stipped spring plate cover bolt hole....help?

coloradoporsche · 06-06-2005, 12:27 PM

I stripped one of the M10x1.5 bolt holes on my spring plate cover.

Naturally, I reached for my tap and die set. I found that there is no M11. Apparently 11 just doesn't exist in the metric world. Can't find any fasters, taps, dies, etc. in this size.

Wanting to keep the pitch the same as the original (which I now realize was a bad idea), I went ahead an tapped a beautiful, new 12x1.5 hole and went to the hardware store to buy the new bolt. Unfortunately, 12x1.5 is a very rare bolt and none of the stores in town have one. Called several on-line faster suppliers, and for $100 I can buy an entire box with a one-week delivery. But I need to get this fixed before the DE next weekend.

I tried an M10 helicoil, but now that my hole is 12mm, the 10M bolt is too loose. And as I mentioned, there is no such thing as an M11 bolt.

Any suggestions before I break out the MIG and weld this sucker on?

rlh930 · 06-06-2005, 12:30 PM

Hmm... drill it out, braze in a plug, then drill and tap the plug back to M10x1.5?

MotoSook · 06-06-2005, 12:34 PM

Folks will want to shoot me for this, but why not go SAE thread size. Just get the appropriate tap that will fit what you alread drilled. No reason to stick to metric.

The bolts do not see much axial force, so I don't worry too much. I did what you did...stripped on of the bungs on my '76 a couple of years ago. There was still some thread engagement, so I threaded the bolt in and tack weld it to make it stay...it hasn't backed out and no funky suspension play.

I bought M10 helicoils but haven't had time to install them.

Good luck Martin!

fred cook · fred cook's Garage

See if you can find a 12 x 1.5 die, buy a 12 mm bolt with a shoulder and rethread it! That way you stay metric and have good threads to hold the spring cover plate in place.

Good luck!

coloradoporsche · 06-06-2005, 12:55 PM

Hey Guys,

Great ideas! I was wondering if I should post this question or not, but within 5 minutes I got three good suggestions that I had not considered. This board is amazing.

Since I already tapped the #$%! 12x1.5 hole, I'm going to give Fred's idea a shot.

Thanks again,

Martin

304065 · 06-06-2005, 01:12 PM

I would advise against rethreading a bolt. The original threads were probably rolled, cut threads are not as strong.

Call our host, Pelican Parts. There are a few 12x 1.5 applications on the car, one of the bolts should work for you. They can search PET for M12x1.5 x N where N is the bolt length and find a bolt from a different part of the car that should work. For example, the springplate bolts to the trailing arm, 900 083 019 08. $3.90.

wevoid · wevoid's Garage

If you wish to stick to M10 x 1.5
Get an M10 Timesert kit - the tap in that kit is close to M12 x 1.5, close enough that you can run the special timesert tap down the M12 hole and install the thread insert. You will have a fresh M10 x 1.5 threadform when you are done.
No welding, but M10 timesert kit is probably about $100, with a handful of inserts. Better still borrow one from a fellow local Pelicanite.

Regards

Hayden

Dennis Kalma · Dennis Kalma's Garage

I'd vote for the timesert kit...basically if done well it will be a permanent repair.

You also may want to check on insert kits comparable to Timesert, my bolt supply guy was showing me some that had variable wall thickness, which may allow you to drill it out even more but with the correct diameter for the "sert" and get an even more perfect repair.

Dennis

fred cook · fred cook's Garage

Over the course of many years I have, on occasion, used a die to rethread bolts. To the best of my knowledge, not one failed because of shear across the freshly cut threads. I have, on occasion, over torqued a bolt causing it to stretch and or break, have twisted bolts in half and on one occasion caused a nut to split. As long as recommended maximum torque levels are observed, it is highly unlikely that the fresly cut threads will deform or strip off. Of course, getting the die started correctly and frequent backing off of the bolt to clear debris from the die and threads will add to the workmanship of the finished bolt. For a task such as holding a spring plate in place, there is no reason to suspect the abilities of a cut thread bolt.

FIN · 06-06-2005, 06:58 PM

I'm add to fred cook's line of thinking.

You are replacing a 10mm bolt with a 12mm bolt. The threads only need to be as strong as the 10mm bolt.

something I've noticed with metric is that european products use different metric sizes than asian products sometime , i.e. there may be a different thread pitch for the same diameter bolt. kinda like UNF v. UNC threads in the SAE system.

therefore if you bought an asian made tap and die set it may not have the same thread pitches that a german set would have.

Good Luck,

Fin

coloradoporsche · 08:05 PM

OK, I am so freaking happy with the fact that I made my own bolt, I just had to post a pic. Due to the sensitive nature of the bolt-making operation, I felt it was best to call my dad in for a consultation. He is an old-school mechanical engineer who grudgingly approved of this method after a couple of cold ones.....

(Note, the long bolt shown in the pic is not the same diameter as the 12mm I used, but I just wanted to give you the idea of what we did.) My Harbor Freight tap and die set made the lower bolt shown.

You can see the new bolt is not perfect, but I got some good torque on it in the new hole, and I’m feeling good about having 4 solid bolts on the spring plate cover.

Thanks to everyone for your help!

coloradoporsche · 06-06-2005, 08:06 PM

Here's the pic....

fred cook · fred cook's Garage

ColoradoPorsche,

Congratulations on your success! Don't you just love it when a plan comes together!

304065 · 06-07-2005, 05:47 AM

Well, I guess you'll find out if it's strong enough on the first high-speed corner of your DE.

While I admire your ingenuity, what you have done is cut eighteen stress raisers across the grain of the bolt you started with. I can't see the marking on the head, so I don't know what property class you began with, but it's definitely weaker now.

Did you use a torque wrench to tighten it? What torque? I don't have the manual handy, but the torque spec for an M10 is in the range of 33-35 foot/pounds. More than that could stretch the bolt and make it worse.

I'm not trying to sound like a jerk, like I said, I admire your ingenuity, but I just read a couple of Carroll Smiths books and he is pretty adamant about not cutting threads.

fred cook · fred cook's Garage

I did a little checking:

8.8 grade mild steel bolt tensile strength = 116,000 psi
Yield strength = 93,000 psi
12mm bolt = .19 sq. in. tensile strength = 22040 psi
Yield strength = 17670 psi

12mm bolt max. recommended torque = 85 ft/lbs.

Keeping in mind that there are also other bolts holding the spring plate cover in place, it would appear that there is a VERY large safety factor at work here!

304065 · 06-07-2005, 10:11 AM

Fred, do you know what load the springplate cover experiences during operation? I certainly don't. Do you know the revised strength of the M12 bolt with threads cut into it? I certainly don't. Therefore, it's impossible for me to concur with you that there's a very large safety factor: I simply don't know whether it's 100% or zero.

Everyone accuses the engineers at Porsche of overkill designs, but I tend to believe that if an automaker specifies an 8.8 property class bolt, it's because a lower property class wouldn't be suitable. We can guess as to the safety margin, but I'm generally of the view that one should replace a factory bolt with the same property class.

Suppose one were to install a different bolt in the lower right hole, against which the springplate rests when the suspension is in droop. If it snaps, the springplate could get caught in between the body and the springplate cover, resulting in unpredictable handling. Also, if the springplate bushings are compliant, the plate cover ends up taking a bunch of the load.

Again, I'm not trying to be obnoxious, but for $4 i'd replace it with a KAMAX bolt from Porsche rather than take any risk at speed.

Jared at Pelican Parts · 06-07-2005, 10:30 AM

Use a time-sert.

fred cook · fred cook's Garage

If it was my car, I would not worry about the strength or capacities of the one bolt, not for street use anyway. If I was to put the car on a track for a DE or whatever, I would probably order a new bolt. In the meanwhile, the car is now driveable rather than being a driveway decoration. Safety is important but let's not get paranoid!