Head bolt thread (block) repair

[tgraham]

One of my head bolts failed to torque before spinning loosely. After removing it, the thread is stripped. I figure I have two options: replace the block or install a Heli-Coil. I'm trying the latter. Has anyone else done this for the DeLorean motor? I've used Heli-Coils before, but not for a head bolt! I know people have done this to other aluminum engines with the same issue... Any advice or recommendations?

Also, should I consider the head gasket toast? I was 1/2 way through torquing to 50 ft-lbs before the head came back off.

Thanks!
Travis

[DMCMW Dave]

I have not tried it yet but did some research. Reported success rate is maybe 50-50 - so it might be worth a try. This is not a common procedure, although the folks at General Motors have been doing this on certain Cadillac engines for a while. Research "northstar head bolts" for what they do.

A standard heli-coil is too short, does not provide enough threads to carry the torque and will likely pull out. You need to find something longer, maybe a TimeSert. (I never found anything). The threads are recessed down in the block so it will be difficult to get it seated correctly as well.

It would be best to replace the head gasket but if you never got to full torque on any of the bolts (and obviously never started the engine) it might be OK. But given the cost, and the labor involved if you are wrong - how lucky do you feel?

[David T]

If you ripped the threads out of the block you probably overtightened ALL of the head bolts. They are all probably stretched and should not be reused. You should probably put inserts into ALL of the holes of all of the bolts you overtorqued. I would not reuse the head gaskets either. Make sure you follow the correct torqueing procedure as it is in the Workshop Manual. You can buy Heli-Coils in different lengths. You will have to find a distributor that has then in stock or can order them for you. You will need the kit of the drill, tap, a bunch of inserts, and insert installer tool. Failure to properly torque the heads will result in cooling leaks and failure of the head gasket. Following the exact torque procedure is necessary because you cannot easily retorque the heads once you run the motor. Overtorqueing is as bad as undertorquing. You stretch the bolts, overcompress the gasket and damage the threads in the block. This is one thing that you have to get RIGHT!
David Teitelbaum
David Teitelbaum

[content22207]

Volvo's opinion on the subject:
3031

Bill Robertson
#5939

[tgraham]

Many thanks for the helpful replies!


...maybe a TimeSert. (I never found anything). The threads are recessed down in the block so it will be difficult to get it seated correctly as well.

Dave,

The TimeSert looks interesting and I considered that as well... however, it appears that they all have a flange on the top - I'm concerned that it wouldn't get deep enough (and/or require a substantial counter-sink hole). Thoughts?


...But given the cost, and the labor involved if you are wrong - how lucky do you feel?

Not lucky at all :) You'll be hearing from me on Monday...



Following the exact torque procedure is necessary because you cannot easily retorque the heads once you run the motor.

David,

I'll admit... I did the 25-50-75 ft-lbs. procedure mentioned by others on this board. I did this because I reasoned that it could be more accurate that following the angle disc, which I do have and used for the crankcase bolts as prescribed.

That said, the threads were scrupulously cleaned (chased them, cleaned them with brake cleaner, blew compressed air through, then repeat) and all bolts that had any thread issues were replaced. One of the bolts that came out was stripped at the bottom... I should have noted which hole it came out of and I think I know now :) Also, I have a good (Snap-on) torque wrench with less than 100 clicks on it since calibration.

The 1-2-3 head went on without a hitch. I'm having second thoughts about my decision, of course, but I don't think the above caused my problem - the book says to torque them to 60 ft-lbs before slackening and doing the angle torquing. The threads gave out between 25 and 50 ft-lbs.


Volvo's opinion on the subject:

Bill,

Thanks! Your Volvo book must have more pages than mine... that really helps - those inserts are 24mm long. Both Heli-Coil and TimeSert make M12-1.5x24 inserts...


Thanks again, everyone!
Travis

[tgraham]

Follow up on this one:

The Heli-Coil worked! I found some M12-1.5x24 inserts at a local fastener warehouse and went for it. I'm not sure what length comes standard in the M12-1.5 tap kit, but it's either 16mm or 18mm...

It took a few tries to get one perfectly settled. It was tough to make sure it didn't skip a thread, because the head bolt threads are so recessed, I had to remove the collar that came with the insert tool (the part used to put some downward pressure on the coil as it is inserted). But, it all worked out. Luckily these are not blind holes!

It seems like a stud kit would be a good alternate solution given the design of the PRV. Alas, after some cursory research (looked at Alpine groups), I couldn't find one.

Attached photos of the standard vs 24mm insert and the engine as of now (after turning the crankshaft several times).

Assembly continues...

Travis

[Ozzie]

GREAT!!
Good job. Engine looks/appears to be coming along great.

[bunni]

I know this is an old thread, but this just happened to me ("old thread" pun mildly intended). I was removing the bolts, #2 on the RHS of the engine came out with about half of the threads filled with aluminum. Has anyone else had success with the repair process?

See picture: http://i.imgur.com/ABSOA.jpg

[David T]

From the picture it looks like the bolt corroded and seized to the block. During removal you ripped the threads, but you had no choice. Once you get the head off you will have a better look but figure on getting some Heli-Coils. Be sure to use the angular torquing method. It is more accurate than just torquing the bolts to a torque spec. I would also measure all of the head bolts before reusing them and chasing all of the threads both in the block and each bolt. BTW, do not use Never Seize on the head bolts to try to prevent this. The torque spec is "dry". The Workshop Manual does not mention using ANYTHING on the bolts so we imply from that the bolts are torqued dry.
David Teitelbaum

[Delorean Industries]

Stepping in before something is done wrong. DO NOT use the angular method for torquing the head bolts. Torque in the order shown in the manual 25lbs 50lb 80lbs (doing each bolt to 25 in the order moving on to each to 50 in the order etc)
A light coat of oil on the tip of the bolt is recommended as well. All remaining block threads need to be tapped to remove any debris and corrosion. You will need an M12 1.50 tap to complete this task. Use penetrating oil while tapping and flush the threads out.
Proven method performed hundreds and hundreds of times here at DPI. So you know it will work!

[jmettee]

I can vouch for Josh's procedure, I used it. I also had a portion of thread come out with 1 of my head bolts. It was tight the WHOLE way out - it turned out being 1-2 threads at the end of the bolt that were seized & ran past all the threads in the rest of the hole on the way out I re-tapped the hole & chased the others. Making sure they were good, I used motor oil & torqued as Josh stated & no problems Even the hole with the bad thread accepted the torque without helicoil (I think I just got lucky). Bite the bulled & get new head gaskets unless you want to risk going back in.

[bunni]

I figured there wasn't a whole lot I could have done, but I should have tried more, oh well. I always see a fight between the angular method and the torque to the few different values method. Josh, why are you so against the angular method? Usually thats done as a constant so that the bolt can be wet or dry when torqued and is always to the correct spec. I would be listen to others opinions (I'm sure there is a thread about it here) on what is the best method. I would gladly doubt the 30 year old shop manual and Volvo engine recond. manual. But it would also be nice to have some hard evidence for one way or the other.

jmette, unfortunately way too many threads came out. This will have to be a repair job. I don't have the right tools to be comfortable with doing it and keeping everything straight. Will probably have a shop do it when I get the valves re-seated, heads blasted, maybe milling if needed, etc. I already have new head gaskets; remember this happened when the bolt was coming out for the first time, wasn't putting anything back in.

[David T]

If Josh, who has done more PRV's than I ever will recommends a particular method, then it has to be alright. He has a lot more experience to go on than I do. I too would like to hear why he does not like using the angular torque method though. As I understand it, it is supposed to be a lot more accurate. Also using any lubricant on the threads will increase the torque on the fastener and the clamping force but as long as you are not stripping threads it should be OK. These procedures were set up because you cannot go back and retorque the bolts after running the motor easily so it was a do one time kind of thing.
David Teitelbaum

[dustybarn]

Interestingly, 1776 has head studs! According to the documentation that came with the car, the engine was built up, and the turbo installed, by Ben Byrd's Scientific Automotive. They are obviously custom studs, origin unknown, and the recesses in the block where the holes come through are filled with what is probably Moroso engine epoxy. It'd be interesting to find out where the studs and nuts came from; a stud kit might really cut down on the potential damage to the block while the head is being torqued.

[David T]

Studs used to be common on motors. They were easier to make and cheaper than bolts. The biggest problem with them is corrosion. If they corrode it can be near impossible to get the parts apart, especially steel studs and aluminum heads. With newer motors and very highly stressed fasteners, in many applications head bolts (and others) are not supposed to be reused more than once. They get torqued so close to their yield strength that they actually stretch and do not spring back to their original length. Once stretched they cannot be reused or they will break. Fasteners and torque is a whole science unto itself. Unless you are ready to do your own research you really have to defer to the people who designed and developed what we already have. Sometimes you experiment and get lucky and hit on a combination that works and/or works better. That may be what happened in this case. Good luck figuring out where those studs came from and what torque they used. Some of the basics behind torque is that at a certain torque you get a certain amount of clamping force. Designers want the clamping force to be more than the stress trying to separate the parts so the fasteners never see cyclic loading which will quickly fatigue the fasteners. This stuff is measured in the hundreds of thousands of pounds. This is what determines the size and number of fasteners and the type of materiel.
David Teitelbaum