Board index Flathead Power-Technical Questions, Answers, and Suggestions Knuckles Knuckle Cases: repairing cracks

Knuckle Cases: repairing cracks

Post Wed Dec 01, 1999 5:29 am

Posts: 27
Location: Minneapolis,MN USA
Hello--I'm new to this site but it is sure nice to see fellow old school bike enthusiasts keeping the old iron running! I have seen some posts refering to S&S making Knuck cases which is news to me--is this true? I'll have to check that out.

On the same note, I have a '37 Knuck with stress cracks on the left side, right below where the cylinder bolts on. Is this a common weakness? I was wondering if a competant welder/machinist could repair such cracks so the cases could be usable with a stock motor re-build--the motor may have to be bored, but that would be it as far as mods. These thin cracks were fixed for a long time with "JB Weld", but I want a more permanent fix, especially since they are the original cases.

Any help/suggestions would be great--thanks in advance.


Post Wed Dec 01, 1999 4:07 pm

Posts: 582
Location: meridian, id. usa
My 47 cases had a crack going from the timing plug hole to the area around the case race and it was repaired using aluminum torch welding. it was filled on both sides inside and out) and then ground down to the level of the surrounding metal. Its visible but doesnt look bad and is probabily stronger then the surrounding metal.
I also have a cam cover that was torch welded that i think was done by someone with one of those MAP aluminum welders and it has holes and pits all through the area of the weld.
it isnt in a structural area so it only looks like crap but it needs to be replaced or redone.
Any way a good welder who has lots of alum experiance should be able to do the deed and leave you with a good part when done. if the crack is through and through across the cylinder surface make sure that you have them milled flat as a pair bolted together.
dont use these cases as hot rod material after unless you desire picking pieces out of your calfes, butt or whereever!!!

[This message has been edited by dotman (edited 01 December 1999).]

Post Wed Dec 01, 1999 6:24 pm

Posts: 607
Location: Menomonie, Wisconsin, USA
JW I haven't seen a case yet that couldn't be fixed as long as all the pieces are there. Just make sure that whoever welds it knows how to weld silica/oil contaminated cast aluminum and only allow them to tig weld it. NEVER let them use arc, wire feed or oxi-acet

Post Thu Dec 02, 1999 12:45 am

Posts: 3158
Location: Central Illinois, USA
How common? I have two '37 ELs and sure enough, one developed a hairline on the left case halfway between the cylinder studs. Lucky I glyptaled the cases, because I'm not ready to take her apart. (Statistically, I'd guess half of 'em are broke) Jerry's right: anything can be fixed, but be certain also that your welder can heat-dam to prevent distortion.

Post Thu Dec 02, 1999 2:37 am

If you haven't already, don't clean them in a parts washer! Must use completely volatile solvent, such as acetone, alcohol, lacquer thinner, toluene, MEK, etc. (do this outside, wear mask and gloves, do not inhale, no contact with skin, do not discard used fluid in drain water). Petro-based solvents will continue to leach out of metal when heated and contaminate weld.

Post Thu Dec 02, 1999 3:03 am

Posts: 582
Location: meridian, id. usa
Dont use alcohol. MEK or one of the others is ok but solvents are spendy per gallon,toxic,need special handling. and do not play well with the fishy's. Hot(as in Hot water heater hot) caustic soap works best like a citric acid base industrial soap. The citric acid will leach the oils right out, Just dont leave alum. in it for long periods of tyme since the citric acid will munch the aluminum. and make sure you rinse parts well in clear water after also DO NOT put bare hands in citric acid based soaps as it will peel the hide right off ya ! and you can dispose of the waste water (NOT THE SLUDGE) down the drain as it is bio-degradeable EPA approved. Works great in clean tanks and ultrasonic tanks even better. and you dont have to suit up like a dork from gork to use it.
The voice of experiance speaks here

Post Wed Dec 15, 1999 11:54 pm

Posts: 540
Location: Kansas City, Mo USofA
Please take a minuite and look at welding 101 on the flathead page. this was my ECACT question during the aluminum seminar I refer to. Having had this same problem with a 40 U I asked and was shown the answer.

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