clearancing with one.;)

    And NO, a sbb 350 stroker isn't a slap it together and down the road, it will have extra work to do to get it together just like some of the other brands when stroke is added but that's the price one has to pay for the extra performance. I wouldn't recommend doing a stroker build for the first engine someone decides to build but the second one, perhaps?
    Mark Demko likes this.
  18. Jim Blackwood

    Jim Blackwood Well-Known Member

    So who makes this mythical reduced base circle cam for the SBB anyway?

  19. Mart

    Mart '71 350 GS, almost stock

    Did a little measuring last nite. My roller base circle is 1.045, and a stock GM flat tappet cam is 1.170
    So the roller already has a smaller base circle than the stock cam. Maybe to allow room for the roller wheel, and to not expose the lifter oil hole.
    There must be some magician that pulls those reduced circle cams out of a hat!:D Anyway, tight quarters going on.
  20. Fox's Den

    Fox's Den 22 years of racing the same 355 Buick motor

    So, Mr D. how many of those stroker sbb have you built. 0 what are you waiting for the heads?

    Just sayin you tell everyone to do this but no one knows if it is worth it or not. Hey, the motor is already stroked why make it harder than it is to build. I would like to know how is the rod cut and how do we know if that might be too much taken away from the bolt threads, and cause a big boom, then what.

    If we have to do this kind of stuff then it should be shown here so the rest of us dumbarses know what we are doing.

    Just comments no pun intended.

    Big John on the other site slaps it together with a blower and goes darn near 8's and asks us what are we waiting for... Christmas.
    PGSS likes this.

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