Little End.

freddie 136 · Post by **freddie 136** » Tue May 05, 2009 11:35 am

Hello again, I have a Little end problem with my 1958 G11 600 twin, after a ride out the other day i noticed on taking off my helmet a knock from the engine top. So i took the rocker covers off thinking it was some sort of rocker noise, but found no fault so took the heads off and could see that the right hand piston [As you sit on the bike]had more burnt oil and carbon on top than the left one so took the cylinder off and to my surprise the right hand piston was loose enough as to think the gudgeon pin had broken,any way i pushed the gudgeon pin out and could see the damage to the little end,[i have put a picture of this in the photo section,post war twins, under the title Little End] The little end bush just pulled out with my finger,it had oil on it as did the con rod.Never seen a little end go before and would have thought this kind of damage to one was unusual. Has any member had this sort of trouble to there machines ?.Does any one have any ideas as to what might have caused such damage ?.Having had a quick look at the con-rod i think the eye has been pulled and been elongated a bit, will have to check this some more.Now, can i have it repaired and a new little end bush fitted or will i need a new con rod? I know the engine has to be striped down to get the fly wheels out [and this is a bit daunting]but i am going to have a go. The other point is the pistons i can't tell what all the markings are can some one help please, on the Piston crown is the letter [A]in the center. Around the top outer edge the are the words and letters [E56] [022226] [W145] in a circle [BHB] and the word front, i know what front means and i think some of the others must be manufacture markings but can't tell if the piston is standard or oversize. Any help and advice would be most welcome,as you can imagine i am a bit sick by this not just the damage but the [barbecue summer is on it's way] and i would like to be out there riding, thanks in advance, Fred

Don Madden · Post by **Don Madden** » Tue May 05, 2009 4:45 pm

First, the easy question answered. Your piston is an original type as fitted at the factory. BMB was a division of Hepworth & LeGrange, (Hepolite) that made the pistons for AMC.

The twin conn rods did not have a small end bush originally & failures are rare. Decades ago there were some problems due to additives in oil crytalizing in the very close space between rod & pin causing siezure but those oils seem to have gone away long ago. If you have a bush, it was installed later to cure a problem.

The major problem with the rods is stretching of the big end eye, especially with the 650cc engines. I would do a complete strip down & inspect all components for problems. I would scrap the troublesome rod, perhaps both if the L/H one has unusual problems, too. Minor ovality of the big end can be cured by a competent engineering shop.

Do have the bottom end assembled by someone familiar with AMC twins, or enlist the aid of a marque expert if you do it yourself. These engines have their own set of pequiliarities.

Cheers, Don.

pto · Post by **pto** » Tue May 05, 2009 4:59 pm

One possible explanation is that the little end bush was a tighter fit on the gudgeon pin than in the con rod. This would cause the bush to wear away the conrod eye, if that is the case I would chance reamering out the eye and fitting an oversize bush, if however the conrod eye has been pulled out of true, I would change the conrod.
I don't claim to be an expert on this engine, my engineering experience is general rather than specific.

freddie 136 · Post by **freddie 136** » Wed May 06, 2009 8:15 pm

Thank you to both of you for the replys to my problem,Fred

Groily · Post by **Groily** » Thu May 07, 2009 7:39 am

If you want a rod checking or rectifying if it's do-able, you could do worse than ask Barry Talbot at T and L Engineering (see ad in Jampot) to have a look. They sorted one out for me last year while doing me a crank grind at the same time and may even have some already- rectified rods around. I think they did a batch for AMC Classic Spares a while back. The big end eye gets rod and cap made true and the eye made round to suit as Don says, and they can also, I'm sure, do a proper bushing job on the little end if your rod turns out not to be scrap.
Usual disclaimers - just a happy customer. Thanks to them I was able to have my Mod 20 back on the road in under a week start to finish.