Loose header bolts

[goldie]

The bolts that hold a set of headers on a 350 V-8 won't stay tight. I have tried tightening them when the engine is hot and I have tried different grade bolts but they still come loose. Any suggestions?

 

[Metalguy]

Are you using a gasket-if so it's probably crushing. I always had problems with the center paired area, but not at the ends. If you have a one-piece flange holding all 4 pipes, you can cut the flange away-I've done that on a big block, and it helped.

Stock cast iron manifolds are thick and hold the flat machined face well, so no gasket is used. But light headers don't hold, so you may have to try and find a harder gasket.

 

[MikeMilburn]

What about using bolts designed for tie-wires..Mike

 

[Metalguy]

Mike,

I suspect the bolts don't actual back out, the gasket just crushes. If they don't squeeze the gasket, they can easily leak and burn out.

The SMC seems very bad in this regard in those 2 center cylinders-at least in my experience.

 

[evelrod]

Header bolts tend to loosed even if NO gasket is used. Locktite, split washers don't seem to be a definative fix. Of course safety wire will work, but who is ambidextrous enough to do it on a big V-8 in a tight engine compartment?
I am not too up on current American V-8 engines but a long time ago I used a set of special bolts that incorporated small "E" clips to secure them after installation. They worked great. I just don't remember which mfgr. supplied them. I was at "Dougs" last year and all they use are standard socket head capscrews. ???????
On my race engines I use studs and bronze(berillium)(sp) lock nuts when I can get them. SS nuts and spring washers have, on occassion worked but great care and proper anti sieze must be used to keep them from galling and thus becoming impossible to remove.

In addition, as Metalguy points out, as the gasket takes a 'set' it would be a good idea to re tighten the bolts.
Unfortunately, you may end up doing this several times.

Also, the newest headers have 3/8 plate flanges that are less prone to this problem. 1/2 inch plate flanges are available on special order but are difficult to install in some cases.


Rod

 

[TheBlacksmith]

One of the misunderstood aspects (and I've as guilty of this as anyone!) of bolts is that they should be stressed to about 90% of their yeild strength. This slight deformation helps prevent loosening. If you use too high a grade bolt, you can't get this before you crush the substrate (in this case the header flange) or strip the mating threads. While I'm not offering a good solution (and I'm not saying use softer bolts than supplied with the headers), I believe that harder bolts are not the answer. The splitting of the flange between the outer ports and the middle two ports is the most common solution I have seen also.

Blacksmith

 

[patdaly]

From my experience, Stage 8 locking bolts do a good job, irrespective of gasket used. They are expensive and are a real pain to use. I currently am running a very thin cross section ( ID/OD ) high collar lockwasher that a guy gave me a set of. I now have 3 years without a single bolt loosening, even with my solidly mounted BBC. The rub is, he dosent know where to buy them, and I have never found a source, perhaps Aeronautical supply? If anyone knows of a place to get them, kindly post it, I would like to get a thousand or so.

 

[Metalguy]

Blacksmith,

"One of the misunderstood aspects (and I've as guilty of this as anyone!) of bolts is that they should be stressed to about 90% of their yeild strength. This slight deformation helps prevent loosening. If you use too high a grade bolt, you can't get this before you crush the substrate (in this case the header flange) or strip the mating threads"

Good thinking, but I think you'll find that the reason for the high stress it so that any "applied load/stress" will always be less than the preload.

For example, correctly tightened cylinder head bolts do not see a varying load (except slightly) as the cylinder fires. People find that hard to believe, but what happens is that the GASKET sees the varying load, but the bolts "don't know" where all that load comes from, preload or firing loads. The stress on them remains almost constant unless the cyl. pressure/vibration exceeds the preload stress. This is why 2 skinny little bolts and survive while a big con rod fractures via fatigue-even a heat treated strong rod. The rod sees all that variance, but the bolts don't!

 

[TheBlacksmith]

Metalguy - I agree with you that the preload should be greater than the operating stresses. I was referring to the ability of the bolts to self-lock due to the slight elastic deformation when properly tightened and that if you use too high a grade of bolt, the normal torque for that part wouldn't be enough to cause any elastic deformation and the torque necessary would damage the parts. Maybe that's not applicable to headers and I should stick to my steam turbines where the bolts are the same diameter as the header primary tubes.

Blacksmith

 

[Metalguy]

Steam turbines, eh? I'm building the things that supply the steam to them-about 1,800,000 HP per pair. That's at 1,800 RPM, so the torque is fairly high. <g>