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Rebuilding 2 piece cranks ?????

windsormw

Aluminum
Joined
Dec 11, 2006
Location
The corner of Hill & William
Greetings P-M ,

I was looking for a little bit of info on rebuilding small engine cranks . Snowmobiles as a matter of fact .

Seems to be a very tight lipped process & it is very costly to the consumer . I have a gang of dirt bikes are around here & have rebuilt 2 cranks thus far . So far so good :)

I am interested in building a 3 cyl. crank ( triple ) that failed on my sled this past winter . Its $700 or so to have the rebuild carried out & it needs to be shipped out of province .

I tend to wonder what those folks in Quebec are doing that is so special , I mean i have a tidy little machine shop set up in my garage to manf. what ever fixtures etc. that are req.'d .

Does anyone have a line on a book or online info ?





Scott
 
I've always wondered how it was done as well. I had a buddy that damaged his connecting rod and needed to be replaced, but because we couldn't seperate the crank and accurately reassemble it, we just picked up a used one on eBay.
 
its done with pressed in rod pins (to hold the rod bearings) and crank weights and end shalf weights, the highend guys tig the journals after truing them. i believe its done on large V-blocks and tapped around with a brass hammer, then indicated, tapped, indicated .......so on till the ends are parallel. try a doing a drag bike motor:nutter:
 
I've done a few (125 & 250 GP bikes)... the cost is mostly time it is not a quick and painless process, there are some things to be aware of like the need for near surgical cleanliness ... your basically building a high speed spindle and putting it between your legs, a failure could be painful.
Cheers Don
 
i read somewhere harley cranks can have upto .012" of runout at the ends. lol. most bikes sould be .0005 - .0015" runout.
 
If these boys had still been in business they would have given you a few tips !

Davycrocket
 

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Scott

Singles are relatively easy but it is not good to rely soley on the cranks centre drillings & just check between centres, these are often damaged or bent by flywheel extractors etc, knife edge rollers provide a way round that in most cases.

Twins these days are rarely splined or keyed between the middle flywheels so indexing requires a little thought & patience. 360 degree twins are much easier than 180's.
Once indexed you build & true a single, then add the other outer assembly & effectively true as a single with one extra long main shaft. Fairly simple for a decent machinist to make up an indexing fixture but not worthwhile unless you are going to be rebuilding that particular crank regularly.

Triples are much more difficult than either a twin or a four to both index & true, 120 degree phasing requires lots of ingenuity & patience, added to which by the time we get to triples things are starting to get quite heavy & truing requires quite some strength & stamina.
Indexing fixtures for triples are a real pain in the arse to devise & end up being very complicated.

Fours are mostly no worse than twins but physicaly very demanding, last time I rebuilt a four (Suzuki GS1000) was about a year ago & I told the customer beforehand that I was not certain that I still had the physical strength to do the job, that one required welding on the main shafts as the reason for the rebuild was that one had slipped, my forearms ached for days after & it did not do my arthritic wrist joints one jot of good.:(

Rough time guide to strip, build & true would be 1-1/2 hrs for a single, 5 hours plus for a twin & a full day for a triple or a four. That's with 30 years experience & having worn out quite a number of Thor #3 copper mallets :rolleyes5:

regards

Brian
 
i read somewhere harley cranks can have upto .012" of runout at the ends. lol. most bikes sould be .0005 - .0015" runout.

I have watched a friend true quite a few HD cranks. It is a 3 piece assembly (not counting nuts and locking washers). He said some mouth breathers just slap them together without truing them...but he snugged, tapped, snugged, tapped, tightened, tapped, torqued, tapped....final result a very low runout between centers.
 
Having done numerous built-up cranks over the years, I would say that these days, $700 is a pretty reasonable sum for a proper job on a multi-cylinder engine.

The stripping and reassembly/truing is only part of the challenge. The really tough part is getting new crank pins, rollers, and sizing the con rod big ends to have the proper running clearance. The rods also need to be checked for bend, and with a two stroke, the crankcase seals must be replaced.

The last crank I did a month ago was from a Matchless G80. The NOS big end bearing assembly required lapping both the big-end eye of the con rod to get the correct interference fit for the outer race, and then the ID of that race lapped to size it for 0.001" bearing clearance ... a very slow process.

.. Gregg
 
i read somewhere harley cranks can have upto .012" of runout at the ends. lol. most bikes sould be .0005 - .0015" runout.

Lol, my friend who is an excellent machinist rebuilt the top end on his 60's era (Pan? Knuckle? anyway, Harley chopper) I think he bored the valves ports out so the stems and holes matched and slid perfectly. It turned out you actually need to leave some slop in a harley as otherwise the valves hit each other when he was out on the road somewhere. It seems to be a whole world of slop in there!
R
 
Is there any books out there for an aspiring crank builder Greg ?

Cheers

Scott

Hi Scott,
Alpha Bearings used to publish a book about rebuilding a built up crank, but I just checked their web site, and it no longer seems to be available. Alpha, of course is the British company that manufactured replacement big end assemblies for engines since the 1920's.

Some of the older publications for enthusiasts (like Nicholson's "Modern Motorcycle Mechanics") did a 30,000' high treatment of the topic, but not detailed enough that you would want to bank on it. And, despite the widely held belief that you can do the job with a workshop vise, you really need a substantial (and accurate) hydraulic press capable of at least 10 tons.

It's one of those things that you can read about all you want, but it takes great care and practice to get it right ... especially during the truing operation where you have to find and correct the cause of the run-out.

.. Gregg
 
Gregg

I agree with your comments. It was probably 25 years ago since I dealt with Alpha but I will always remember my first visit & "factory tour" particularly in one area there was a very old & very substantial oak desk, glancing at it the very thick & heavy lid appeared to be bowed but on closer inspection the lid was in fact worn down towards the centre by at least half an inch:crazy:.....that desk could tell some stories I bet.

A "substantial & accurate hydraulic press" is indeed essential & not I would suggest one of those bolted up flat bar things, I would suggest that 20 ton hydraulics in a much higher rated frame is required to accomodate most crank work, a later Ducati bevel twin with 38mm crank pin can be well over 30 tons to get apart.

Over the years I have taught several employees to build cranks some of whom just never got the nack & required continuing guidance on anything more tricky than a simple 2 stroke single, from memory only one bloke ever mastered the more complicated cranks.
Perhaps the others had more sense than to subject their wrists to such abuse:)

regards

Brian
 








 
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