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BS350 lab seal? Part 2


OK, here are 3 links/videos from today.  The issue may have resolved itself...unless I'm missing something.

This first video is a better visual of the wobble in the lab seal and bearings:

The second video has the crank in the lower case showing the wobble:

In the third video, I decided to put an O-ring on the lab seal and the result surprised me.  Now I'm thinking that maybe the whole crank unit is stabilized once the O-ring is in place and the top part of the case is bolted on.  The "slop" that I was worried about my be built into the tolerances.  Of course, I could be all wrong.  As stated previously, the only other time I've built a 350, I didn't have any of the wobble issues.

               You've got me worried now  ;D and I have a few questions for you at the end of my post.
       My observations :-
      Video 1, Movement you see on bearings and seal is perfectly normal.
      Video 2,  Lab seal may have been designed with a clearance fit on outside, without the oring, you can see it turn slightly. Was probably to allow for greater expansion of the alloy.
        Previously, I was not sure if the oring was stopping the crank from fully seating but without it, the crank is still not seating fully (IE, bearings can still be rocked) which may be due to an interference bearing fit in the cases. In fact I've taken the left 'c' ring out and I can slip a 0.5mm feeler under the bearing, but I have the oring fitted. So I now believe the crank bearings do not fully seat until the top case is bolted on, (with or without the oring). I can also rock some of the bearings slightly with the crank placed in the lower case. When you spin the crank it may be imperceptibly rocking in the case and moving the bearing outers, as in the videos
      Video 3, Fitting the new oring makes things seem worse. Its groove is only 2.6mm deep, there is a lot of rubber to be compressed which the weight of the crank can't do. The crank is able to rock even more. Try giving each bearing a light tap to better seat them and see if things improve.

      On old or reused cranks the oring is already squashed, /deformed and allows the crank to seat better, the bearings will also have bedded in, maybe why this wasn't noticed before on rebuilds.
 A bit of a ramble (as usual for me ;)) but I've tried to stick to the main points. I'm sure both our cranks will be just fine when the top case gets bolted up.

    Now, with my crank correctly located on the right c ring I have a gap of approx 0.7mm between the left bearing and c ring. Seems a lot. Do you have same. My whole crank seems biased to the right, based on case clearances and centre bearing positions. All the crank shims seem to be in place. Are you able to measure the flywheel gaps across the rods and centre bearings on yours. I have 8.9mm on the rods and 50.5mm on centre.

   Tried a dry fit of top case, will close to about 1/16'' gap, not happy about flipping the cases to put the bottom bolts in as knock pins could displace. Will maybe support on trestle and bolt from bottom. Cases might close up better if 2 seals on left of gearbox are pushed in after assembly.
  Are you intending to apply sealant with shafts in place, be easier without, but threebond I'm using advises assembly within 1 minute.
    Like yourself, I rebuilt these engines, twice, 45+ years ago without issues. Are we just more cautious or experienced now ?
                                                                                                       Brian, we'll get there.


I'll have to go the shop and take some measurements to get you all the info.  I probably won't get back to the engine until tomorrow.  Thanks for the time and effort of your responses.



I’ve gone through your last response in detail and will address the issues it raised to me as I come across them.

 I am unable to get my left C ring out but with the crank seated in the lower case and the O-ring in place  I am able to get a .025 feeler gauge underneath the right outer bearing.   I did notice however, that the feeler gauge wil only fit at the bottom.  As you go up the sides the space narrows, which makes me wonder if it doesn’t completely fill in and fit tightly with the top case bolted on.

With the O-ring fitted and the crank in place I can also get a very slight rocking which I attribute to the height of the O-ring. I then fit the upper case to the assembly and there is a small amount of space at the front of the engine which I also believe is from the O-ring.   Beforehand, I also lightly tapped the assembly with a rubber mallet in order to make sure things were seated completely. When I assembled my last 350 engine, I just put the bolts in it to hold things together and at that time the crank did not spin smoothly.  It wasn’t until I loosened them and then tightened them down in accordance to the sequenced outlined in the Service Manual that the case came together completely and the crank would spin smoothly.

The space between my left outer bearing and the left C ring is almost 1mm. I seem to recall this being about the same gap I had in my other engine assembly.  I don’t know if there is a median specification for this anywhere.

I’ve never had the dowel or knock pins be loose fitting in the bottom of my crank case.  I’m not sure if I could get them out if I wanted to. A lot of times I will use a thick grease to hold parts in place during assembly.

On my last 350 assembly I put the sealer on with everything in place.  I use Yamabond 4 semi dry liquid gasket, which also says parts can be assembled after one minute.  Before applying sealer to the cases I also seal the O-ring with Hylomar blue.  It is a non-setting joining compound.  You have to wait for the solvent to evaporate before assembly, which gives you the time to apply the case sealer.

I tend to be cautious to the point that I am borderline OCD. Four years ago I couldn’t even change a spark plug, so all of this is still pretty new to me. Overall, I think we’re OK and I am going to go ahead as is.

 Thank you very much for all the time and effort you’ve put into this.


     Hi Rowland,
                       Many thanks for detailed reply, I also find I have to work steadily and carefully these days or I overlook things  :D. Don't think I'm OCD.....................yet  ;D

       Your findings confirm (to me) that a) the bearings do not seat fully until the top case is bolted on and are an interference fit, b) the crank sits high on the center Oring when crank is first placed in lower case and will rock
        I am reassured by your 1mm gap between C ring and left bearing. Seem to remember the bearing sat right against the C ring on my original engine in 60s.
      I did try fitting the crank with drive shaft on the left, the centre bearings and flywheels were centred much better but I can't work out why. Should be the same either way ? I know the rods should be centred in the bores, mine might be 0.5 to 0.8mm out but there's plenty of side clearance on the small end/piston and it's not a racing engine.
      Anyway, this crank is going in and I will put the top case on tomorrow.
     Good luck with the rest of your rebuild.                                                  Brian


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