Hi guys happy 2020 to you...I have put in a complete new UK made crank pin assembly. I have aligned tuned the crank. The problem I have is that when I rotate the crank in the tuning jig the Conrod crips to the timing side of the crank. The Conrod has less than 0.020” either side of the two crank halves. I’m worried that if I put it back in the bike it will cause a problem.
Is it usual?
I think what you are tying to say is that you have clearance between either side of the rod and flywheels. But when rotated the rod works its way over to one side and rubs the flywheel. If this is the case then the roller clearance is a little tight. This is what happens when the clearance is a bit tight the rod will work over one way. It may need the rod eye honing or it may be ok when it has run a while. It all depends on how tight it is. Ian will confirm this and will give you better guidance.
Let us know how you get on.
Tim W
email (option): t.j.walker@btinternet.com
I would concur with Tim that a slightly tight clearance for the rollers is the problem you have...I am about to go into town to pick up some parts but will post some dimensions later on today for comparison with what you have....(including the side float figures for the conrod)....Ian
email (option): ian@wright52.plus.com
I think there should be 10-12 thou of side clearance on the rod. Just be aware that BSA were none too careful when throwing these bikes together. The last one I did had 45 thou of side clearance when assembled.There were no signs of wear on the flywheels,they just hadn't been machined to tolerance. Alpha bearings were incredibly helpful, I sent the crank pin back to them and they taper ground the pin to get the correct clearance.When I put it back together it was exactly 10 thou. They didn't even charge me for the postage ! And a lovely chat to the workshop foreman who told me about moving to Bradford on Avon because his Father was sent there by Royal Enfield to work in their underground works.
[' Just be aware that BSA were none too careful when throwing these bikes together...']
I can't say I agree and have found little reason to believe that is a correct statement...
Having built scores of BSA engines and gearboxes and mixed components from engines that were manufactured often decades apart I have found BSA production tolerances to generally be 'bang on'...
BSA were one of the leading engineering companies in the UK at that time and the idea they were 'throwing' bikes together simply does not conform to the way a company involved in mass production at that level works...Take a look at the tolerances specified throughout the M20 standards book...That is what they manufactured to..
Most BSAs at this distance out from their date of manufacture have had numerous owners, shops and organisations working on them with different skill levels, resources and knowledge and I can attest to the fact the results are variable and frequently incorrect...Not to mention the introduction of non standard parts into the mix...
I spent decades in manufacturing and I can't understand why people believe that because something was manufactured a while back no one involved bothered to apply even basic engineering standards...
My father, who was involved in early jet engine manufacture, worked at Armstrong Siddley, Austin Cars, Alfred Herbert Machine Tools etc. etc. and was eventually the chief production engineer at British Aerospace would turn in his grave at such a suggestion...:laughing: ...
In answer to the original post the correct tolerances for the big end are as follows...
Side clearance of the conrod .010"-.012"
Crankpin roller track 1.2700"- 1.2702" O.D
Big end rollers .250" dia.
Con rod big end eye 1.7702"- 1.7704" I.D
So, effectively no tolerance on the rollers and two tenths of a thou on the other parts, with 2 thou. on the end float...The end float figure is not critical to the thou. but it is not advisable to have less float than the lower figure quoted...Ian
email (option): ian@wright52.plus.com
Ian, maybe I need to explain. I don't mean to in any way rubbish long standing and respected British manufacturing. I too have spent years rebuilding and working on all sorts of bike engines for the last 44 years,and I'm "only" 55 now... So I have seen all the good and bad examples of build and bodging.And I think bikes attracted the most hamfisted spannering compared to all the industrial,agricultural and 4 wheel drive stuff I normally work on.
But I have to say I've seen plenty of stuff that has clearly never been right from new,please don't think I'm singling BSA out for criticism.Most of the bad things I've seen were Leyland derived and JLR are still selling new parts that fit badly or don't work.The throwing it together comment actually were the words of the chap at Alpha Bearings.An M20 owning friend told me once that the wartime bikes had a life expectancy of 6 weeks.Now if this was true you could understand why maybe tolerances / standards may have slipped. Another chap who I got to know as a teenager was a development engineer at Moulton Developments in Bradford on Avon where my Father was contract felling dead elm trees.His name was Chris Hillier,lovely chap and brilliant engineer.he told me lots of stories of his time underground at Royal Enfield as a hard up apprentice,many of them about how things were ignored or let go.
So, I'm just trying to be realistic about how things may have been,quantity rather than the normal quality we had enjoyed.The lucky thing is now we can take our time to get these bikes to run probably better than they ever did.
Thanks for clarifying. Yes it doing exactly what you have so clearly written. The new assembly went in tight not a plug fit. Rotated freely until side movement
Hi Ian...thanks for information. I don’t really want to pull it apart again. Before I put the Conrod and crank pin assembly into the flywheels, it was tight but I could lightly tap with a small hammer to position the assembly in the Conrod. The Conrod rotated freely around the crank pin when I held either side of the pin with my hands. Given this do you think the crank pin assembly will wear-in when the bike is running?
Bryce,
I know your referring this question to Ian, but it sounds a little tight to me. From what you are describing i would pull it down and check Ian's tolerances. You will be surprised as to how much difference a little hone will make.
email (option): t.j.walker@btinternet.com
Yep...your right I’m just being lazy about it. Stupid to put in a new crank assembly the wrong way and expect it the work
It took two mins to pull it apart and I got the following measurements:
Conrod eye bush 1.762 about 0.0082 different
Roller 0.248 about 0.002 different
Pin/roller track 1.273 about 0.0029
If I allow 0.002 for the fact that I have a cheap callipers I think I need to get 0.006 out of the Conrod eye. Is it possible to emery cloth it out?
Strange that the rollers are down! I take it they are new ones? Ron
email (option): ronpier@talk21.com
Yes all new but I figure my calliper could be out 0.002 so the rollers would correct for 0.250
That's OK then Bryce as the rollers should be very precise! For certain applications they can be supplied in increments of 1/2 thou under or over size. Ron
email (option): ronpier@talk21.com
I agree with Tim...Some time (and inconvenience) spent now will pay dividends in the long run..If the bearing is too tight as it sounds it is you may end up with premature wear of the track, rollers and big end eye...
The tolerances tell you this is a precision assembly and it is best treated as such...Slightly too much clearance can be tolerated and is in fact applied to big end assemblies used for competition..However, too little clearance is not correct or desirable...The big end parts really need to be measured with micrometers that are graduated in tenths of a thou. to make an accurate calculation of what is required...Vernier calipers are completely unsuitable!!!....The big end eye should be professionally honed to ensure the required level of accuracy and importantly to ensure the parallelism of the big end eye bore..
In Johns post regarding production tolerances you have to think about this logically...A con rod end float clearance of .045" is four times greater than the specified tolerance...If that line of thinking was actually applied in the factory to a range of components due to the the short projected life span of the finished machine or wartime contingencies, then it would become impossible to fit many of the parts together on the assembly line. Additionally, there would be many problems with the replacement of worn or damaged parts in repair workshops....Tolerances are applied, and held, purely to enable mass production in the first instance and also to enable an interchange between parts/machines at a later date...It's the basis of production engineering...
In fact, it's worth remembering the whole reason BSA was formed as a company at the outset was to ensure dimensional standardisation and thus compatibility between infantry weapon parts at a time when logistics were becoming impractical as guns were individually made by hand and each part could be/was dimensionally different to the next one..Ian
email (option): ian@wright52.plus.com
time to show my ignorance :thinking_face:
regarding side float of the conrod; I can see that if it is too small then it can foul the flywheels but what are the consequences of it being too large? Only thinking about a few thou here and not such that the flywheels contact the cases or the piston/rod is not central. What might be acceptable max clearance?
cheers
Pete
email (option): petercomley@web.de
The conrod should drop under its own weight from a horizontal position. If it does not, I would certainly remove the crankpin to fix this. Expecting it to "wear in" is just asking for trouble.
email (option): pvlietstra at gmail
It moves freely and drops under its own weight. I think I have found the issue, perhaps. There is a two thou cap between the crank pin and the drive side flywheel, so I think it is sitting up slight on the keyway side. The Dave Plumb info in the tech section says I can emery the lip off where the crank pin and bolts into the flywheels, this should make the new big end bearing assembly seat correct. Will try it this weekend