It is currently Sun Aug 20, 2017 7:49 am

All times are UTC - 5 hours [ DST ]


Forum rules


Please click here to view the forum rules



Post new topic Reply to topic  [ 21 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: 912 3rd piece cracks
PostPosted: Sat Aug 12, 2017 3:09 pm 
356 Fan
User avatar

Joined: Mon Mar 31, 2008 11:43 pm
Posts: 183
Location: Seminole, Florida
I would like to get some opinions on an issue I just discovered. I'm getting ready to put together a replacement engine. I have decided to use a 912 case I had laying around. Looking at the 3rd piece I noticed moderately sized cracks in the two lower bosses where the 912 rear motor mount mounts. However, this engine will be used in a 356, so there will be no mount there. In my uneducated opinion, I can see no reason why these cracks would cause a problem in my application. The third piece has numerous bolts holding it in place. And there is no mount to support any longer. The only stress on these damaged areas will be just pressure of the two (of many nuts) holding it on the 3rd piece on. Can anybody think of a reason why this is not a usable 3rd piece. (FYI, if it matters, it appears to be a replacement part with no number and a R after the stamping inside.)

Glen
64Cx2


Top
 Profile  
Reply with quote  
 Post subject: Re: 912 3rd piece cracks
PostPosted: Sat Aug 12, 2017 3:25 pm 
356 Fan

Joined: Wed Nov 23, 2011 10:52 am
Posts: 2611
Location: Cleveland, Ohio
Could we see a close-up picture?

_________________


Top
 Profile  
Reply with quote  
 Post subject: Re: 912 3rd piece cracks
PostPosted: Sat Aug 12, 2017 4:34 pm 
356 Fan
User avatar

Joined: Mon Mar 31, 2008 11:43 pm
Posts: 183
Location: Seminole, Florida
Didn't know how to do this, and then poof, I figured it out. Here's some photos.


Attachments:
101_2261.JPG
101_2261.JPG [ 1.04 MiB | Viewed 297 times ]
101_2260.JPG
101_2260.JPG [ 1 MiB | Viewed 297 times ]
Top
 Profile  
Reply with quote  
 Post subject: Re: 912 3rd piece cracks
PostPosted: Sat Aug 12, 2017 4:56 pm 
356 Fan

Joined: Wed Nov 23, 2011 10:52 am
Posts: 2611
Location: Cleveland, Ohio
I would use it, maybe add a regular flat washer on first under the wave washer.

_________________


Top
 Profile  
Reply with quote  
 Post subject: Re: 912 3rd piece cracks
PostPosted: Sat Aug 12, 2017 5:53 pm 
356 Fan
User avatar

Joined: Wed Mar 26, 2008 2:18 pm
Posts: 1770
Location: SoCal
Personally, I'd weld it up.


Top
 Profile  
Reply with quote  
 Post subject: Re: 912 3rd piece cracks
PostPosted: Sat Aug 12, 2017 9:29 pm 
356 Fan
User avatar

Joined: Wed Mar 26, 2008 11:45 am
Posts: 5383
Location: San Francisco
jim nelson wrote:
Personally, I'd weld it up.
Either that, or if they are shallow enough, machine down until there is no crack in the remaining material.
Cracks propagate; that's what they do.

_________________


Top
 Profile  
Reply with quote  
 Post subject: Re: 912 3rd piece cracks
PostPosted: Sat Aug 12, 2017 9:39 pm 
356 Fan
User avatar

Joined: Mon Mar 31, 2008 11:43 pm
Posts: 183
Location: Seminole, Florida
Back in my flying days, I remember them drilling holes at the end of any crack they found on the air craft to stop the crack from propagating (I'm sure there were some cracks they were allowed to do that with and some they weren't. What I didn't know wouldn't hurt me.). My point or question is would drilling a hole at the end of the crack stop the crack? That is something that I can do in my garage. Thanks!
Glen
64Cx2


Top
 Profile  
Reply with quote  
 Post subject: Re: 912 3rd piece cracks
PostPosted: Sat Aug 12, 2017 10:09 pm 
356 Fan
User avatar

Joined: Wed Mar 26, 2008 11:45 am
Posts: 5383
Location: San Francisco
In a piece of sheetmetal, you can be sure that a well-oversized drill left none of the crack if you drill normal to the surface. I'm saying you can't be sure of that in a casting of some thickness where the crack may well have wondered out of plane to the visible end of the crack, and the 'length' of the crack may not be equal inside and outside. Suffice to say, I would not trust a drilling to "end" the crack in a casting.
It is not a high-stress location, but if you race with it, and it lets go, the guys following you will probably pay you a visit in the pits after you're black-flagged.

_________________


Top
 Profile  
Reply with quote  
 Post subject: Re: 912 3rd piece cracks
PostPosted: Sat Aug 12, 2017 11:10 pm 
356 Fan

Joined: Wed Nov 23, 2011 10:52 am
Posts: 2611
Location: Cleveland, Ohio
With the nuts only at 15 ft/lbs I would call it an extremely low stress application as long as it is not in a 912. Of course I am not a mechanical engineer.

_________________


Top
 Profile  
Reply with quote  
 Post subject: Re: 912 3rd piece cracks
PostPosted: Sun Aug 13, 2017 1:14 am 
356 Fan

Joined: Tue Mar 25, 2008 9:28 pm
Posts: 2903
The four holes in the 912 timing cover are where the rear engine mount brackets attach. Probably is a fair amount of stress in those four points in the 912 engine mount application. The holes are counter bored to take shouldered nuts, 616-115-171-00. See photo below. Welding and re-bore should be the fix.


Attachments:
Timing cover 912.jpg
Timing cover 912.jpg [ 71.25 KiB | Viewed 216 times ]

_________________
Top
 Profile  
Reply with quote  
 Post subject: Re: 912 3rd piece cracks
PostPosted: Sun Aug 13, 2017 7:25 am 
356 Registry Member
User avatar

Joined: Tue Oct 27, 2009 5:02 pm
Posts: 5927
Location: SE Michigan
In a 356 application, I would do NOTHING to those cracks. There is no stress on them in a 356; they will not propagate because there are no loads being applied. Don't waste any time, or money or worry about them. If you're still concerned, I'll send you another; do you want one with or without a number? I have one wall covered with about thirty of those things.

_________________


Top
 Profile  
Reply with quote  
 Post subject: Re: 912 3rd piece cracks
PostPosted: Sun Aug 13, 2017 8:47 am 
356 Fan

Joined: Sat Jun 21, 2008 11:16 am
Posts: 1347
Location: Santa Monica, CA
If there is no stress, why did the crack form?

_________________
#6386


Top
 Profile  
Reply with quote  
 Post subject: Re: 912 3rd piece cracks
PostPosted: Sun Aug 13, 2017 9:01 am 
356 Registry Member
User avatar

Joined: Tue Oct 27, 2009 5:02 pm
Posts: 5927
Location: SE Michigan
The crack was caused by some incident in the past life in a 912. It did not FORM; it was CAUSED.

_________________


Top
 Profile  
Reply with quote  
 Post subject: Re: 912 3rd piece cracks
PostPosted: Sun Aug 13, 2017 9:33 am 
356 Fan
User avatar

Joined: Wed Aug 27, 2008 6:38 am
Posts: 729
Location: England
The 3rd piece of the engine was stressed in the previous 912 application with the rear engine mounting cross member. Why did Porsche have the 356 engine unsupported and mounted only on the bell housing of the gearbox and yet felt the need to have the rear of the engine supported in the 912? I feel that the engine must have been constantly trying to escape at that rear mounting, thus the stress cracks.
Jay

_________________
 


Top
 Profile  
Reply with quote  
 Post subject: Re: 912 3rd piece cracks
PostPosted: Sun Aug 13, 2017 10:02 am 
356 Registry Member
User avatar

Joined: Tue Oct 27, 2009 5:02 pm
Posts: 5927
Location: SE Michigan
"the need to have the rear of the engine supported in the 912?"
Because there was no rear transmission hoop to support the transmission and engine.

_________________


Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 21 posts ]  Go to page 1, 2  Next

All times are UTC - 5 hours [ DST ]


Who is online

Users browsing this forum: Ashley James, Google [Bot], tim noble, Yahoo [Bot]


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
Powered by phpBB® Forum Software © phpBB Group