Power plate & stripping bolts??

[GNSteve]

Hi all,


I'd like to know if anyone happens to know the size for the bolts that hold down the plenum to the manifold. While installing my Powerplate, I noticed that the bolt at the very rear of the plenum was *not* holding torque. (ie. stripping) The other four bolts were OK.
After running the car, I noticed no problems. After a few WOT blasts, I noticed a loss of about 2-3lbs of boost.
I'm thinking that boost could be escaping through the back bolt hole. However, the car acts fine during the WOT blasts. In fact, she drives well all of the time. I was concerned about the subtle loss of boost. However, I'm thinking that boost escaping through the back bolt would be noticable, especially at WOT.
It seems to me the factory bolt length becomes *borderline* when installing the plate with its 2 gaskets. Perhaps longer bolts would help here. I'm not sure how bad the threads on the back hole are, but maybe a longer bolt will utilize more usable threads without the need for a heli-coil. Maybe.
Has anyone else experienced this? I've heard of others stripping bolt holes, but my search of the archives came up empty. I'd like to hear how they handled their problem. I also e-mailed Jason for his opinion.
So....can anyone tell me the size of the bolt or heli-coil that I'm going to need? Please? Thanks!


Steve

 

[sam87]

Original bolts are M8 - 1.25mm x 25mm. You could try one a little longer in the hole where you have the problem and see if it bites enough to hold the upper plenum. I got some of those bolts at NAPA.

 

[FJM568]

The problem with just putting in longer bolts and is that only the threads at the very end of the bolt will be holding...Let's say you put in a bolt about 1/4" longer...You may only then have 4 or 5 threads engaging tightly instead of the whole length of the bolt...That is 4 or 5 threads holding all the torque of what 20 to 25 threads would be holding normally...

You really need to heli-coil that hole...The slightest bit of boost leaking past the PowerPlate/doghouse will have you scratching your head a year or two down the road on why you have slow spoolup, why you can't build 20# boost anymore, etc...Installing a heli-coil isn't very hard to do either...

Hope this helps...

 

[Red Regal T]

Now would be the time to replace those bolts holding down the doghouse with studs. Aluminum threads aren't very forgiving. You might try installing a stud in the rear hole with some red loctite. Allow it to cure and it may hold.

 

[RealFastV6]

I stripped 2 of mine a long time ago, just gave up and HeliCoiled all 5 of them. M8x1.25 is right.

 

[GNSteve]

Guys,

 

[GNSteve]

Guys,

Thanks for the info! For the guys that went the heli-coil route, did you re-use the orginal bolts or did you go with longer ones?
Sure seems to me that the bolts could use a little extra length when installing the plate and 2 gaskets.
I'll probably go with the heli-coil now that I'm sure of the correct size. How about the bolts? Re-use stockers, or go with ones with a little more length?
Thanks for all the help!


Steve

 

[Two Lane]

Studs eliminate this problem...why worry with the "in-out" of bolts?

HTH

 

[QuickWE4]

My pass-side bolt, the one that holds the vacuum line bracket is stripped. In fact, I noticed it was stripped when I removed the doghouse.

Anyway, I helicoiled that one. First time I inserted the bolt, I guess I had the helicoil in crooked or something and it stripped the threads right off the bolt. I put a new bolt on, and it's ok, but not perfect. I used the same length, but only because the bolt that stripped was the funky one that has a little bit of a stud on top of it. THis, above all else, would be the one bolt that I would "stud".

At any rate, if all the rest of the bolts are torqued down, I wouldn't think that you'd have a leaking problem with just one bad bolt.

Also, Eastwoods sells another type of helicoil system that looks, to me, like it might be better for that alum manifold. It's like a threaded sleeve rather than a springy coil.

The only downside to studs is the look. That's just my opinion. Not as clean as bolts.

Rich

 

[QuickWE4]

Another thing...

If you put the two gaskets on - one under and over the pp, you shouldn't have anything leaking through a bolt hole (even with one bad bolt) because the bolt hole is outside the sealing edge of the plenum.

Rich

 

[Red Regal T]

Originally posted by Two Lane
Studs eliminate this problem...why worry with the "in-out" of bolts?

HTH

IT'S FUNNY HOW YOU CAN LEAD A HORSE TO WATER BUT........................

 

[Two Lane]

Amen, John!

LOL!

 

[salvageV6]

A bolt that size you only need about 3 threads worth to hold the proper torque.

Proper torque is also important.

Longer bolts work fine.

 

[GNSteve]

Well, ran down to the hardware store and picked up some longer bolts. The size I used was M8X1.25mmX30mm.
The stockers were the same, except they were 25mm long.
Seems like the extra 5mm made the difference. The back bolt torques down nice and snug like the other 4.
Seems to have solved the problem for me. Thanks everyone!


Steve

 

[TulsaGN]

what are the torque numbers on those bolts for the plenum?

 

[tta583]

25lbs if memory serves.

Also, next time you pull the plenum, Heli-Coil the thing. Ihad one in the same shape, it only lasted one more removal. The only reason I put it back on was because I could not get a Heli-Coil "right then".


Latter,

Brent

 

[salvageV6]

I wouldn't go anymore than 15lbs. on them myself.

Similar to the throttle body to plenum torque spec. I would think.

Can't find it in the book however.

Carb. screws to plenum are only 1 1/2 ft. lbs. kinda strange.

 

[FJM568]

The doghouse bolts have to hold the doghouse down while the motor is under boost...When you are at 20# of boost, I believe that means you are putting 20#/sq.in. of pressure inside the intake trying to blow the doghouse off of the top of the intake...How many sq. inches of surface is there inside the doghouse??? That is how much those 5 little bolts have to hold to keep the doghouse clamped tight to the intake...If even one of the bolts isn't engaging on the whole length of threads, soon whatever threads are left will be stripped too...Spreading the force along 15 to 20 threads is a whole lot better than on just 5 threads...

Think of it this way...You have a thin rope...It breaks at 20# of weight...If you take 5 of those ropes, you can now hold 100# of weight...The weight is distributed over all 5 ropes...The same principle is involved with threads...The torque is distributed over the whole length of the bolt if all the threads are good...If you are only engaging 5 threads, then the 25 FT/lbs of torque is only on 5 threads and not on all of the threads...It may hold now, but I would bet that the next time you have problems with idle and driveability, that you have a vacuum leak and it will drive you nuts trying to find it...Fix it now before you forget about it and then you won't have to worry about it later...

Just my .02

 

[GNandGS]

On those intake threads that were only slightly damaged I used some "form a thread" which worked fine... helicoil is the best fix if you can.

As for torque, keep in mind that part of the reason the threads are along the entire length of the bolt is so that its easier to install/manufacture and not just for clamping.

S

 

[salvageV6]

Been looking around for some torque spec.s. Haven't found any application for an M8 bolt over 25 ft. lbs.

Now I know how they all get stripped.