Author Topic: Small end bush replacement  (Read 781 times)

Offline Degsy

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Small end bush replacement
« on: 25.04. 2021 15:40 »
I was planning on replacing the small end bushes on my A7 engine since I am putting in new 060+ pistons, but my replacement bush is just a plain bush with no oil hole and no oil groove (see photo).

I guess drilling the hole is no big deal but I am not sure about making a groove, is the groove required? do I have the wrong bushes?

Also the new bush feels like it will need to be reamed out when I try the new gudgeon pin, it looks like it would be difficult even with the pin out the freezer and a warm rod and bush.  Is it normal for the bush to need reaming out?
The new Gudgeon pin can be pushed into the old bush by hand.

Degsy

Offline RDfella

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Re: Small end bush replacement
« Reply #1 on: 25.04. 2021 16:38 »
Yes, new SE bushes always need honing to size. Drill the hole, don't worry about the scroll. I don't recomment reaming, as that's difficult to keep perfectly square and therefore risks having a tilted piston. Also, I find reamers are generally only accurate to around .001" whereas I like gudgeon pin to bush fit to be a moderate push fit.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Offline Swarfcut

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Re: Small end bush replacement
« Reply #2 on: 25.04. 2021 16:41 »
  I've only ever seen plain bushes, and BSA state plain bearing sizes after reaming or grinding (as appropriate).   For the A7 Gudgeon Pin Bush it's 0.6881"/0.6878."   Ref BSA Service Sheet 702/June1959.

 As you've already spent on the new bushes it would be a shame not to use them. Having said that the end result may not be that much better (if at all) from new pins in the old bushes. That small end eye on an A7 rod always looks a bit fragile to me, so take care if you  do swap them out.

Swarfy.

Offline Degsy

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Re: Small end bush replacement
« Reply #3 on: 27.04. 2021 09:16 »
So the plain bearing is ok I just need to drill the oil hole, that's good.

I was tempted to just use the old bearings but when I put the new gudgeon pin in and hold it in a vice I get about 1mm play rocking back and forth at the big end (see photo)  that feels like too much?

RDfella how do you do the honing if not with a reamer, I have an appropriate sized hand reamer on order.

Cheers
Degsy

Online berger

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Re: Small end bush replacement
« Reply #4 on: 27.04. 2021 12:01 »
the rock is not so important unless of course it's ACDC!! it's the up and down slop that matters

Offline UKlittleguns

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Re: Small end bush replacement
« Reply #5 on: 27.04. 2021 12:08 »
Hi,

Had to replace one of my small end bushes recently.  Easy enough to get the block pistons etc off but taking the rod out for a 'proper job' was deemed a step too far.

Old bush pulled out with a simple nut bolt and spacer arrangement.  Rod eye warmed with heat gun and new bush fitted.

I used an adjustable reamer.  It was set to enter the new bush half way and then pinched up to just touch the surface.  One turn, reamer out and pin fit checked.  Reamer back in and process repeated.  The nice thing about the adjustable reamer used in this way is that it self aligns with the bush and only takes out a tenth at a time.  Took some 15 minutes but eventually gave an excellent pin fit.

Not for the purists amoungst you all but it worked a treat and saved an eyewatering amount of work.

Len.


Offline UKlittleguns

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Re: Small end bush replacement
« Reply #6 on: 27.04. 2021 12:25 »
Hi again,

Just remembered how to post pictures.


Offline Degsy

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Re: Small end bush replacement
« Reply #7 on: 27.04. 2021 12:32 »
Hi Guys

I already have the bushes and the adjustable reamer arrived in the post this morning so I think I will go for it and change the bushes. I read somewhere that you could use an old piston to help align the reamer, I tried it (see photo) and I am not sure it helps.

Len I think your careful incremental approach is the way to go.  I hope the bushes come out and new in OK, should be easier as I have the conrods off the crankshaft.

Cheers

Degsy

Online Colsbeeza

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Re: Small end bush replacement
« Reply #8 on: 27.04. 2021 13:34 »
Hi Degsy,
I was advised not to try to ream the small ends myself without sufficient experience, as it is thought hard to achieve absolutely perpendicular alignment to the rods, or in other words parallel to the crankshaft. From the efforts of a previous "mechanic", I had one piston tilted to the right by 10 thou and a subsequent smoky exhaust - the small end was not parallel. Took me two attempts to remove the top end and replace the rings, re-hone etc without success before I discovered the piston tilt. Opinions vary but generally the tilt should be less than 2 thou over the piston width, which means about 0.5 thou tilt over the width of the small end bush. Now that is a tall order from hand reaming!
I asked the machinist if he could replace the bushes without removing the rods. He said "I want the rods". Although I do have the reamer, I decided to give him the rods, so had to dismantle a newly rebuilt motor to give him the rods.
He machined the small ends perfectly perpendicular, and I think it was well worth it.
No matter how you decide - as a final check, I was advised to get hold of a 0.750" Bright alignment rod and pass it through both small ends. If you are a good machinist, make your own alignment bar.
I purchased a 300mm length of 19mm bar ( 0.748" ) and a 19.05mm bar (0.750" ). That larger (oiled) bar squeezed through nicely with very little force. I took a photo for reference (Sorry about the background clutter).
Col
Colsbeeza
Australia

Offline Degsy

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Re: Small end bush replacement
« Reply #9 on: 27.04. 2021 16:38 »
Hi Col

Thanks for the good advice and you speak from experience and I don’t doubt that we should be working to these levels of precision and accuracy. 

I joined the forum recently and I have read a lot about the need to work to tolerances of less than a thou, did these bikes really leave the factory with this level of precision, I can’t help feeling the quality control was not great back then and largely dependent on what kind of day the 1950s machinist was having  (yes I have seen "Saturday Night and Sunday Morning" and wasn’t Albert Finney good in that!).

This is me just making excuses for planning on reaming the bushes myself and when my BSA goes bang one day half way up some Scottish mountain in the middle of nowhere I will regret not taking your good advice.

Cheers

Degsy
(Edinburgh, Scotland)

Offline RDfella

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Re: Small end bush replacement
« Reply #10 on: 27.04. 2021 20:09 »
Degsy - a reamer is a cutter, a hone uses stones.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Offline tomkilde

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Re: Small end bush replacement
« Reply #11 on: 27.04. 2021 23:20 »
I recently tried using an adjustable hand reamer on new small end bushes for my A10.  I had never done it before, but the concept seemed simple enough.  The result was a complete disaster.  I bought another set of bushes and took everything to an experienced specialist who will do it correctly.  Looking forward to picking up the completed rods this weekend and resuming my restoration project.  Good luck to you Degsy if you decide to go ahead!  (Also, is anyone out there interested in buying a slightly used adjustable hand reamer? ;))
1958 BSA A10 Super Rocket

Offline Degsy

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Re: Small end bush replacement
« Reply #12 on: 28.04. 2021 08:47 »
*conf*Starting to have second thoughts about doing it myself, I'm looking for good machine shops locally

Cheers
Degsy

Offline Swarfcut

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Re: Small end bush replacement
« Reply #13 on: 28.04. 2021 09:43 »
Degsy. If you decide on the professionals, have the rods checked first for straightness and big end eye ovality. Make sure these aspects are in order before spending time and money on bush installation and finishing. Alloy rods are more "elastic" than conventional steel rods and are known to deform.

 Standard size big end eye for small journal crank is 1.6055"/1.6060"   Metric 40.78mm/ 40.792mm.

 Swarfy

Offline Jules

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Re: Small end bush replacement
« Reply #14 on: 28.04. 2021 13:20 »
yep, agree, my experience with an adjustable reamer was a disaster too on my gearbox, if you want to read about it!! but that was my own fault really for not understanding the correct way of using it ie VERY small cuts at a time!!