Thread: Urgent: How can I stop bolts from continually coming loose

My guess is that the item that is held on by the 4 bolts is no longer flat to the block and flexing action is undoing the bolts. Thread pitch will have a marginal effect on the loosening of these bolts. Look carefully at the item and see if an extra brace can be added if you can't fix the mounting surface.

Originally Posted by bee utey

My guess is that the item that is held on by the 4 bolts is no longer flat to the block and flexing action is undoing the bolts. Thread pitch will have a marginal effect on the loosening of these bolts. Look carefully at the item and see if an extra brace can be added if you can't fix the mounting surface.

The item is the bracket that the twin cylinder Compressor sits on, this bracket is cast iron and is also the drain back for the compressor sump oil, I have had both mating (one to block other to comp. base) surfaces skimmed to true up, the block surface is perfectly flat and has the correct type gasket (paper, very thin) fitted.
The compressor that it holds up probably weighs 15k's and I was considering an extra brace to help support, but I was worried about harmonic vibes causing even more problems. Thanks for your advice, I may try replacing the cast iron bracket with a fabricated 1/2" steel bracket that would hold up the Harbour Bridge, Regards Frank.

which loctite are you using and do you want to be able to undo them?

you could replace the bolts with bolts that have the toothed underside that bite into the metal.

you can use a spring washer

if the bolts are going onto bare metal wihtout a washeryou can coat the underside of the head only with 680 or 690.

being cast iron what the minor and major diameter of the bolt holes?

are you trying to use 60 degree thread form (which will be undersized bolt) into a 55 degree thread form hole (which will be over size)

are you putting a flat washer under a spring washer?

are the bolts stretched?

if your mating with a gasket is the gasket compressing with use/exposure?

your probable best option for redrill and rethread is to go into a 2xd UNF helicoil if the compressor housing is going to limit the overall bold diameter.

in place of lockwire why not use loctabs or make a pair of small straps that you can use in lieu of loctabs and give the bolt heads a tack weld?

I used red loctite the first few times, also hardened flat washers and spring washers and very thin paper type gasket with adhesive sealer (liquid) on the block side and nothing on the bracket side, as per Perkins instructions, still came loose so used heavy type gasket with a very light smear of a special Sikaflex sealent recommended by a Perkins rebuilder.
There must be a vibration from the back of the injector pump shaft driven compressor, cannot feel it with your hand on the comp. at idle or revs.
I am contemplating studs with shanks with a slide/tight fit in the bracket holes (bracket holes are approx. 75mm deep and then fit anti-vibration serrated washers and lock nuts and some fold up tabs, would be near impossible to put a tack weld on 2 of the nuts as you can not even see them, much obliged for the advice, always helps to have some fresh ideas, anymore would be welcome, Regards Frank.

Are you able to isolate the vibration of the compressor from the bracket? I would think that if it is a standard fitting, there is something other than the bolts/loctite etc causing the problem.

Tom.

Loctite gasket compound is also red and has no retaining qualities.

The colour doesn't determine it's characteristic.

Check that you are using correct Loctite......you can do it on internet search.....it's all listed.

G'day Tank,
Gee you have done all the right things and still have a problem.
I suggest that you may have a problem with your torque wrench. Has it been calibrated or tested? and can you get a lend of another one to try?.
It maybe under torquing the bolts. When you torque the bolts does it feel right?. Also remember Loctite has a max. temperature rating and can soften if it get to hot. You can get High temperature Loctite. Also what grade of bolts are they Grd5 or Grd8?.
Here is a link for a correct torque chart and note this is for nuts and not tapped holes.
Torque Chart
Here is another link for bolts into cast iron torques. Scroll down for Imperial data.
http://www.allpar.com/fix/body/fasteners.html
Good luck
Chris

I wish I had made a blue, but everything's kosha.
I'm going to get the local engineering shop to make me some studs with tight fitting (slide on) shanks to actually support the weight of the compressor.
Perkins should have fitted at least 2 dowel pins to locate and support this bracket, I believe that the bolts can not support the weight by clamping force alone, a couple of factory fitted dowel pins would have solved this problem from the start.
Again thanks all for your helpful advice, Regards Frank.