Head bolts

[18Fiatsandcounting]

I'm not an expert on bolts, but I was cleaning up some old head bolts and it got me thinking. The ones I have are grade 10.9, and a casual search on the web indicates that the maximum recommended torque for these M10 fine thread bolts is about 60 ft lbs. Given that the cylinder head bolt torque for the DOHC engines is around 58 to 61 ft lbs, doesn't it seem odd that this is right at the maximum torque? Could this be the source of some of the elongation issues that some of us have experienced?

Maybe these bolts really should be M12, and there just isn't room in the block or head? Or am I missing something? Or maybe this is why some of us switch to studs?

-Bryan

[kilrwail]

The maximum torque for M10 bolts of strength 10.9 is 55 lb-ft. You should use bolts having a rating of 12.9, for which the maximum torque is 63 lb-ft.

[aj81spider]

I switched to studs and now it appears I have a good explanation for why!

Even 63 ft/lbs seems like a low rating for something you torque to 61. If your torque wrench is off by 4% (which would be pretty good for something that rarely gets calibrated and kicks around the garage for years) then you are over the rating.

[Nut124]

I switched to studs and now it appears I have a good explanation for why!

Even 63 ft/lbs seems like a low rating for something you torque to 61. If your torque wrench is off by 4% (which would be pretty good for something that rarely gets calibrated and kicks around the garage for years) then you are over the rating.

Do the studs have a 1.25 or 1.5 pitch nut at the top?

Max torque specs for stud is no different from that of a bolt of the same grade.

I think the Fiat factory bolt is a very bad design as it has almost no free thread, putting all stress on a very short section of the bolt. Good designs have long free thread or waisted shanks where shank diameter matches that nominal diam minus thread depth.

[18Fiatsandcounting]

Thanks for the insight, guys. The spare head bolts I have are Fiat originals, and they are stamped "Fiat" and "10.9" on the head. While you may be able to buy 12.9 head bolts, the originals (at least the early years) were 10.9, and I think that's what the vendors sell.

My guess is that Fiat just didn't see the need to put in larger bolts, and by and large, they worked just fine. I haven't worked on as many engines as some, but I can honestly say that I have never had a problem with the head/block interface. Skim the head, use a good quality gasket, clean the block surface, clean and lubricate the bolts, torque in the order specified, re-torque after a few hundred miles, and all has been well.

-Bryan

[davidbruce]

Skim the head, use a good quality gasket, clean the block surface, clean and lubricate the bolts, torque in the order specified, re-torque after a few hundred miles, and all has been well.

I looked at both Fiat shop manuals I own. A Haynes for the early models and an IAP reprint I believe for later years. Both have a page dedicated to engine torque specs and neither indicates dry or lubricated threads. There was also no mention of lube or sealant on the threads on the assembly pages. The spec in the manuals coincides with the "dry thread" torque I found on several online torque charts. This chart from Fastenal shows a torque value of only 40 foot pounds for a lubricated 10mm 10.9 bolt.
https://crafter.fastenal.com/static-ass ... teners.pdf
Even using sealant on the threads will add some lubricity and may alter the recommended torque value.
So if we lubricate the bolt and torque to the dry spec that may be why it is a common problem. Throw in a torque wrench that reads low and we compound the problem. And that is on a new fastener. If we reuse a fastener it may have been subjected to one or both of these scenarios. Perhaps more than once. Or perhaps a PO was having head gasket problems so decided a few more pounds may help. I was probably guilty of lubing the threads in the past on many american V8's. I never had an issue but that was probably dumb luck.

[18Fiatsandcounting]

Dave, I can see the plot thickening! I agree with you that there may be an issue with dry vs. lubricated, but I actually don't know what that chart from Fastenal is showing. The chart is labeled as to show the torque-tension relationship, but I don't know if it's showing the nominal torque, the maximum torque, or the torque to reach the 8115 lbs clamping force value listed for a grade 10.9 M10x1.5 bolt.

Actually, I wasn't very clear when I said I "lubricated" the bolts when assembling. I put a very light coat of oil on both sides of the washer, but the threads are cleaned and left pretty much as they were. So there may be some residual oil on the threads, but I do not use any thread sealer or lubricant.

I guess my feeling at this point is that the recommended torque for the DOHC bolts is very close to the maximum value for that size bolt, so care should be exercised not to exceed this, and it may call for new bolts if they have been used extensively.

-Bryan

[davidbruce]

Ì realize these generic charts are guidelines written by people with a lot more education than me. And just that, generic. And a head bolts job is different from say a front suspension component. But I did find it surprising how much difference there was between dry and lubed to presumably accomplish the same clamping force. Not suggesting we deviate from what the Fiat engineers determined was optimal for the application. But it does make one wonder. I checked two domestic manuals I had and the asembly instructions both said to apply sealer to the threads. They didn't specify what kind but at least we know it's part of the equation. I'm no expert either but sometimes the quest for knowledge raises more questions than answers.

[wetminkey]

My understanding is that all torques are measured with clean, dry bolts and washers. No lubricant.
Fiat head bolts require thread sealant.