? On Bronze bearing Id and od fitment

Discussion in 'General Engine Discussion' started by Naiveambition, Dec 29, 2018.

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  1. Dec 29, 2018 #1

    Naiveambition

    Naiveambition

    Naiveambition

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    Looking for info regarding bronze bearing fits. I looked around for some info, and wasn't able to find a situation similar to mine, so thought I could ask you all.
    These are bearing for a hit and miss crankshaft, and with bolt down caps. I'm wondering about the squeeze on the bearing, and if it translates the inside diameter. One site I found suggested a less than press fit, and another one at .001+
    My plan is to first lap the bearing to the shaft and then lap od of bearing to the bearing blocks. Right now I have 1.0055 od bearing and block size is 1.003. Wall thickness is .125. I do believe there should be some interference to keep the bearing from spinning. But how much without over affecting the crank clearance
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  2. Dec 30, 2018 #2

    Cogsy

    Cogsy

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    Why not go the other way and go for a tight/light press fit for the OD, then lap the bore to what you want it so you know it's correct? You're definitely going to get squeeze on the bore so it makes sense to me to do it last for best roundness and size. The other option would be to either chemically or physically lock the bearings from spinning and then you don't require a super tight fit. It shouldn't be overly difficult to generate a tang on one bearing half to fit a recess in the bearing cap, along the lines used for automotive bearing shells.
     
  3. Dec 30, 2018 #3

    Charles Lamont

    Charles Lamont

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    So you have 0.0015 clearance between the bearing and its seat? That is more than you would want for perfection, but will do fine. Especially as the caps will hold the bushes in place anyway. You might put a small radial pin or dowel in the bottom of the seat to be sure the bearing will not rotate, meaning that the caps would not have to be clamped down hard. (You drill the oil hole in situ, right through the cap, both 'sides' of the bush, and into the bearing seat.) Ideally, the bushes should be both be finish bored in situ at one setting. I ream bronze bushes with variable results. Sometimes they come out perfect, and sometimes they don't. Nearly always, the shaft needs to be a little oversize for a good running fit in a reamed hole. Lapping the bores could be a good option for a superior job.
     
  4. Dec 30, 2018 #4

    Hopper

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    I wouldn't lap a bronze bushing. The abrasive particles will stick in the bronze and forevermore wear away at the shaft. +1 on reaming should be good enough for a crankshaft on an old banger (or a newly made model thereof). Or even boring. But as Charles Lamont said, it's best to ream the pair in situ in line.

    The outside should be about right. Just don't overtighten the pinch bolts enough to make the bushing contract inside. Bit of loctite would be the easy way to stop the bush from spinning, and minimise clamping pressure needed on the pinch bolts. Just put a drop or two on the lower half so the cap can be moved if wanted later.
     
  5. Jan 1, 2019 #5

    chrsbrbnk

    chrsbrbnk

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    as said before a lot of the split bearing half's are prevented from rotating by a pin seem's to help if the pin is a tad shorter than the thickness of the bearing!
     
  6. Jan 1, 2019 #6

    Charles Lamont

    Charles Lamont

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    If a suitable material for the lap is used (eg annealed copper) with a suitable lapping compound (not diamond or silicon carbide!) a bronze bush can lapped perfectly satisfactorily without embedding abrasive. I am all in favour of Loctite in the right place, and as a method of preventing the bush rotating, it might be easier than using a locating pin, but in this instance it would not be a better method.
     
  7. Jan 2, 2019 #7

    steveastrouk

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    Lapping soft metals needs special non-embedding compounds, my favourite is called "Timesaver", you can buy it in big tins, but there is a nice sampler set of their hard and soft compounds available for $40 IIRC.
     
    xpylonracer likes this.
  8. Jan 3, 2019 #8

    davidyat

    davidyat

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    I agree with Charles Lamont. Put a dowel pin in your setup. When I was rebuilding VW engines that's how they kept the main bearings from turning in the case halves. See if you can see a diagram of a VW engine and how it's done.
    Grasshopper
     

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