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ZB 31 1950 bottom end assembly. #724049
02/02/18 4:13 pm
02/02/18 4:13 pm
Joined: Aug 2001
Posts: 3,987
argyll. scotland, uk
gavin eisler Online content OP
BritBike Forum member
gavin eisler  Online Content OP
BritBike Forum member
Joined: Aug 2001
Posts: 3,987
argyll. scotland, uk
Questions about ZB31 crank , big end and main bearings.

Big end was replaced by a marine engineer, looks new, there is no axial movement at all, is this correct? Doesnt seem right to me,
, side clearance 0.008" DS con rod to crank , 0.038 " other side.
Also when the cases are assembled with the new RHP roller mains they nip up and the crank is trapped, are RHP mains thicker axially than the original spec?

This is a customers bike at the LBS , he already paid a "marine eng " to assemble the big end/ flywheels, the big end does not feel good to me , no radial play but not smooth either.
I have little previous experience with pre unit singles , so putting this out there seems like a good idea.
Thanks in advance.
Gavin.

Last edited by gavin eisler; 02/02/18 8:55 pm.

71 Devimead A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
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Re: ZB 31 1950 bottom end assembly. [Re: gavin eisler] #724084
02/02/18 8:54 pm
02/02/18 8:54 pm
Joined: Aug 2001
Posts: 3,987
argyll. scotland, uk
gavin eisler Online content OP
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gavin eisler  Online Content OP
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Joined: Aug 2001
Posts: 3,987
argyll. scotland, uk
Follow up, called a friend who has pre units, he called Phil Pearson, Phll confirms this is wrong, the conrod should have axial movement, he suspect poor assembly of the big end is to blame. now have service sheets and other good infoto back this up The ZB goes back to the customer untouched, he can speak to the big end butcher and try to get a refund. A hard lesson learned for the customer, dont let boat fixers near BSA big ends.

Last edited by gavin eisler; 02/02/18 8:57 pm.

71 Devimead A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
Re: ZB 31 1950 bottom end assembly. [Re: gavin eisler] #724086
02/02/18 9:11 pm
02/02/18 9:11 pm
Joined: Feb 2014
Posts: 467
Cork Ireland
C
chaterlea25 Offline
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chaterlea25  Offline
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C
Joined: Feb 2014
Posts: 467
Cork Ireland
Hi Gavin,
The main bearings are identical in size,
A spacer is fitted on the main shaft between the drive side roller and ball bearing
This spacer is often worn due to engines being run with the cush drive nut not tightened properly
When new bearings are fitted with a worn spacer the roller bearing can get pinched against the outer race lip when the cush drive nut
is tightened

It sounds like the flywheels are not fitted fully / properly to the crankpin ? which maybe is the reason of the .046 total gap between the rod and flywheels
It also sounds as though the rod was not honed after fitting the rod race?

John

Re: ZB 31 1950 bottom end assembly. [Re: gavin eisler] #724087
02/02/18 9:19 pm
02/02/18 9:19 pm
Joined: Aug 2015
Posts: 1,196
Bolton Lancs UK
A
Andy Higham Online content
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Andy Higham  Online Content
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A
Joined: Aug 2015
Posts: 1,196
Bolton Lancs UK
BSA pre unit single crank assemblies are pretty horrid. The mainshafts are held in with little rivets that work loose. The crank pin has short tapers that fit in the flywheels and held in place by thin nuts. The tapers must fit tightly in the flywheels while the shoulders abut the inside of the flywheels. Imagine machining that!! then imagine trying to get it to fit correctly 60 years later


1955 BSA B31 500cc "Stargazer"
1962 Greeves 200cc "Blue Meanie"
1962/67 Greeves 350
1967 Greeves 360 Challenger
1984 Suzuki GSX1100 EFE "Sorcerers Apprentice"
GM500cc sprint/LSR bike "Deofol"
Jawa 500cc "Llareggub"
2003 Aprilia RSV Mille "Lo Stregone"
Re: ZB 31 1950 bottom end assembly. [Re: gavin eisler] #724105
02/03/18 12:37 am
02/03/18 12:37 am
Joined: Aug 2001
Posts: 3,987
argyll. scotland, uk
gavin eisler Online content OP
BritBike Forum member
gavin eisler  Online Content OP
BritBike Forum member
Joined: Aug 2001
Posts: 3,987
argyll. scotland, uk
I had a good look at the rivets on the crank, I wonder if BSA had a job lot of 4 inch nails to use up?maybe someones idea of belt and braces with a press fit combo, then the braces got slack and the press fit became looser to speed assembly leaving the poor rivets to do it alone.? Imagine re riviting your crank between races at the IOM! What undistilled joy that must have been for goldie racers.
Ali test assembled the crank without the spacer on the drive side to rule that out, it certainly seems like the big end was not assembled or honed correctly.
We has a quick look at new big end prices on line , quite a spread! Hopefully a strip , hone and check on the key fit will put things right, its been clocked between centres and is not terrible 1-2 thou, but thats for the owner to ponder. If Ali disturbs anythiing now it then becomes his fault, so time will tell what happens next. The perils of running a small shop , glad I am just a bystander.
For future reference the critical dim. according to Mr Pearson( written on the shed wall) is 2.935 " between flywheels. Dont know the patients results yet will post later when measured up

Last edited by gavin eisler; 02/03/18 12:43 am.

71 Devimead A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
Re: ZB 31 1950 bottom end assembly. [Re: gavin eisler] #724801
02/08/18 8:39 pm
02/08/18 8:39 pm
Joined: Aug 2001
Posts: 3,987
argyll. scotland, uk
gavin eisler Online content OP
BritBike Forum member
gavin eisler  Online Content OP
BritBike Forum member
Joined: Aug 2001
Posts: 3,987
argyll. scotland, uk
Results in, the width across the flywheels was about 80 thou over spec, big end assembly at fault.


71 Devimead A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod

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