Crank Rebuild Problems

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Crank Rebuild Problems

Postby Farky » Thu Jul 11, 2019 9:56 pm

I’m continuing with my 53 model 18 rebuild and have some more questions if someone can help. It’s a bit long winded I’m afraid.
I stripped it due to lots of bad noises after a run, at first I thought piston slap and sure enough about 0.5mm. However then took the barrel off and found I also had about 0.5mm lift on the conrod so the bigend was goosed too.
Split the crank and found the pin, rollers and conrod bearing surface all in a bad way.
Bit the bullet and ordered a new crankpin from Jampot spares. Then took the crank, new crank pin and conrod to an engineering company I used 35 years ago when rebuilding bike engines. Anyway after a discussion the old guy said he’d done loads over the years (I was also getting a rebore at the same time) and they would call when it was done. (I made a mistake by not taking the new crankpin out of it’s plastic covering to check it first so I don’t know if I started with problems or if the problems were caused during the building up of the crank – I suspect the latter).
A couple of weeks later I went to collect it. He said the conrod seemed to get tight after it was turned a few times but it shouldn’t be a problem.
When I got home I had a closer look and feel. It was as if the big end was wandering across the pin (Which could only be a wander of a couple of thou) then got tight enough that there was no way the conrod would rotate down under it’s own weight. I also noticed that the Drive side nut on the crankpin still had about a thread width to go
NewCrankPinDriveSideText.jpg

First I put it in a lathe and turned the crank while holding the conrod straight with a bunggie – thinking this may free it off, ran it at about 100 rpm for about a total of 10minutes stopping to feed in more oil every so often via a syringe through the timing side oil hole. But still the bigend would tighten up every four or five turns.
So next I split the crank again. I couldn’t really find anything except the rollers and conrod were very tight to get off. Next I looked more at the crank pin length. It measured the same as the original but the shoulder for the timing side was smaller
ShoulderSizeText.jpg

So how could this be?
Next I found a misalignment of the oil holes in the new crank pin axially, you’ll see in the next picture with a pin dropped through at the edge of the hole, so misaligned axially by about 1mm. Additionally the pin wouldn’t drop through the other pair of holes (There are 4 holes in 2 pairs 90 degrees apart). So I presume that the outer is rotated 90 degrees from where it should be as well.
OilHoleAlignment.jpg

Once I found this I tried the pin in the old crankpin and found it wouldn’t go through any of the 4 oil holes – so this probably explained the original trouble with the big end.
Next when checking the new bearing surface in the big end of the conrod I thought the conrod was bent and sure enough checking on a flat surface it definitely was.

So now the questions:-
Has anyone had problems with 2 part crankpins where the outer part has moved with respect to the inner shaft?
Can anyone recommend someone that has the tools and know-how that could rebuild the crank correctly pushing out the bearing race from the conrod bigend and into a new (Second hand) conrod I have, along with any machining that may be required to get the big end moving freely and then balance the whole lot with the new Gandini 40 thou oversize piston I have which is heavier than the original wirewound?
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Re: Crank Rebuild Problems

Postby Pharisee » Fri Jul 12, 2019 7:38 am

I had a crankshaft rebuilt by T & L Engineering last year. The new crankpin assembly came from the club spares and like you, I handed it over without taking the protective plastic coating off. I was very happy with rebuilt crank as the con-rod was free to rotate without any noticable tight spots so I assembled the engine without putting it under any further sctutiny. The reassembled bike has covered around 1500 miles since then without any problems. T & L certainly have the tools and know-how to rebuild a crankshaft but whether they actually check oil hole alignment... only they know!

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Re: Crank Rebuild Problems

Postby Dave T » Fri Jul 12, 2019 9:13 am

That looks like the fitter has tightened one nut completely then the other and not each nut equally a small amount in turn until tight. He has thus pulled the centre pin to one side. You should be able to redress the problem by tightening likewise on the short side. I am sure a good engineers (like T&L) will do that no problem, also balance the crank which I would recommend you do for a modern piston most emphatically.
Have fun with it. Dave.

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Re: Crank Rebuild Problems

Postby Mick D » Fri Jul 12, 2019 9:35 am

Dave T wrote:That looks like the fitter has tightened one nut completely then the other and not each nut equally a small amount in turn until tight. He has thus pulled the centre pin to one side.


That get's my vote

Regards Mick

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Re: Crank Rebuild Problems

Postby SPRIDDLER » Fri Jul 12, 2019 10:40 am

Yep, that sounds highly likely.

Edit.
Voila! Have just found this reference on page 49 of F W Neill workshop manual....... http://archives.jampot.dk/book/Workshop ... dition.pdf

Crank pin nuts (568x229).jpg


Since I'm housebound at my son's this morning with now't better to do ( :roll: ) here's pic of the big end ass'y. Note that the oil hole in the inner pin must register with the feed hole in the timing side flywheel (but you probably know that):
Big end assembly (508x514) (2).jpg
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Re: Crank Rebuild Problems

Postby Farky » Sat Jul 13, 2019 10:59 am

Thanks for all the answers on this.
Certainly that explanation of how the crankpin must have been tightened up all makes sense now.
Does anyone know if all 4 oil holes should be open to the central oilway or is it the case that only 2 will line up?
I've just talked to Barry at T&L Engineering and he's filled me with hope, he knew straight away what was probably done and what wasn't. So I'm packing it all up and sending it to them.
I just hope I don't have to buy another crank pin assembly!

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Re: Crank Rebuild Problems

Postby Mick D » Sat Jul 13, 2019 11:30 am

Hi

This photo would suggest none of the holes in the sleeve 'need' to align with the drillings in the pin:

http://www.jampot.com/pic_display.asp?id=509

Regards Mick

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Re: Crank Rebuild Problems

Postby Farky » Sat Jul 13, 2019 12:54 pm

Thanks for that picture. It explains a lot as well.

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Re: Crank Rebuild Problems

Postby Allen_F » Tue Jul 16, 2019 1:18 am

everything I have read for 57 model anyway suggests pressing flywheels together not pulling them together with the nuts and then carefully tighten the nuts evenly to avoid pulling the pin through .
This picture shows if the pin is pulled through too far the holes in the pin wouldn't line up with oil channels inside the outer component.

Also if a new OUTER" bearing race on the conrod is fitted it needs to be ground to size or it will bind . use the old one if it's ok.
each ROLLER bearing is supposed to be measured individually.
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Last edited by Allen_F on Wed Jul 17, 2019 12:36 am, edited 2 times in total.

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Re: Crank Rebuild Problems

Postby 56G80S » Tue Jul 16, 2019 7:17 pm

Great thread. Told me a lot about this part of the engine. I've only bui;lt up the crank once when I was 19 and really only watching and expert (Frank Reynolds in Middlesbrough) do it.

Cheers

Johnny B

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