To Go 4" BigBore Or Not?

Discussion in 'Engine, Fuel and Exhaust' started by Rod Stewart, Jan 21, 2016.

  1. Rod Stewart

    Rod Stewart Active Member

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    Early last year I did a 103" BigBore kit (3-7/8") with SE 203 cams on my 2007 96" Road King Classic. I would have gone 110" or bigger, but the 103" was the biggest MoCo bore you could get without having to bore the cases. I did not want the bother and expense of case boring, so 103" was the way I went.
    Man what a difference! That simple kit really woke the bike up, and works well with the K&N air cleaner and true dual Rineharts that are on it. I downloaded the canned map for the Power Commander that is also on it, then tweaked the 5-20% throttle-open range to get some cruise mileage back, and all is well. A completely different beast. Nicely broken in now, it gets 38-43 MPG, which is fine by me.
    Then this year the MoCo comes out with the new 4" BigBore kit in the SE Accessories catalog, that does not require case boring! Wouldn't you just know it! I was just a bit too early.
    I thought hey, no big deal, I'll just swap in the 4" kit this spring, and bingo, 110"!
    So the other day I go into the local Arrowhead Harley dealer to get the lowdown on the 4" kit. A mere 1200 smackers I am told, plus tax, and I can take the kit home that day, since they had four in stock. Wait a minute; 1200 bucks for a kit of two barrels, 2 pistons, pins, and rings? What? The kit does not include heads, cams, or much else. Other similar BigBore only kits go for around $700. Where is the extra $500 coming from? Something does not add up.
    So I left empty-handed, but with my Visa intact. For that kind of money I will stay with the 103" thank you very much, or go with a 3.94" 107" aftermarket kit instead. So what have I missed here? The parts guy at Arrowhead could offer no explanation as to why this kit is so over-priced. Anyone know anything about it?

    Rod
     
  2. dolt

    dolt Senior Member Contributor

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    Where is the extra $500 you ask? What you are missing is the fact that the MoCo 4" BB kit is the ONLY way you can get to 110" without case boring. So factor in the additional labor and down time to pull the lower unit, to and from shipping for case boring and the cost of case boring and your cost to get to 110" will exceed the $500 premium for the MoCo 4" kit. Not to mention the convenience factor of being able to "take the kit home today" and via a simple top end rebuild; wala 110".;)

    Gotta hand it to the MoCo; going after a share of the performance aftermarket is a smart move. The MoCo just introduced a similar kit for the 110" motors to jump to 117" without case boring. These kits, installed by a dealer, also come with a warranty; pretty tempting for many.

    Save your $$, there are shops like Hillside Performance and VeeTwin Performance that will sell a 107" kit, complete with your cylinders bored, new pistons fitted, rings gapped and gaskets for about $600 and will do it on an exchange basis to minimize down time. Set up with decent heads and cams with the right exhaust (your RH TDs are fine, best with the 3.5" baffle IMHO) a well thought out 107" motor will perform as well or better than the MoCo Stage V "Tire Shredder" for a lot less $$.:)
     
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  3. Rod Stewart

    Rod Stewart Active Member

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    dolt; thanks for the reply.
    Just so there is no misunderstanding; I am a big fan of the MoCo's bolt on engine mod kits, and what they can do for a bike. I did a similar Bigbore kit on my older 2004 Classic TC-88 to upsize it to 95" along with 203 cams, and a Dobeck tuner. All in all great bang for the buck and I have been pleased with how that bike runs ever since.
    So I do not want to seem like I am bad-mouthing the MoCo here, but I also do not want to throw money away unnecessarily. What is it that the MoCo does to build a 4" bolt-on kit that others cannot do? Logic tells me that if 3.937" is as big a bore that others say will work without making the insertion sleeve wall too thin, but that will still fit into the standard machined case spigot hole, how does the MoCo make a sleeve that has an inner ID 63 thou bigger? It sounds like the sleeve wall must be 32 thou thinner?
    I guess I could go with an exchange 107" kit for sure, but I am not sure the expense would be worth the modest gain over 103". Have to think that over I guess.
    Interesting about the 117" Bigbore kit for the 110" motor. What is the standard bore and stroke for the 110", and what bore is the 117" kit?
    Rod
     
  4. Rod Stewart

    Rod Stewart Active Member

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    Further to this, I thought the standard bore & stroke for the 110" is 4.00" x 4.375", no?
    So to give you a 117" without using a stroker crank would require cylinders with a bore well over 4"; something like 4.125"
    How does this happen without case machining; or is the standard case for the 110" already machined with a larger spigot hole?

    Rod
     
  5. dolt

    dolt Senior Member Contributor

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    I did not mean to indicate that you dissing the MoCo; just answering your question. Although I can find plenty to fault the MoCo for, these new "drop in" kits are pretty cool. All the aftermarket builders are not sold on boring OEM cylinders to 107"; some have a problem with the thin spigot but some do not. At the end of the day, comparing the cost of the 110' drop in kit with boring to 107" is an owner decision based on budget and whether or not the owner shares the concern some builders have with the thin spigots on the 107" kits.

    What the MoCo has done is replace the ductile iron liner with a high strength steel liner which should be stable in the case bore even with the thin spigot wall. I guess time will tell if that is the case. The downside is that these new liners cannot be bored, so if there is a problem, the whole cylinder needs replacing.

    In both the 110" and 117" kit, the hi strength steel liners are the "trick" that allows a bigger bore to fit in the case bore where previously a case bore would be required to accommodate the larger cylinder bore. The standard bore for the 110" motor is 4"; 4.125" for the 117". The stroke is 4.375"; same as 96"/103" motors. Check out the videos.



     
  6. Rod Stewart

    Rod Stewart Active Member

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    OK, makes sense.
    Looking at those videos, that spigot wall is HYPER thin for sure. But in normal operation there is very little stress on it; it just keeps the piston straight at the bottom of the stroke. (Just don't drop it accidentally on the shop floor during installation!!)
    I can understand where switching to a forged steel liner should make it workable. That also helps to explain at least in part the higher cost of this kit. It would be considerably more costly to machine and bore high tensile steel compared to cast iron.
    I think for anyone with a stock 96" this would be a great way to go (with the right cams etc), since you are jumping up 14 cubic inches! (YAHOO, hang onto your shorts!)
    In my case the cost to go from 103" to 110" just does not make the same kind of sense, so if I do anything I think it will be an aftermarket 107" bored out kit.
    This also retains a little bit of a safety margin, since the stock bottom end is just the pressed-in pin crankshaft, not welded. I think if you are building a 110 to 120 hp monster you should likely go with a beefed up bottom end, just so your day doesn't end badly. Just my opinion.
    Rod
     
  7. dolt

    dolt Senior Member Contributor

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    Not hard to build a 110-120HP 107" motor, so if you are concerned about the health of your crank, check run out before proceeding with the 107" plan. So you will have baseline to compare with run out after you have rolled up a few miles on the new build. If run out increases, you know addressing the issue is in your future.;)