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Mercruiser 6.2 Broken #5 head bolt before reaching torque spec

Docksidemarineservices

Docksidemarineservices

Silver Medal Contributor
Replacing heads on a 6.2 liter, while torquing the new head bolts the #5 bolt broke before the final torque 0f 66lbs. The broken bolt was easily removed with the smallest easy-out so the bolt was not bound in the cylinder deck threads.

Bought another set of bolts from the local machine shop. Started in again with another new head gasket. This time on the final torque pass 66lbs, the # 3 short bolt broke. Again removed easily with the smallest easy-out.

It's not my torque wrench. Guaranteed. Tried two different ones.

So, does anyone have some insight on this?

On the final assembly, I used a torque angle gauge. Have not fired the engine yet.
 
Head Gasket thinner than previous ones? Are you lubing the head bolts? Have you used a die or thread chaser on the holes to clean and check them? are the bolts new? Have you checked for Stretching ? ( they do )..
 
Docksidemarineservices said:
Replacing heads on a 6.2 liter, while torquing the new head bolts the #5 bolt broke before the final torque 0f 66lbs. The broken bolt was easily removed with the smallest easy-out so the bolt was not bound in the cylinder deck threads.

Bought another set of bolts from the local machine shop. Started in again with another new head gasket. This time on the final torque pass 66lbs, the # 3 short bolt broke. Again removed easily with the smallest easy-out.

It's not my torque wrench. Guaranteed. Tried two different ones.

So, does anyone have some insight on this?

On the final assembly, I used a torque angle gauge. Have not fired the engine yet.
Click to expand...

.........
 
what does the Merc. manual say to apply to the threads? What about the underside of the head. Don't apply anything they don't prescribe.


Are the bolts necking down where they break? Are they breaking at root of first thread? Any pics of broken bolt available.
 
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Too much protein and carbohydrates at breakfast, Chris.
Cut back, and switch to chocolate covered or glazed doughnuts.
Lay off the orange juice or anything healthy... black coffee only.:D


On a more serious note, if you're hell bent on finding out why these failed, I suppose that the bolts that made it could be sent out to a testing lab.


.
 
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The best approach we have used is get "old fashioned" ARP bolt kits and use their torque specs.

If using the TTY one-time use bolts, the receipe for success we found is: 1) only get GM packaged bolts, 2) use only PST on the threads, & 3) use the "torque-angle" method...too many variables to use only a torque wrench with the TTY bolts...

FWIW, the old school - ARP route was a suggestion from the auto world...
 
makomark said:
The best approach we have used is get "old fashioned" ARP bolt kits and use their torque specs.

If using the TTY one-time use bolts, the receipe for success we found is: 1) only get GM packaged bolts, 2) use only PST on the threads, & 3) use the "torque-angle" method...too many variables to use only a torque wrench with the TTY bolts...

FWIW, the old school - ARP route was a suggestion from the auto world...
Click to expand...

Broke the TTY bolts and broke the ARP bolts. The ARP bolt snapped well before 66lbs.
 
with new bolts, there are only 4 possibilities I can think of:
1) faulty torque wrench
2) faulty bolts (simply looking at the failed bolt will tell most of the time)
3) underside of head or threads lubricated with something instead of clean and dry (I suspect perfect seal on the threads is all the book says in this case)
4) wrong spec. published, used or interpreted incorrectly

3 and 4 are the most common fastener failures I see, then 2 more common then you'd think. #1 almost never. (even the cheapie beam type are usually bang on)
 
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hystat said:
with new bolts, there are only 4 possibilities I can think of:
1) faulty torque wrench
2) faulty bolts (simply looking at the failed bolt will tell most of the time)
3) underside of head or threads lubricated with something instead of clean and dry (I suspect perfect seal on the threads is all the book says in this case)
4) wrong spec. published, used or interpreted incorrectly

3 and 4 are the most common fastener failures I see, then 2 more common then you'd think. #1 almost never. (even the cheapie beam type are usually bang on)
Click to expand...
Two different torque wrenches.
Two different sets of head bolts, TTY from Quick silver and your run of the mill grade8 ARP bolts.
Only PST on the threads.
Never hit the torque spec with the wrench, Felt like it was stripping the threads as you turned it.
 
"Never hit the torque spec with the wrench, Felt like it was stripping the threads as you turned it."
I'm sure you checked everything for flatness. (no warp in the head)
What bolt sizes are they?
A couple GM guys I know absolutely hate the tty stuff. They said to go with a stud kit and be done with it. But since your breaking Gr 8s (or Cl 10s?) now..............:confused:

"while torquing the new head bolts the #5 bolt broke before the final torque 0f 66lbs"
Maybe try more steps to get there?
 
I can't believe the ARP bolts broke @ less than the OEM spec....

last time the cal on the torque wrench was checked?
 
daves69 said:
"Never hit the torque spec with the wrench, Felt like it was stripping the threads as you turned it."
I'm sure you checked everything for flatness. (no warp in the head)
What bolt sizes are they?
A couple GM guys I know absolutely hate the tty stuff. They said to go with a stud kit and be done with it. But since your breaking Gr 8s (or Cl 10s?) now..............:confused:

"while torquing the new head bolts the #5 bolt broke before the final torque 0f 66lbs"
Maybe try more steps to get there?
Click to expand...

They are 7/16 std GM head bolts. New head from Mercruiser Double checked by my local machinist.
 
When removing the broken bolts while drilling they TTY bolt it ribboned out a nice cut in one piece from the drill bit. The Grade 8 bolt made short ribbons with the same drill bit. Almost as if the bolts were not hardened.
 
PST is probably part of the issue, and head bolts should be VERY difficult to drill out, so there's another issue- The T stands for teflon and that's slippery. (all loctite is very slippery when wet, in my experience) That extra slip converts the 66 foot pounds into way more tension than the bolt was designed for.
Merc calls for perfect seal I bet. ps is not very slippery.

PST might be taking all the energy out of the green triangle and handing it to the yellow one
piechart.gif
 
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hystat said:
PST is probably part of the issue, and head bolts should be VERY difficult to drill out, so there's another issue- The T stands for teflon and that's slippery. (all loctite is very slippery when wet, in my experience) That extra slip converts the 66 foot pounds into way more tension than the bolt was designed for.
Merc calls for perfect seal I bet. ps is not very slippery.

PST might be taking all the energy out of the green triangle and handing it to the yellow one
piechart.gif
Click to expand...

Merc Manual Calls for PST. Service Manual #31 page 3A-75 Loctite 565 PST
 
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It's sure nice when the customer has an un-restricted budget. :D

Just curious...... when we last spoke, you mentioned that you bought only Mercruiser engines. I assumed new.
The 383 cu in Stroker version (actually a 6.3L) is a re-manufactured engine.
 
This is a Reman. A .30 over 6.2 makes 383CID. 3year warranty.

I let the boat owner know before we started the job that he may end up with a new engine at the end. He was fine with that being that the port engine was replaced when he bought the boat last year with a 383. So now he will have a matched set.
Good thing is, there is no core charge. I will take the old engine and have her rebuilt. Then I will have a complete MPI Drop-in for stock.
 
Ahhhhhh

I see....at first I thought you were showing your completed rebiuld..........haha

If I had a replacement motor for every bolt that broke..............just saying........

WHat are the basic specs for that motor? cost? was it a long block or complete drop in assembly?
Where did it come from? Company?
 
The engine with the broken bolt was questionable before I started the Job and was no longer questionable after I ran it on the water for a while.

The new engine is a Mercruiser Reman 383cid/350hp. It came as complete drop in with a 3 year warranty.

The boat is a 28 Formula BR the port engine (6.2) was replaced a year ago with a 383/350 by the previous owner. So now he will have a matching pair.
 
Several years ago there was a report of counterfeit bolts made in China that failed well before their torque specs. This was first found in some MIL spec bolts sold to MIL sub contract and maintenance customers. Perhaps these are some "trickle downed" resells of these bolts. Typically parts that do not meet customer spec in the "MIL " world are returned to the vendor.
 
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