Author Topic: Big end shells and main bearing - sizing them up  (Read 1772 times)

Offline owain

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I'm about to order som new big end shells and main bearing for the 1950 A10 plunger. My question is how do I know which size to get? Looking at draganflys page there seems to be std, +0.010, +0.020, +0.030, etc. It seems a bit of a risk to just assume that I need to replace everything with the std size. Do I need to measure the diameter of the crank and small end? If so, then does anyone know what the standard diameters are?
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Offline JulianS

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Re: Big end shells and main bearing - sizing them up
« Reply #1 on: 19.03. 2017 14:59 »
You need to establish the condition of the journals as well as the size. If the journals are worn or damaged you will need to get the crank reground to an appropriate under size.

The big end shells are normally marked with a part number or undersize which you will see when shells removed from the rod.

The standard big end journal measurement for the small journal crankshaft is 1.4595-1.4600 inches. If more than 0.002 inch worn or is oval or damaged then it will need a regrind.

The standard main bearing journal measurement is 1.375 - 1.3745 inches.

The attached photo from the BSA service sheets shows regrind sizes, small journal crank to the left.

Not shown is the main bearing jurnal -0.020 measurement which should be 1.3540-1.3534 inches .

If fitting a new main bearing bush then it may need to be line bored or line reamed or honed to size and to ensure alignment.

If in doubt as to size or condition best to consult an engineer familar with old bikes.

Offline coater87

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Re: Big end shells and main bearing - sizing them up
« Reply #2 on: 19.03. 2017 15:10 »
 The crankshaft journal (where the big end shells ride) wears out over time. It gradually gets smallers until the shells are too sloppy to work correctly. You lose oil pressure, you may get knocking noses, and eventually the motor pukes its guts all over the road.

 You must use a micrometer to measure these journals. There is a wear limit to these.

 Once you have reached the wear limit, you take the crankshaft to a machine shop and they will "grind" your crankshaft to the next undersize. From "standard" (factory new) size, your first undersize would be +10 (10 thousandths thicker shell than standard), then when thats worn out, it gets ground to +20, and so on until +40 is reached. After that you have a few options to restore the crankshaft but no larger shells are available.

 So the very first thing that needs to be done is get the journals accurately measured, you may be able to just fit a new set of the appropriately sized shells. But you may need a crank regrind....

 Also, dont neglect to the clean the sludge trap.

 Lee
Central Wisconsin in the U.S.

Online RichardL

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Re: Big end shells and main bearing - sizing them up
« Reply #3 on: 19.03. 2017 15:51 »
If buying a micrometer to take measurements yourself (a caliper isn't accurate enough), be sure to get one that reads in 1/10,000 of an inch. There are some cheapo versions that only read in 1/1000ths.

I have previously seen the sheet posted by Julian and find it simultaneously useful and sorely flawed. It gives large and small-journal sizes in text while providing small-journal dimensions (including end radius) in the drawing. It refers to "B" with no "B" in the drawing. It shows how the web should be stamped without considering stamps from previous grinds. It fails to note the original dimensions. It implies that the main journal must be ground in 0.010" increments while the accepted method is to grind to clean-up, then ream/hone the main bearing to fit (thus preserving meat on the journal).

All this said, I, too, am flawed, deeply flawed.

Richard L.
Plan on signing up for the world-wide 2020 DISTINGUISHED GENTLEMAN'S RIDEon September 27, 2020 (if it's not cancelled and we are free to move about by then). Watch website at https://www.gentlemansride.com for details.

Offline owain

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Re: Big end shells and main bearing - sizing them up
« Reply #4 on: 19.03. 2017 17:28 »
Haha, it's never as simple as I had hoped. It might be worth mentioning that I had a bit of lateral play with the conrods attached. I assumed that perhaps just a replacement shell would do  *conf*. The number I have on the big end shells is W1454+. It doesn't appear to be a parts number so I'm a bit confused as to what it means.
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Offline coater87

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Re: Big end shells and main bearing - sizing them up
« Reply #5 on: 19.03. 2017 21:19 »
Haha, it's never as simple as I had hoped. It might be worth mentioning that I had a bit of lateral play with the conrods attached. I assumed that perhaps just a replacement shell would do  *conf*. The number I have on the big end shells is W1454+. It doesn't appear to be a parts number so I'm a bit confused as to what it means.

 It honestly does not matter what the shells say, and they are not usually stamped with a big "+20" or anything. It all comes down to a very carefully measured crank pin.

 That is the only way you are going to know what you have, and what you need to do about it.

 Lee
Central Wisconsin in the U.S.

Online KiwiGF

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Re: Big end shells and main bearing - sizing them up
« Reply #6 on: 19.03. 2017 21:49 »
Most important measurement with big end journals, assuming scoring is not present, is the amount of journal "ovality" as generally the journals only wear on the top/bottom not the sides. Opinions will vary but 002 ovality means it's probably time for a re grind.

You can measure ovality without needing to know the correct journal size. A good machinest will grind the journals to fit new shells anyway, and not go strictly by the "book" size. You may get asked what clearance you want, I went for slightly over a thou.
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Offline duTch

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Re: Big end shells and main bearing - sizing them up
« Reply #7 on: 19.03. 2017 23:49 »

 When I stopped mine down a while back and measured the crank pins with digital caliper and micrometer, it measured fine, but when I took it to an engineering shop the guy measured it with fancy ' three point ' (?) micrometer, he found it Oval in a triangular way- kinda like a Rotary engine rotor.
 I thought hewas full of $hite but did some searching, and either he said, or I found reference to it being not uncommon in airplane engines (citation required), so maybe worth researching that and have a machine shop check it (unless you can so it yourself).
                       *dunno*


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Offline muskrat

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Re: Big end shells and main bearing - sizing them up
« Reply #8 on: 20.03. 2017 09:10 »
As Richard said it's easy with a 1/10th of a thou mic. Any machine shop will be able.
On my Rhonda the difference between one size and the next is 1/3 thou" and you know if you get it wrong!
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Offline owain

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Re: Big end shells and main bearing - sizing them up
« Reply #9 on: 23.03. 2017 20:47 »
Righto, I've taken your advice and the crank is being grinded down to fit 0.010" oversize big journal shells and the main bearing is being enlarged to 0.020". The machinist also said that it's standard to buy new big end bolts for the con-rods...at £20 each and with the current bolts looking pretty solid (+ I'm bait skeptical about the quality of modern bolts compared to the older ones). I'm wondering whether it'd just be an unnecessary expense. Thoughts?
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Online chaterlea25

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Re: Big end shells and main bearing - sizing them up
« Reply #10 on: 23.03. 2017 21:33 »
Hi Owain,
There's really no way of knowing if the old bolts have exceeded their elastic limit ?
At present there is only one reputable manufacturer of big end bolts, ARP bolts made in USA
Good news is that the crank is only on its first grind,
MAKE SURE the machinist knows the correct radius information for the journal grinding  *warn*
The normal method for the main journal is to grind until its true, Theres no point in grinding to a "figure"
The new main bush MUST be line bored to the drive side main bearing
There are lots of previous threads on this subject

John

1961 Super Rocket
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Re: Big end shells and main bearing - sizing them up
« Reply #11 on: 24.03. 2017 08:10 »
"the main bearing is being enlarged to 0.020".

To empathise what chaterlea said, the accepted way of grinding the main bring is to grind it unto it until it is round, then measure it (it might be say -014 undersize) and make a special journal to suit, a standard undersized journal of the next size could be purchased and bored to suit (so for a journal of -014, you could bore a -020 journal).

It is common for the Timing side journal housing in the crankcase to be worn oval, in which case it should be bored out slightly to a larger diameter, in which case a standard journal cannot be used, and a special one will need to be made, to suit the case I.d. , and crank o.d.

The final line boring of the journal to size, to acheive the correct clearance is a specialist job, but an experienced machinest should be able to work out what to do, as said in other posts this topic is covered very well on other threads on this forum.

Depending in funds it may not cost much more to get new billet con rods, so you might want to look into that, by the time old rods have been crack tested, shot peened, sized, new small end bushed fitted etc it may come to more than 50% of the cost of new rods.





New Zealand

1956 A10 Golden Flash (1st finished project)

1949 B31 rigid “400cc hot rod” (2nd finished project, + favourite bike)

1949 C11 rigid, but why!!! (cos it was cheap)

1937 B21, project missing parts, mission impossible?

GL1800 Goldwing, well, the wife likes it

KTM 950 ADV, cos it’s 100% nuts

1952 Armstrong Siddeley Whitley for rainy days (with wife

Online mikeb

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Re: Big end shells and main bearing - sizing them up
« Reply #12 on: 24.03. 2017 10:19 »
Quote
MAKE SURE the machinist knows the correct radius information for the journal grinding
and to further emphaise what chaterlea said - the radius is the curve at the edge of the journal. that is to say if it is ground to 90 degrees you have no radius (as its square). there are lots of posts showing how crankshafts weaken and break when the journal is ground out to 90 degrees  eg http://www.a7a10.net/forum/index.php?action=post;quote=76053;topic=10273.0;last_msg=76244 and it likely was the cause of my a10 cracking a crank.

the problem is some machinists tend to grind them out to save wear on their equipment. this is very important and unless the guy 'gets' old bike don't assume he knows... tell him quick!
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