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As I was putting things back together with the new Ford motor mounts installed, I got a little too torquey on the last 13 mm bolt and twisted it off. It looks like only about 1/2" of the bolt remains in the hole - the rest came right out. So, it is pretty deep in there for any type of drill out.
I have never drilled out a broken bolt before, and I am not sure that the motor mount is the best place to learn, especially for a bolt that is buried so deep. So do i leave as is or give it a shot? I would be more concerned if it was one of the 17 mm bolts, but both of those are nice and tight on that side, plus the other 13 mm, and the far side mount is all nice and tight.
Any thoughts? Is it better to leave the mount alone or to try and drill out and replace?
Bill,
If you were to put a shorter bolt in the hole how much threaded part would be engaged?
If you have enough threads that would be my solution and add some locktite.
Other than that work out how long the part in the hole is (original length of the bolt less the broken part) and get yourself a set of easyouts.
Only drill to less than the length of the part left in the hole or enough to lcate the easyout.
The size of the drill will depend on which size easyout you are using.
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Does it have the "Do It Yourself" manual transmission, or the superior "Fully Equipped by Porsche" Automatic Transmission? George Layton March 2014
928 Owners are ".....a secret sect of quietly assured Porsche pragmatists who in near anonymity appreciate the prodigious, easy going prowess of the 928."
That's a good example why threads should trace tapped, where possible. I don't know how you could ever get a drill up there to perform the extraction. If it were me, I'd leave as is (then obsess over it until the engine is pulled).
I am leaning toward leaving as is, and obsessing over it - I highly doubt the engine is going to break the bolts loose as they were all nice and tight when I removed them in the first place, and that was with the motor mounts fully collapsed and vibrating on the mount frame. However, better safe than sorry, and it doesn't look too bad to get a drill in there.
Rog - as you can see, i was wrong about the length of the broken bit, so unfortunately no threads are exposed to get a shorter bolt in there - too bad...it looks like the hole for that bolt goes all the way through the mount though, so i hopefully wouldn't have to be too worried about the length of the easy out...
Here are the pics - it looks more like about a full inch of bolt is left up there - the nice thing is that the bore of the hole from the broken bolt is unthreaded, so if the drill slips it won't damage any threads. I guess this is as good of time as any to learn how? (oh yeah, the last pic is a little shelf and pulpit for reading my WSM's - it's my own little shrine )
IIRC, one or both of the M10 bolt holes are drilled through the block 'ear'. Stick your finger up on the top side to check. If this rumour is true, juice it up with penetrant .... and with a regular drill bit, it may loosen and spin on through.
Alternatively, if it is a blind hole, a left handed drill bit may catch enough to back the broken portion out without concern of snapping an extractor off ... in a place you don't want to be!
Bill,
It is worth a try to get it out providing you can get the drill to it.
Use a centre punch to mark the center of the bolt as a starting point for the drill.
If an easyout does not work I would leave as is. Three bolts will hold the mount well enough.
Roger
Just as Roger said give the center a good punch and drill a hole in it. Use a small extrator tool and it should come right out. There is no tension on the remaining bolt and it should come right out.
If the broken bolt looks as nice as the unbroken one then it should come out easily. Looks like it fatigued and snapped. Torque on 13MM is not supposed to be very much. Lefthand drill bit if you can find them. Extractors - I don't like EZ-out type, but this looks like you should not be in any risk of snapping one. There are some square extractors that seat in a drill hole without being threaded in. They are safer than EZ-out. http://cgi.ebay.com/ws/eBayISAPI.dll...category=50383
I am leaning toward leaving as is, and obsessing over it - I highly doubt the engine is going to break the bolts loose as they were all nice and tight when I removed them in the first place, and that was with the motor mounts fully collapsed and vibrating on the mount frame. However, better safe than sorry, and it doesn't look too bad to get a drill in there.
Trace tapping is running a tap through existing threads to clean them before reassembly.
There are four bolts per side, and missing that smaller bolt on one ear is not going to cause any problems IMO. I have the bits, easy outs, special extractors, etc, and have learned to use them only when necessary. The block material is pretty soft compared with the bolt, so the opportunity to booger up the hole is fairly high since the bolt is recessed in there and it's in a very tight location. Of course it might come out with only a little persuasion, but those bolts get frozen in there because of crud buildup in the threaded area - and can be difficult to unbolt due to the added friction caused by the crud. Assuming there was enough dirt to create the resistance required to actually break the bolt, you'll be very lucky if just a nudge breaks it loose. Also, that ear is pretty thick and threaded all the way through, so you have a lot of drilling and/or crud to overcome.
It would be very satisfying to get the broken part extracted, but in the end you have to weigh the benefits to the risk. I don't think there is much risk to leaving as is compared with the work and potential for more trouble by trying to drill it out. Maybe I'm just a wimp?
Go easy on the center punch. Too heavy a hand with the hammer will expand the exposed end of the bolt, making it even harder to get out. I'd suggest you start with a very light tap on the center punch, just enough to make a visible mark, then use a small drill to make a pilot hole ~2mm dia x ~2mm deep. THEN attack it with a LH bit of an appropriate size for an extractor. Also be careful with the extractor, as it's a fine line between pounding it in there enough to "bite" and pounding it in so much it expands the bolt against the threads. YMMV.
Also what JP refers to as "trace tapping" is also commonly referred to as "chasing the threads".
I did the exact same thing last year. I ended up getting a drill bushing and drilled the bolt completely out. I do not think it is possible to drill this our straight without a drill bushing. I then bought a longer bolt and but a nut on it. When I drilled the bolt out, it took out the theads in the block too.
11-24-2005, 10:34 AM
Overagressive with the Motor Mount bolt
George Layton March 2014
(oh yeah, the last pic is a little shelf and pulpit for reading my WSM's - it's my own little shrine 
)
