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Connecting rod dowels vs bolts

Discussion in 'Technical and Troubleshooting Torque' started by Ljiljan, Oct 25, 2013.

  1. Is there any particular reason bike connecting rods use dowels rather than bolts? Seems rather unnecessary.

  2. When you say dowels, do you mean studs? If so, I'd surmise that they are there because a stud offers a superior means of fastening, with fewer potential failure modes, than a bolt when a joint is likely to be dismantled and reassembled during the life of the machine. A stud and nut is an engineer's decision. A bolt is an accountant's decision.

    On the other hand, if you mean the precisely sized sleeves that surround the bolt/stud in the rod and fit into a corresponding, precise hole in the cap, they're there to accurately locate cap to rod and ensure that the bearing is a true circle rather than two half circles with a step between them. Bolts are not, generally, an accurate means of location. If you faff around to make them so, fitting them is then rather more involved than whacking them in with an automated torque wrench. Cheaper and easier to use a standard bolt and a standard dowel and put the locating holes in the rod and cap as part of the automated machining process.
    • Like Like x 2
  3. Nicely put, as usual, Pat.
    Plus, in some cases, studs precisely mate the cases together so the bolt holes line up exactly...
  4. I'd say so. Never really thought about it.

    Is it really necessary though? It wouldn't be the first time an engineer has gone to the n'th degree for the sake of "doing it properly", I know I personally have always prefered the design of stud and nuts simply because I don't really like threads. But it's not often you hear of a production rod in a stock engine failing due to the bolts. In fact, last time I heard of that happening was roughly never.

    Way I see it, conrods are twice the weight they could be. Now you can say that is adding factor of safety, but it's also adding significant load without which bolts would suffice just fine. If you have the means Pat, have a read through SAE 2005-01-0987. I think you'll find it rather interesting
  5. Bolt stretch is how/why bearing shells spin. There are oher ways of positively locating the mating of the conrod and big end cap, for example, the Ford Zetec engine used sintered metal conrods and the big end cap was cracked off the rod after it was formed in one piece, The mating faces were the break line and the irregularities kept them aligned.
  6. Engineers might do that in a flagship car such as the Bugatti Veyron, displaying a company's engineering prowess. But in most cases, I suspect they do what they believe is necessary.

    You're right. Rods & bolts normally do not fail nowdays, because manufacturers have learned from past failures, and are "doing it properly" now so that there are no failures.

    This may have been the case in a 1969 850 mini, but not now in a high performance motorcycle engine. Also keep in mind that bikes are built to a price, so maybe they could make rod assembly's a few grams lighter but at twice the price???

    Also have you seen how Yamaha makes their rods now? They use the fracture split method, where the rod is made in one piece and then they break the cap off the rod, which leaves a rough surface between the cap & rod. When refitted they line up perfectly, and the rough surface prevents any movement of the joint. This means they do not need studs or dowels to maintain cap alignment.
  7. #7 Ljiljan, Oct 26, 2013
    Last edited: Oct 26, 2013
    There are a few papers on reducing weight of connecting rods. Look them up.
    There is no way to ever verify this, but I got $10 that says no f1 or motogp conrod from the last several years has weighed more than 250 grams. Here's a rather unique one from the 60's.


    Typical use of C70 for this method has a significantly reduced endurance limit to usual forged steel. Can't remember off the top of my head, but something like 410 MPa down to 340 MPa.
  8. Studs are a "better" way of doing things, not because they're stronger, but because having a thread which might be reasonably expected to be dismantled and reassembled in an expensive and difficult to replace component is a duff idea. If you want to do it properly you put a stud into that thread. Under normal circumstances the stud never needs to come out and so the thread in the component remains pristine and sparkly new. Then, when Mr Hamfist-Mechanic overtightens or cross-threads the nut, all you need to replace is a cheap, easily swapped stud rather than a complete rod.

    In the real world of compromises, bolts into steel rods I can live with. What really makes me want to lay about with something heavy is bolts into unsleeved aluminium; an utterly shithouse practice but one very common in the retention of motorcycle heads and barrels. Again, fine from a single-use strength point of view but inviting expensive disaster if the thing ever needs to be taken to pieces by anyone other than the factory.
  9. That looks like it's from one of those twin cylinder 50cc Honda gp bikes which revved to 20,000rpm. Horses for courses. That same rod design might not work so well in a 1800cc v twin cruiser that is built to a price.

    Also being one piece suggests the crank was a pressed together roller bearing type like all single cylinder bikes are. But when you try and do this with a four cylinder engine, like Suzuki did in the 80's with their GS series, you introduce a few new problems. Sure the conrods may be lighter with this design, but they are not easily rebuildable for the average mechanic, and crank may suffer from flex & twist if pushed to the limits.

    Yes sometimes manufacturers will make a heavier rod using a weaker material, and for a variety of reasons. If it works and is reliable, why is this an issue?
  10. RC166
    Obviously, but that doesn't mean the 1800 v-twin needs to run 750 gram conrods. Also, horses for courses, the context is fairly evident, it's not about 1800 v-twins.

    Point is, there are different options, manufacturers don't have to conform to a standard design from the 50's.

    In this day and age it is relevant to everything from fuel consumption to bmep. Utilising studs significantly detrimentally effects bottom end mass, bearing film thickness and pressure, inertia, parasitic losses, power and fuel efficiency.