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Username Post: Tapped main bolt threads on new block... What to do?        (Topic#361563)
Chevytu 
Senior Member
Posts: 244
Chevytu
Loc: Ushuaia, Tierra del Fuego...
Reg: 12-29-02
06-29-20 01:59 PM - Post#2797457    

After lots of issues with an old Mark IV I decided to sell it and bought a new Dart Big M BBC.

Had it completely machined and when picked up asked if everything went fine. Guy told me "we ran a tap on 3 main holes as the bolts were hard to turn giving incorrect torque readings.." I cringed... No chaser, a regular tap.

Finally took the block home after arguing and getting nowhere.. Whats done it's done I guess. Left it there for a while.

Last week bolted all mains up with the stock Dart bolts. Torqued up to 100 using darts lubricant. All OK, nothing felt funny and held up just fine.

Had a set of new ARP studs and installed those. Again torqued up in 3 steps to 110 as per ARPs instructions. All OK. Also clearances did not change at all with the studs, so will use these for piece of mind.

Returned to the shop, much to my regret, and got the taps dia measured. They are 0.012" larger than the studs dia.

Also measured the "side to side play" of the studs (without the main saddles on of course). Screwed them up all the way and then backed 1 turn. Measured as close to the main surface as possible. All of them move 0.0015" to 0.0025". The ones these ppl redid (the outer on the 3rd cap and the inners on 4th and 5th cap), have 0.003 to 0.004" side play (they of course felt "looser" just by feeling while screwing them on...

Now my question is: Should I leave as it is or fix it? I know the studs are clamping as they should, but once you spin a 75 lb crank to 6.000 or 7.000 rpm there will be a lot of punding down there, isn't it? So clamping force is one thing but I suppose the mains have to endure the extra forces a running engine will have... For (crossed fngers) a long time... This is not a race engine that will get torn down every weekend to check on things.

I have access to a nice large mill. Was thinking of drilling these threads out and inserting 1.25" helicoils tightly in place.. Am I overthinking this? So afraid to break up something I will never be able to afford again that I'm getting paranoid.

Sorry for the long post.. any opinions appreciated. If there's another way to check these holes please let me know.

Thanks

1966 Chevy II SS, BBC


 




grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17506
grumpyvette
Age: 72
Loc: FLORIDA USA
Reg: 03-16-01
06-29-20 06:33 PM - Post#2797481    
    In response to Chevytu

well my first thought was why not call and talk to dart and ARP as it is their block and their studs.
but I've used several DART big M blocks and dozens of sets of ARP main cap studs , and honestly, I don,t think your running any risk.

https://www.jegs.com/p/Dart/Dart-Big-Block -Chevy-B...

https://www.competitionproducts.com/Dart-Big-M-Mac...

https://rehermorrison.com/product/bbc-blocks/

https://www.summitracing.com/search?SortBy=BestKey ...


https://arp-bolts.com/kits/product.php?PL=29&S...

https://arp-bolts.com/kits/make.php?_Make=301&...

https://www.amazon.com/UNC-Thread-Chaser-Ret hreadi...




IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


 
Rick_L 
Member #409
Posts: 27731
Rick_L
Loc: Katy, Tx, USA
Reg: 07-06-00
06-29-20 07:57 PM - Post#2797489    
    In response to grumpyvette

  • Quote:
I know the studs are clamping as they should, but once you spin a 75 lb crank to 6.000 or 7.000 rpm there will be a lot of punding down there, isn't it? So clamping force is one thing but I suppose the mains have to endure the extra forces a running engine will have...



Clamping force is what lets the block/mains endure all those "extra forces".

I would definitely get a comment from Dart before putting helicoils in those holes. Drilling and tapping for a helicoil is going to make the wall thickness from the hole to the outside edge thinner. I doubt if that's a problem either but it doesn't hurt to ask.



 
IgnitionMan 
Valued Contributor
Posts: 3762

Reg: 04-15-05
06-30-20 07:53 AM - Post#2797524    
    In response to Chevytu

I'm sorry, I am going to be the devil's advocate here on this.

I know for a fact after working for GM and other manufacturers, that a CUT thread tap, or chaser is NOT the cutter/cleaner to use in a ROLL thread hole, as the cut thread removes the end areas of the threads in the material from a roll strong end, to a sharp, weakened end thread, weakening the threads.

Those threads may well hold for a while, or cold, but what will occur when the fasteners get to optimum stretch, vibration, AND heat levels?

I may be wrong on that particular block, but if it were mine, two things would happen with it, the "machine shop" wouldn't ever machine anything for me again, and, it would become a destroyed table support for my front room.

I would also have been very suspect of the three threads in the first place.



 
Chevytu 
Senior Member
Posts: 244
Chevytu
Loc: Ushuaia, Tierra del Fuego...
Reg: 12-29-02
06-30-20 05:39 PM - Post#2797581    
    In response to IgnitionMan

Thanks for the replies.

I was getting some piece of mind before this final post... :sad : but better to get as much information as possible based on your experiences.

Of course I will never set foot on that shop again, just can't believe they would do that, at least they should have told me before.

No matter what I will have to either build it like this or helicoil it, there is no way I can get another block sent. I live overseas and with the pandemic and the economy taking a huge fall it is impossible now, and will be for quite some time (maybe years... The USD has quadrupled in price since I bought it and it continues to climb, even daily).

I understand your last explanation about how the threads are rolled in the block. It is a fact these have weakened, just can't check by how much, that is why I was going for the helicoils. The block itself looks like a tank, a lot beefier than the stock Mark IV, so I don't think a helicoil could mess it up, it takes very little material. I thought it could be the only solution. Had some head bolt holes repaired like this on a stock GM block and they worked perfectly. Of course they were rusted out so it was either that or throwing the block to the thrash..

I will send Dart an email and see what they tell me (if they respond, sent them an email when I bought the block to ask for correct cam bearing position and p/n and they never got back to me).

1966 Chevy II SS, BBC


Edited by Chevytu on 06-30-20 05:43 PM. Reason for edit: No reason given.

 
IgnitionMan 
Valued Contributor
Posts: 3762

Reg: 04-15-05
07-01-20 01:22 PM - Post#2797654    
    In response to Chevytu

I have to add that myself, I would NEVER Heli-Coil a main, or head bolt hole, they are just not worth it, nor strong in those situations.

I would ask DART if a larger stud/bolt could be used in place of the ones initially tapped, and, would they might make a stepped thread stud for that sort of application?

Tierra del Fuego, We always thought there should be some sort of combined Grands Prix and Easter Egg Hunt there, for our motorcycle road races.



 
Chevytu 
Senior Member
Posts: 244
Chevytu
Loc: Ushuaia, Tierra del Fuego...
Reg: 12-29-02
07-01-20 05:06 PM - Post#2797670    
    In response to IgnitionMan

Well... Lots of strange things happen down here, but I have yet to see something like that!

Got an email from Dart, and of course they tell me it should be checked at the factory or replaced... Not going to happen soon I'm afraid.

Helicoils are made of case hardened stainless steel, my belief is that is a lot stronger than brittle cast or even nodular iron... Just checked they are 220.000 psi (?)

I got in contact with a well known race shop up north that does racing engines for a popular road race series here. These are all destroked classic Chev, Ford, and Dodge straight sixes. The Chevrolet motors spin to 9.500 rpm and crank around 350 hp with a restrictor plate. Races last for about 1 to 2 hours. They told me they helicoil them all the time as these blocks are extremelly old 2-bolt main gray iron and with all the crank whip they take a big toll... I know, different scenario as once the race is over practically the whole thing gets replaced, but still that's a lot to endure.

Since there is not much I can do, I will try this route on a practice SBC I have that is not salvageable and see how they work. Also seen the time-serts that are available in stainless now, as opposed to the original carbon steel sets that I believe are around Grade 5... Not even close for this kind of repair.

A stepped stud also sounds like a great solution but I don't believe they or ARP would special make 3 studs, or even certify that as a repair. I could be wrong, doesn't hurt to ask. Have not seen something like this offered anywhere that I can think of.

Thanks for all the help, much appreciated.

1966 Chevy II SS, BBC


 
IgnitionMan 
Valued Contributor
Posts: 3762

Reg: 04-15-05
07-01-20 05:16 PM - Post#2797673    
    In response to Chevytu

The Heli-Coil is the weakest of thread inserts, they are subject to racking in the threads, which is, pulling opposed threads to different sides of the threading, load axis to load axis.

I use a good quality thread insert, one that is a full piece, not a coiled piece of metal, they just work better in everything I ever had to fix a thread in.

I do hope whatever you do works great, sir.



 
Chevytu 
Senior Member
Posts: 244
Chevytu
Loc: Ushuaia, Tierra del Fuego...
Reg: 12-29-02
07-01-20 05:48 PM - Post#2797675    
    In response to IgnitionMan

Thanks for the info

By just looking at the design I would assume that a solid insert would be the strongest, but of course looks is one thing... Plus I know nothing on thread repair.

I believe a stainless time sert would be the best, but I don't think they are available here. In any case I might order a set later on when we get out of this Covid pandemic, as all regular flights are cancelled thus there is no international mail at the moment.

Thank you again for your time, I dearly hope something works out..

1966 Chevy II SS, BBC


 
CNC BLOCKS N/E 
Senior Member
Posts: 983

Loc: NORTH EAST
Reg: 12-12-03
07-05-20 07:45 AM - Post#2797954    
    In response to Chevytu

  • Chevytu Said:
After lots of issues with an old Mark IV I decided to sell it and bought a new Dart Big M BBC.

Had it completely machined and when picked up asked if everything went fine. Guy told me "we ran a tap on 3 main holes as the bolts were hard to turn giving incorrect torque readings.." I cringed... No chaser, a regular tap.

Finally took the block home after arguing and getting nowhere.. Whats done it's done I guess. Left it there for a while.

Last week bolted all mains up with the stock Dart bolts. Torqued up to 100 using darts lubricant. All OK, nothing felt funny and held up just fine.

Had a set of new ARP studs and installed those. Again torqued up in 3 steps to 110 as per ARPs instructions. All OK. Also clearances did not change at all with the studs, so will use these for piece of mind.

Returned to the shop, much to my regret, and got the taps dia measured. They are 0.012" larger than the studs dia.

Also measured the "side to side play" of the studs (without the main saddles on of course). Screwed them up all the way and then backed 1 turn. Measured as close to the main surface as possible. All of them move 0.0015" to 0.0025". The ones these ppl redid (the outer on the 3rd cap and the inners on 4th and 5th cap), have 0.003 to 0.004" side play (they of course felt "looser" just by feeling while screwing them on...

Now my question is: Should I leave as it is or fix it? I know the studs are clamping as they should, but once you spin a 75 lb crank to 6.000 or 7.000 rpm there will be a lot of punding down there, isn't it? So clamping force is one thing but I suppose the mains have to endure the extra forces a running engine will have... For (crossed fngers) a long time... This is not a race engine that will get torn down every weekend to check on things.

I have access to a nice large mill. Was thinking of drilling these threads out and inserting 1.25" helicoils tightly in place.. Am I overthinking this? So afraid to break up something I will never be able to afford again that I'm getting paranoid.

Sorry for the long post.. any opinions appreciated. If there's another way to check these holes please let me know.

Thanks



I machine and sell a lot of Dart blocks and to date never had any issues with the bolt holes in the decks or mains. When the mains are tapped at Dart its the same tap for all 20 holes, I would say the shop messed something up and is trying to blame Dart.

Going to studs often changes the housing bore size and should be addressed with an align hone.




Edited by CNC BLOCKS N/E on 07-05-20 07:45 AM. Reason for edit: No reason given.

 
IgnitionMan 
Valued Contributor
Posts: 3762

Reg: 04-15-05
07-05-20 08:56 AM - Post#2797964    
    In response to CNC BLOCKS N/E

CNC, it ISN'T that the bolt holes were tapped to clean them, the key issue is thread strength after the wrong thread design tap was used to do that operation at a machine shop long after DART built the block.

At least, that is what I am getting from all of it, so far.



 




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