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#806366 04/23/20 11:39 am
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Hi all. I know this subject has been flogged more than an an Inqisition victim. So I apologise in advance, and am ducking the dog turds and rotten eggs already.
Nevertheless... I can get repeatable end float with everything done up and sealant applied, cam (and spring!) fitted, and the crank nut done up. How small can the end float be? My gauge shows 0.015mm or around 0.0006”. This is with the dial stuck to the flywheels, and referencing on a barrel stud.
Again, sorry about trying to revive this dead horse...

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When in doubt, consult your manual. I believe it says .0015" -.002"
or
I could be wrong, it may not be in the manual.

Last edited by Nick H; 04/23/20 11:57 am.

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Aim for 0.002“-0.003”

If you know what shim size you need, get some shim steel and make your own. 1 shim of the correct size is better than several.

Don’t forget to shim the crank sprocket to align it with the clutch basket.


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The spec is .0015" - .003". Anywhere in between is fine. Less is really bad, more is not so bad - but it will start to knock at low RPM if you get too much.


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Thank you for your replies. I have the original specs, but I recall that Mr Mike sets his to the minimum possible measurable end float. I wonder why that is, and if he still believes in that setup.

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I have seen experiments in less than .0015" end play. It really isn't pretty.

There was a local around here when I was younger who fashioned himself as a BSA mechanic. He set several engines up at less than .001" end play. You simply do not want to pinch the rollers against the race lips or gall the thrust washer to the crank. You really don't. It gets ugly fast.

Like Allan, I aim for .002". But, in too many years playing with these engines, I have seen some seriously sloppy engines with WAY too much end play that ran fine, they would just knock at low RPM. Sounded like a rod ready to depart captivity, but they ran ok.

Get your end play in spec, then make sure the clutch sprocket and crank sprocket are inline. IMO, that is frequently overlooked and the primary chain can force a lot of side loading into the crank if it is not aligned.


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Thanks Rich B. Duly noted.

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Chas.A50,
I have just started reading this site again after years of being away. I do recall setting the end play on the very short side of specifications. I am not opposed to the .0015 " to .003' as a general specification. That allows for errors in measurement, thickness of glue in seams ect.. And CERTAINLY there must be clearance for an oil film and to avoiding preloading the bearing. When I experimented with .0005", I assembled and disassembled several times being absolutely sure I had .0005". I even heated up the entire assembly in an old BBQ grill with an electric hot plate to see if my clearance held. As soon as the motor begins its running life that clearance starts increasing...so my thinking was it will stay inside its outside limit (when it starts knocking) longer. If you are CONFIDENT in you measurements, I would go with the tighter spec, but as a word of caution, measurement to tenths of a thousands takes care and patience. I am not sure the BSA repair shops back in the day had the time or patience to get it right and therefore the .0015-.003 is appropriate.

Just talking out loud.

Mr Mike

PS A ball bearing conversion eliminates all this discussion and makes the reassembly simple. Recognize though that you give up the load capacity that the roller provides.

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Logic tells you as long as you have some clearance then you are ok, aluminium expands faster than steel. But what if the combustion heat of the cold running engine gets to the crank first, it then expands earlier than the cases and what little clearance you had dissappears.

I have never seen any tests but its suggests keeping to the published minimum is safer than just enough so the crank spins cold.

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The other thing is even with a filter the sludge trap still fills up (unless you use synthetic oil but that’s another thread). At some point your going to need to clean that sludge trap out, your also going to shim the crank at that same time. The clearance does grow over time, I don’t know if it’s the bearing wearing from side thrust or shims falling apart... don’t know. But I’ve seen a standard motor doing nearly 20,000 and not been pulled apart yet.


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I agree with Kommando's comment, i don't think i've ever really spent time measuring end float, if
the crank turns ok, then the sealant added on the cases for assembly will give you a thou anyway.
If you are stupid enough to cane the living daylights out of a cold engine, thus making the crank grow
faster than the cases, then allow a bit more clearance.
When warmed up the float is always a few thou plus anyway, like the distortion in the bearing alignment
you'll never get these old crates perfect or even near it. The cases move about in all sorts of directions
as they heat up and cool down, ribbed sand cast alley does that, even without being split vertically.

I never want to be taking one of my street motors apart before it's done 30k miles, if they don't last that long,
i didn't build them correctly, simple as that.

Last edited by NickL; 04/24/20 11:23 pm.
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I actually calculated the differential expansion and then measured with a dial indicator that the clearance had increased when the assembly was heated. I surely can't make any claims as to how the engine heats up and I surely wouldn't recommend doing this without using dial indicators. On the other side of the argument I think that assembly at the high end of the clearance (.003") is likely to invite shorter life. In the end each of us gets to choose how we do it and we live with the result.....that's what makes this interesting.

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Running a rebuilt engine at .003” does not shorten engine life. Contrary to interweb lore, a BSA unit twin runs just fine with too much end play. They run noisy at low RPM, but they run. Once the wear gets to about .015”, the knock gets too annoying to ignore. BTDT.

The unit twin only runs a short period of time with too little end play. Very short.


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