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Author Topic: Rodeck 481 T main cap  (Read 6951 times)
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jdincau
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« on: June 02, 2012, 07:35:12 PM »

     I was unable to find any anywhere so I am making a replacement. We only need one but I learned in my apprenticeship that as long as you are making the setup you might as well make two. With no blueprint I am having to copy from a good one (relatively good). The precision work is done ( steps and dowel pin holes ) now all I have to do is bore the bearing tunnel undersize, reduce all that expensive excess metal on the top side to chips and drill the angled stud holes.


* caps in a row.jpg (189.59 KB, 2848x2144 - viewed 496 times.)
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RichFox
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« Reply #1 on: June 02, 2012, 08:03:01 PM »

That's not suspossed to hapen. shocked
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jdincau
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« Reply #2 on: June 02, 2012, 08:28:40 PM »

     I know Rich but it was new in 1988 and we have abused it mercilessly.
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DND
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« Reply #3 on: June 03, 2012, 05:59:14 AM »

Did you break a crank and that is what broke the main cap?

You are right about the set up and making spares.

Don
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jdincau
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« Reply #4 on: June 03, 2012, 08:52:47 AM »

Crank is fine, the bearing from the broken cap looks used but not abused.
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kiwi belly tank
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« Reply #5 on: June 03, 2012, 04:16:31 PM »

That's some goofy crap with there being no other real damage, looks like the center thrust took a pounding. I would be building a full set for it if it was mine. What's the original material, 7075-T6?
Amazing how that did not spin a bearing.
What are you building the new ones out of?
Had you considered going to steel?
Have you dialed the crank??
  Sid.
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interested bystander
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« Reply #6 on: June 03, 2012, 06:28:53 PM »

You do know about Evans  (Gen Ohly and rite hand man Jaime) in El Monte, do you not?

They may or could be helpful - they were  good enough for George Bentley - and that Elwin kid!
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« Reply #7 on: June 03, 2012, 06:29:00 PM »

Can't tell for sure but from what I can see of the fracture line, I would suspect a fatigue failure starting at the outside of the cap from over stress perhaps from a small imperfection in that outer surface.

It looks like it started from the bolt hole ??

If that is the case, add a bit of material on that dimension (make cap slightly deeper) and make sure you de-burr and smooth any nicks or sharp edges.
Since aluminum is subject to fatigue failure from repeated stress, I would also suggest going to steel if those original ones were aluminum if you can.

Larry
« Last Edit: June 03, 2012, 06:31:25 PM by hotrod » Logged

jdincau
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« Reply #8 on: June 03, 2012, 07:02:04 PM »

Kiwi
     It's a BB Chevy replica, the thrust is on the rear main. The damage you see is from a main bearing  pushing out due to an oil pump seizure at 7,000 RPM back in the 90's.
     The material is 7075 T651 stretch leveled plate with the grain running thwart ships.
     Crank has been to grinders and pronounced straight and crack free.

IBS
     I know Evans well, back in the 80's Gene repaired several  iron blocks for us. Making and building this kind of stuff is why I do this.

hotrod
     It looks like the crack may have started at a divot left by a broken rod in the past or from the spot face for the nut on the stud, it passes thru both. I think it was the spot face as it looks like the radus isn't smooth. As I told Rich Fox we have abused this piece since 1988. We canít give up on it though it is a one off that John Rodeck made for us. The sleeves have his standard dry setup OD at the top and KB wet sleeve OD at the bottom. We had sleeves made with with O ring grooves at both ends and a 1/4 inch cut back in the ring travel area so we could run water in the block. We put the water in between the cylinders in the side of the block and I drilled angled holes in the deck to mate with the standard BB Chevy head water transfer location intersecting the area behind the sleeve at each cylinder.
     In 1976 it was easy to run a .030 over 366 truck block with a 427 forged crank to make the 372 inch limit for C motors. We had to do so much work to those blocks to maintain cylinder sealing we went to the Rodeck  so we could run a 10 bolt layout and 9/16 studs. We have been running Arias Hemi-Chevy heads since 76.
« Last Edit: June 03, 2012, 08:12:35 PM by jdincau » Logged

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SteveM
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« Reply #9 on: June 03, 2012, 08:53:21 PM »

I see a shiny spot in the photo, on the right hand part of the broken cap, near the outside edge.  The fatigue striations seem to point to this spot as being the origin of a fatigue failure.  It's tough to say from just a photo like the one shown, but with fatigue fractures, you should be able to trace the striations (ridges) back to the point where they began.  The striations are like a road map of a fatigue failure.  There is also a shear lip on the surface of the cap, immediately adjacent to where the bearing rests.  That is the last part of the cap that fractured.

I'm a Metallurgical Engineer by day.  My only recommendation would be to make sure that all machining marks, sharp corners, steps, etc. are smoothed out before putting the new main caps in service.

Steve M.
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kiwi belly tank
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« Reply #10 on: June 03, 2012, 09:11:40 PM »

Bystander,  George & that Elwyn kid, thats funny $hit.  cheers Reminds me I'm over due for a chat with "that kid" & his girl.
  Sid.
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jdincau
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« Reply #11 on: June 04, 2012, 07:00:01 PM »

They are done, two days work total.


* maincapsdone.jpg (195.1 KB, 2848x2144 - viewed 271 times.)
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Jack Gifford
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« Reply #12 on: June 05, 2012, 12:16:25 AM »

Done? I don't see any threads in the slide-hammer hole (may be there and not visible?). When I built some 7075 caps for my hemi I suffered a delay of needing to EDM a broken tap out of that hole. I just HATE tapping that alloy!
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fastman614
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« Reply #13 on: June 05, 2012, 01:16:24 AM »

Bystander,  George & that Elwyn kid, thats funny $hit.  cheers Reminds me I'm over due for a chat with "that kid" & his girl.
  Sid.

Yeah... Several of us, as well!
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« Reply #14 on: June 05, 2012, 06:34:37 AM »

I would put at least a 1/16 to 1/8 chamfer on the outer edges. Aluminum, unlike steel, will crack eventually; it has no endurance limit.7075    translation - at a given number of cycles it will crack, not maybe. Predicting the number of cycles is near impossible but i would guess if you could add up every revolution the crank has made since 1988 you would have a good starting point. For that reason it might be a good idea to change all caps.

T-651 is especially prone to cracking. Chamfering the edge can delay the inevitable and greatly extend the working life by reducing stress risers.

Fred
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