crankshaft bondo?

Jason Luther /

the needle bearing on my crankshaft dug themselves a nice ditch in the crank. im thinking finding a replacement might be tough. can anyone think of anything to fill in the ditch? i know its pretty much hopeless, but i thought id check

Re: crankshaft bondo?

Steamboat Aka J. R. Stevens /

A good machinist could turn the crank and build it up. This could be expensive or maybe he could turn it and install larger bearings. Jim.

Re: crankshaft bondo?

The only thing that would work would be to take the crank to a machine shop and have it flame sprayed and ground back to standard. This would be expensive. It would be cheaper to buy a new crank or a used engine on ebay. Bondo or JB Weld will not work so do not even try.

Sorry for the bad news.

Re: crankshaft bondo?

Jason Luther /

thanks guys. i was just hopeing there is something i have never heard of.

Re: crankshaft bondo?

which bike was it? not the cady i hope!

Re: crankshaft bondo?

Jason Luther /

yeah it is. but its something you couldnt have known about, until the engine starts (which it did after the correct plug was put in).

Re: crankshaft bondo?

You basically have two choices.Buy a new or good used crank,this would be the cheapest way to go if you can find one.The other choice is to have a machine shop weld it up and regrind it.This will be expensive,but is your only other option if you cannot find another crank.There is NO other way to repair a crank.This kind of repair is quite common on antique cars and motorcycles where parts are no longer available.A crank repaired this way should last as long as a new crank.

Re: crankshaft bondo?

Jason Luther /

i have brazed it with a nickel-silver brazing rod. so today i'll shape her up and see.....

Re: crankshaft bondo?

Steamboat Aka J. R. Stevens /

I think those steel needle bearings will chew through that brazing like it was never there. Jim.

Re: crankshaft bondo?

Jason Luther /

indeed they did. _sigh_

Re: crankshaft bondo?

I think having it welded and reground will be your only choice outside of buying a new or used one. Maybe one will pop up on ebay.

Grey

Live to ride, Ride to work

Re: crankshaft bondo?

I do not know if it would work but you might be able to replace the needle bearing with a solid bronze bearing. I think alot of mopeds have solid bronze bearings.

Re: crankshaft bondo?

Jason Luther /

yeah, i was thinking about that too. the small end on a lot of peds just have a bushing. anyone think it would be okay on the big end?

Re: crankshaft bondo?

Plain journal bearings are superior to roller bearings as far as resistance to load stresses.. Rollers are superior as far as low-friction.

Plain bearing may work .. i don't think you'll get away with some common bronze bushing.. precision and the correct alloy is needed.. All crank/bearing surfaces must be polished smooth but a few pits or grooves shouldn't matter.

Perhaps an oil channel will need to be somehow cut so enough oil gets in there.. Perhaps drill a hole through the con-rod big end.. and chamfer the openings of the drilled hole. Also some sort of shallow feed channel on the inner bearing surface.

i dunno all the fine details (clearances??) between designing a crank that runs rollers as opposed to plain bearings but it's worth a shot, imo.

Re: crankshaft bondo?

Jason Luther /

i really dont think the tollerances were all that tight, since the con-rod is only held on to _the side of the flywheel_ with a snap-ring. its really a chincy set up. unlike most cranks that have flywheels on either side of the con-rod, there is only one main crank bearing, and one flywheel half. you can take the con-rod off by just removing the side cover of the engine, still on the bike, if you wanted to.

Re: crankshaft bondo?

i was wondering if or how you managed to disassemble the crank.. but that explains it.

i'm clueless about that engine.. can't even picture it's design from the description, but although chincy might apply it sounds easy enough to play with.

I'd try the plain bearing .. even a common bronze bushing. My guess is such a bushing would be press-fit into the connecting rod and have about a 0.001 clearance on the journal. I'd still be thinking about provisions for adequate oil feed..

Re: crankshaft bondo?

Jason Luther /

yeah definatly. most small ends i have seen have oil holes and narrow grooves like you described for lubrication. the crank design definatly makes it easy to work on, but i suspect its also the reason it failed. have you any idea where to look for this type of bushing/bearing? or, in fact, for any part of this nature?

Re: crankshaft bondo?

A good hardware store with lots of nuts, bolts, cotter pins, woodruff keys, etc.. like an industrial or agricultural oriented hardware store should have a few bronze bushings.

But the bushings i've come across are SAE (not metric) and the chance of finding one that fits are like zero. So I'd expect to bore it out, turn down the outer diameter and trim it's length on a lathe (which is a simple thing if you have a lathe :) Common bushings are like 2 inches long with outer diameters of 3/8, 1/2, 5/8, etc. up to 1 inch or larger, with a wall thickness of 1/8 inch on the larger ones.

It probably wouldn't hurt to try a small, good auto-parts store or small engine, outdoor equipment repair shop.. a place where the guy knows something and has the time to search out a suitable bushing.

Re: crankshaft bondo?

John Joedicke /

Go to a bearing supply house. and ask for oil lite bushings, Have your sizes with you. You can get metric sizes.

Re: crankshaft bondo?

Steamboat Aka J. R. Stevens /

Clevite? Engine Bearings

Since the development of the first precision insert-type bearing in 1929, Clevite 77 engine bearings have set the pace for engineering quality. Clevite 77 engine bearings are built above industry standards for strength and durability ?even exceeding the highest standards set by Heavy Duty engine manufacturers. The majority of Clevite 77 engine bearings utilize a superior TriMetal? design first developed by Clevite 50 years ago and perfected over time. This design incorporates the strength of a copper-lead alloy layer on a precision steel back and finally, a precision electroplated white metal "babbitt" surface layer. TriMetal is an ideal design, producing a degree of conformability, embedability, slipperiness and fatigue resistance, not found in many original equipment and competitive bi-metal bearings. -- This type might work. Jim

Re: crankshaft bondo?

Jason Luther /

thanks guys

Re: crankshaft bondo?

Bronze is used for bearings/bushings because it is somewhat porus,oil will actually soak into it.I seriously doubt that you could replace a roller bearing with a plain bearing in an engine designed for a roller bearing,and if you did,it probably wouldn't last very long.An engine is a precision made assembly with very exacting tolerances.

Re: crankshaft bondo?

If you do decide to try a plain bearing,you're going to have to figure out the proper clearance between the bearing and the crank journel.If its to tight,it will not be lubricated properly,and will seize up when it gets hot and expands.if it's to loose,it will knock,and hammer itself to pieces.Good luck with it.

Re: crankshaft bondo?

Jason Luther /

well, how much clearance do you suspect there should be? just a rough guess...

Re: crankshaft bondo?

to get some idea, look at a connecting rod small end clearance.. from a '83 PA50 manual:

New Factory piston pin OD tolerance is from 9.994 to 10.000 mm. (So, the roller bearing is something less than 10mm ID)

Service limits:

Pin OD minimum = 9.97mm (0.3925")

So, service limits on the little end pin allow a difference of a bit more than 0.03mm ( 0.3937 - 0.3925 = 0.0012" of slop)

If you cut a bearing to fit with 0.001" clearance it would be superior to little end factory service limits. The big-end is a larger diameter so limits are greater.

::

Generally, 0.001" per inch of journal diameter is the clearance goal on plain bearings on big engines with oil pumps. MAny replacement journal bearing shells are sized in 0.001 increments.

Anyway, I dont think you're gonna be able to size a bushing ID more accurately than about 0.0005 (half a thou) .. I'd shoot for 0.001" clearance or slightly less and see what happens.

Re: crankshaft bondo?

The rule of thumb in industry is .001" per inch of journal diameter. The bushing should be about a +.003" shrink/pres fit into the rod.

The rod should have a large oil hole drilled thru the top surface to let the oil into the bearing. The bearing should have X shaped grooves cut into it from the hole to around the ID to distribute the oil.

These type of bushings worked on OLD engines that were low rpm and horepower. They were also mostly used on splash lubed crankcases but some were used on two strokes.

It is worth a try but go easy when you get it running. If the clearances are too tight open them up only .0005" at a time.

Good Luck

oops ..more than less than.

meant to say:

New Factory piston pin OD tolerance is from 9.994 to 10.000 mm. (So, the roller bearing ID is something more than 10mm ID)

(a 10mm pin won't fit into a 10mm hole so the roller bearing ID is something more than 10mm.)

Re: crankshaft bondo?

Jason Luther /

great info guys. at this point i have nothing to loose. except money. and time. and headache. but its a moped, those are the three main ingredients

Re: crankshaft bondo?

Jason Luther /

wait a tic. how do you shrink it .oo3" put it in the freezer, and heat up the mate?

Re: crankshaft bondo?

Heat and cooling and pressing it in with an arbor press may do it .. but my guess is that after the bushing is installed it should then be bored to size. The reason is the inner surface of the bushing might no longer be perfectly round due to the stresses. This boring operation couldn't be done on a lathe.

The objective is to get the bushing's fit so tight it cannot rotate inside the rod. On auto crank rod journals the (2-piece) bushings are peened or pinned or have little ears sticking out so they don't spin...

if i was you i'd machine the bushing about 0.002 (or less) oversize and do the freezer - heat thing and try to press the bushing in without deforming it. If that won't work, go to Plan B.

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