350cc Heavyweight Single Big end

matman · Post by **matman** » Mon Dec 17, 2007 11:35 pm

Thank you for all the help until now!!

I have just posted some pictures on which you can see the whole disaster:
[img]http://www.ajs-matchless.com/pic_display.asp?id=754[/img]
[img]http://www.ajs-matchless.com/pic_display.asp?id=755[/img]
[img]http://www.ajs-matchless.com/pic_display.asp?id=756[/img]
[img]http://www.ajs-matchless.com/pic_display.asp?id=757[/img]
[img]http://www.ajs-matchless.com/pic_display.asp?id=758[/img]
[img]http://www.ajs-matchless.com/pic_display.asp?id=759[/img]
[img]http://www.ajs-matchless.com/pic_display.asp?id=760[/img]

I have not yet identified the type of crankpin and hope that the pictures might tell anything about it.

Thank you for any further comment!!

Cheers, Jan

matman · Post by **matman** » Tue Dec 18, 2007 9:30 am

...there is still another aspect I´d like to mention:

On the driving side of my engine´s crankcase I found two ball bearings. As an experienced engineer (not an AMC-, but a BSA-, Norton- and Triumph-specialist) told me, on Single-engine constructions like the AMC H/W single there should generally be one ball- and one roller-bearing. This can as well be identified on one drawing in the original spares catalogue.

The crankshaft should be fixed in position by one of these bearings, to avoid axial forces on the timing-side bush.
But when dismantling the engine, I could pull the flywheel out of the crankcase resp. the bearings without any resistance, which means that I did not have to heaten the crankcase.
Could this have caused axial motion of the crankshaft, and might this have been the cause for the engine failure?

Thanks once more, and cheers - Jan

Biscuit · Post by **Biscuit** » Tue Dec 18, 2007 9:33 am

Definitely a one-piece pin. The radius is quite clear to see, as is the lack of chamfer on the flywheel bore. The area where the pin has broken, ie.just beyond the radius seems to me to suggest the pin was not pressed fully home.

Be interesting to know whether the other end is fully home, ie. can you get a feeler between the pin and the flywheel cheek.

matman · Post by **matman** » Tue Dec 18, 2007 9:55 am

Thank you Alan, I will check this out!

Biscuit · Post by **Biscuit** » Tue Dec 18, 2007 10:41 am

The driving side has two ball races, 021872 and 0171191, the roller race depicted, 022351,quite in the wrong place, is for the timing side on competition models. Check 1959 spares list, they have got it right on this one.

The bearings should be an interference fit in the crankcase and should not slide out easily. Never met this myself, but the manual refers to knurling the housing where the interference fit has been lost. This is open to debate, but I feel Loctite will suffice.

I don't think axial movement woukd have caused the crankpin failure.Edited by - Biscuit on 18 Dec 2007 10:41:44 AM

GSAX1 · Post by **GSAX1** » Tue Dec 18, 2007 10:58 am

I agree with Biscuit, looks like the lack of chamfer in the flywheels is the culprit in this case. You where lucky though, no catastrophic damage it seems.

Loctite should do it if the clearance is not too great. If the bearing slides out but there is no play evident then use "Bearing fit" Loctite. Do not heat the case when using Loctite, as it will grip immediately before bearing is fully home. Don't ask me how I know..

Knurling is not satifactory for such high-load applications. Phil Irving even called this practise "barbaric".

Michael

wilko · Post by **wilko** » Tue Dec 18, 2007 9:25 pm

i think jan means shaft to bearing fit, no? douglas

itma · Post by **itma** » Tue Dec 18, 2007 9:42 pm

This is a typical old type Alpha big end failure, you are lucky not to have destroyed the drive side case as well.
The received wisdom was that the two piece pins had a degree of flex in them, certainly with the smaller pins pre war breakage was unknown in normal use.
Malcom Saggers also advised that with modern fuel one could run on considerably less advance, without loss of performance, and giving the big end an easier life. I Reduced mine to 5/16 from 7/16; He reckoned that you could even use 3/8.
A matter for individual experiment.

I too, also used to wonder about the two ball bearings, and on occasions used in a similar engine to yours, a roller on the inside, nothing bad happened, but nothing remarkable either,
The bike was still set up like that when I sold it.

Vern Brady · Post by **Vern Brady** » Tue Dec 18, 2007 9:54 pm

Very interesting info guys. So tell me, which crankpin should I use? 030134-single piece or 900595- two piece. Both seem to be available thru Jampot spares. I have a 1967 G80CS- eng #5563.

Biscuit · Post by **Biscuit** » Tue Dec 18, 2007 11:09 pm

Go for the two piece every time.

Wilko - Jan specifically mentioned with out heating the crankcase, which implies bearings out of their housing, not shaft out of bearings.

Ken - are you saying 3/8" is smaller than 5/16"?