Author Topic: Bearing and piston clearances  (Read 129 times)

Offline Truckedup

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Bearing and piston clearances
« on: 22.08. 2020 13:49 »
  Repair manuals usually have clearances specified...But sometimes clearances used might be different  based on experience
   My A10R isn't a Triumph, but I put the rod bearing shells in rods torqued to the ft lbs listed...At the machine shop the rods are measured and the crank machined to the clearance . On my street and race Triumphs it's  .0015....I assume the A10 is similar on the rods and timing side journal?
  The piston clearances listed in manuals for stock type pistons are ok? Mine will have an iron head on it, it that makes a difference... thanks
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1961 Super Rocket, 79 Triumph T140D, 96 Ducati 900M

Offline Swarfcut

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Re: Bearing and piston clearances
« Reply #1 on: 22.08. 2020 14:37 »
 Truckedup.
     Bearing catalogues publish the housing dimensions and the shaft diameter. The bearings are  produced to run at pre -deternined running clearance and as long as the machining is done to the specified dimensions and within tolerance, that's it. From my Glacier catalogue for a big journal crank, crank pin is 1.6865/1.6870". Big end eye 1.8435/1.8440" Shell thickness 0.078" I'll leave you to consider the figures for max and minimum clearance, you may be fine with 1.5 thou.

  Lots on the forum about the timing bush and the different types and approaches to sizing and accurate machining. Some folks convert to a needle roller type with modified oil feed to the crank end. Clearance here is based on shaft diameter, bearing type (pressure fed) and bearing material.  Suggested figures on the Forum, and the bearing bore shrinks slightly when installed into the case.

  Pistons will come with specification for the finished bore size suitable for the average intended use. This takes care of the running clearance. BSA published piston clearances for standard factory parts. Over time the metallurgy has advanced so better to use present day piston manufacturers' recommendation for new pistons. For genuine time warp parts, in their original model application,the BSA spec will do.

 Looks like you have built a fair few motors and this one is no different in basic engineering terms.

 Swarfy.
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Offline Truckedup

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Re: Bearing and piston clearances
« Reply #2 on: 22.08. 2020 17:42 »
 Thanks....Reading the timing side bush threads it appers it's  best to have it  line bored ..No arguement with that...
 
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1961 Super Rocket, 79 Triumph T140D, 96 Ducati 900M

Online muskrat

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Re: Bearing and piston clearances
« Reply #3 on: 22.08. 2020 22:22 »
G'day Trucky.
1.5 thou is good for both t/s and big ends. You probably know that when the shells are put in the rod and the cap is put on there will be a gap of a few thou between rod and cap. This is to give the shells crush. If the bolts are only done up to half their spec torque they will bind on the shaft. Once torqued to spec all is good.
As Swarfy said it depends on the piston manufacture. Std BSA A10 pistons 3 - 4.5 thou for solid skirts, 2 - 4 thou for split skirt pistons (A7 split skirt goes down to 1.1 thou). New modern pistons can  be anywhere from 1 to 3 thou. Go by what's on the box.
Cheers
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