Author Topic: 186 heads - rocker arm studs  (Read 4826 times)

vabeach56wagon

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186 heads - rocker arm studs
« on: November 14, 2019, 03:44:04 PM »
Please spare me the effort of removing a valve cover: are the 186 heads equipped with screw-in rocker arm studs? Memory fails me.

Mine are Dec 1 68 and Dec 2 68...




Thanks

paul

bertfam

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Re: 186 heads - rocker arm studs
« Reply #1 on: November 14, 2019, 03:54:00 PM »
No. Screw in studs didn't start until late 1969 (the 1970 model year).

Ed
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william

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Re: 186 heads - rocker arm studs
« Reply #2 on: November 14, 2019, 03:54:45 PM »
They weren't built with them but they were often added.
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69Z28-RS

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Re: 186 heads - rocker arm studs
« Reply #3 on: November 14, 2019, 03:55:05 PM »
I believe the 186 heads were equipped with 'screw in rocker stud' beginning with the 1970 model year.  Before 1970, 186 heads had press in studs.

Note:  Anyone rebuilding their '68 or '69 186 heads who want to keep their heads appearing stock (press in) but use screw in studs should be aware that machine shops have access to rocker studs that screw in but without the integrated 'nut'.. so it would appear to be press in but would be screw in...
Gary W / 09C 69Z28-RS, 72 B 720 cowl console rosewood tint
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60 Corvette white/red, 72 Corvette coupe, '70 Mach I 
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vabeach56wagon

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Re: 186 heads - rocker arm studs
« Reply #4 on: November 14, 2019, 04:48:34 PM »
Thanks for the response...that helps in the planning/budgeting of the build.

We'll machine the bosses for the studs plus an extra 1/8" for the pushrod guide plates - and elongating the pushrod slots in the heads slightly.

This block is a July 25 stamped DZ. Ours is a June 18 car. Does anybody have a good block with an April or May casting and DZ stamping? As long as the engine is out, may as well find a more realistic date. I DO NOT RESTAMP and will always disclose the reality of the car, i.e. the M22 is an all-new transmission built by Riverside Gear in Eaton Rapids, Mi and stamped to match this car.

All Corvettes I've done have been matching numbers cars and any restamped elements (including blocks) were always disclosed to the buyers.





Do internal improvements to a DZ engine impact the perceived value of the car?

paul

Stingr69

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Re: 186 heads - rocker arm studs
« Reply #5 on: November 15, 2019, 04:04:16 PM »


Note:  Anyone rebuilding their '68 or '69 186 heads who want to keep their heads appearing stock (press in) but use screw in studs should be aware that machine shops have access to rocker studs that screw in but without the integrated 'nut'.. so it would appear to be press in but would be screw in...


I considered doing that myself but the more I dug into it, the less exciting it was.  Seems they can be failure prone at the stud boss.  I actually bought a set of those replacement threaded studs but decided against using them.  Chose to go with the known better option of milling the boss and using a stud with a hex base with the guide plate and push rod hole enlargement.  It is much more robust that way.

If anybody wants to try a set you can have them for the cost of postage.  I will not use them.

69Z28-RS

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Re: 186 heads - rocker arm studs
« Reply #6 on: November 15, 2019, 05:25:13 PM »


Note:  Anyone rebuilding their '68 or '69 186 heads who want to keep their heads appearing stock (press in) but use screw in studs should be aware that machine shops have access to rocker studs that screw in but without the integrated 'nut'.. so it would appear to be press in but would be screw in...


I considered doing that myself but the more I dug into it, the less exciting it was.  Seems they can be failure prone at the stud boss.  I actually bought a set of those replacement threaded studs but decided against using them.  Chose to go with the known better option of milling the boss and using a stud with a hex base with the guide plate and push rod hole enlargement.  It is much more robust that way.

If anybody wants to try a set you can have them for the cost of postage.  I will not use them.

WHY would this type of screw in studs be more prone to breakage at the boss than the original press in studs?   

When I purchased my Z28 back in '76, one of the heads had a 'poorly welded' stud and a poorly welded valve cover (where the original press in stud had come loose and broke the valve cover as well.   Back then, when I rebuilt the engine, I replaced the heads with angle plug heads (good screw in studs) and a new valve cover (but I still have that 'welded up one' as a reminder.. :)     When I pulled the motor (many years later), I sold the angle plug heads and after that found good dated '69  186 heads for rebuilding and using on my engine.   I was 'torn' between the originality of the press in studs and the strength advantage of the '70 type screw in studs, so I 'compromised' and had my head shop install the 'stock looking' screw in studs.  I really don't expect I will drive the same way when I put it back on the road as I did in the 1970's... :)
Gary W / 09C 69Z28-RS, 72 B 720 cowl console rosewood tint
69 Corvette convertible, silver/black 350 hp,
60 Corvette white/red, 72 Corvette coupe, '70 Mach I 
90 ZR1 red/red #246, 90 ZR1 white/gray #2466
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william

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Re: 186 heads - rocker arm studs
« Reply #7 on: November 15, 2019, 06:02:41 PM »
Press-in studs don't break; the press-fit can loosen over time and the stud lifts. Makes some racket and can ruin the valve; if on an exhaust valve it will pop back through the carb. I have heard that hot-tanking heads can compromise the press-fit. I bought a Z/28 in the '70s with freshly rebuilt heads and it pulled a stud a few weeks later. The simple fix is to re-seat the stud; drill and pin it. That works ok but I have heard the stud boss may eventually crack and break.

Were I to ever have another 302 rebuilt, I would never use press-in studs.
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70z28lt1

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Re: 186 heads - rocker arm studs
« Reply #8 on: November 15, 2019, 06:09:43 PM »
Just like many things the answer is "it depends" on the type of screw in stud.  Rocker studs have 3/8-24 fine threads for the nut.  With replacement screw in studs that do not require the head to be milled, the part that threads into the head is usually 7/16-14.  The minor diameter (root of the thread) for 7/16-14 is 0.3499 in diameter.  This is less than the 3/8 diameter of the stud between the 3/8-24 and where it meets the head casting boss.  So the weakest spot in now the minor diameter of the 7/16-14 thread.  With a screw in stud that has a flange and the head is milled there is a generous fillet that transitions to the hex to tighten down against the machined head.  This is MUCH stronger than the aforementioned replacement thread in studs.  This also results in a more stable valvetrain.  It's hard to tell, but when the valve opens the stud actually flexes back and forth a small amount. You lose a bit of lift due to this.  Hence the need for shaft mounted rockers and stud girdles when high lifts and heavy springs are used.
Maybe a better way for you is to have the standard press-in studs machined for pins to prevent the studs from backing out.  The pins are generally 3/32 to 1/8 in diameter and the head is cross drilled about mid-way down the head casting boss and the pins are then pressed in.  This can't be seen and prevents the stud from backing out.  Not many do this as the real, machined screw-in studs are better than this so most racers don't bother with pinning the stock studs.

69Z28-RS

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Re: 186 heads - rocker arm studs
« Reply #9 on: November 15, 2019, 07:37:30 PM »
For racing purposes, there's no discussion... that's the topics over in TC.    Here in TC, we discuss originality (for the most part).. keeping cars original (or at least original looking). 

PS.  I don't think many of us with original 69 Z28s are going racing with them nowadays.. :)
Gary W / 09C 69Z28-RS, 72 B 720 cowl console rosewood tint
69 Corvette convertible, silver/black 350 hp,
60 Corvette white/red, 72 Corvette coupe, '70 Mach I 
90 ZR1 red/red #246, 90 ZR1 white/gray #2466
72 El Camino, '55-'56-'57 Nomads, '55-'57 B/A Sedan

70z28lt1

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Re: 186 heads - rocker arm studs
« Reply #10 on: November 15, 2019, 08:37:03 PM »
I believe the 186 heads were equipped with 'screw in rocker stud' beginning with the 1970 model year.  Before 1970, 186 heads had press in studs.

For dates, keep in mind the heads that used the screw-in studs in 70 were only on the LT-1 Corvette and the 1970 Z28.  Both of the those had late intros in January of 70, so in reality there would be very few true '69 dated '186 heads out there with screw-in studs.  Most heads and blocks only pre-date the build of the engine by 2 or 3 weeks generally.

z28z11

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Re: 186 heads - rocker arm studs
« Reply #11 on: November 15, 2019, 09:06:18 PM »
I believe one of the problems using the screw in stud without the hex may stem (pardon the pun) from the smaller diameter stud (as was mentioned before), but also the fact there is no radius to re-inforce the stud instead of creating a built-in stress riser at the smallest diameter of the 60 degree V thread profile. Hex head studs have a generous root radius at the bottom of the upper stud, which can withstand a long slot rocker and high lift cam, plus heavy spring pressure that puts a lot of force on the stud during any kind of rpm (much less a solid lift camshaft motor). Probably why big blocks have screw-in studs (which are also compromised by the mounting angles of the studs themselves).

I have a spare set of 186 heads that I picked up a long time ago, screw-in studs already in place - I've been considering using them rather than alter my originals by pinning, or drilling/tapping. Remember, they're only originals once, as some of us are quick to point out.

Regards,
Steve

1968 Z28 M21/U17 BRG/W
1969 Z28 X77/M20/VE3 LeMans/W
1969 L78 X66/N66 Cortez/BVT
1969 Z11 L48/M35/C60/C06  1949 3100 5wd 235/6

69Z28-RS

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Re: 186 heads - rocker arm studs
« Reply #12 on: November 15, 2019, 10:42:46 PM »
I believe one of the problems using the screw in stud without the hex may stem (pardon the pun) from the smaller diameter stud (as was mentioned before), ...


The screw in stud (that I referenced) is the same diameter shaft as the original press in studs with the ONLY difference being the 'fine' threads cut in on the bottom so it screws into the head boss.
Gary W / 09C 69Z28-RS, 72 B 720 cowl console rosewood tint
69 Corvette convertible, silver/black 350 hp,
60 Corvette white/red, 72 Corvette coupe, '70 Mach I 
90 ZR1 red/red #246, 90 ZR1 white/gray #2466
72 El Camino, '55-'56-'57 Nomads, '55-'57 B/A Sedan

vabeach56wagon

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Re: 186 heads - rocker arm studs
« Reply #13 on: November 16, 2019, 02:43:37 AM »
For racing purposes, there's no discussion... that's the topics over in TC.    Here in TC, we discuss originality (for the most part).. keeping cars original (or at least original looking). 

PS.  I don't think many of us with original 69 Z28s are going racing with them nowadays.. :)

In your experience, do internal engine modifications to improve performance and longevity impact the value of an original engine in an original car? That would include the aforementioned screw-in studs, pushrod guide plates, stainless valves, matched ports, blueprinted cams, etc.?

paul

69Z28-RS

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Re: 186 heads - rocker arm studs
« Reply #14 on: November 16, 2019, 05:03:14 AM »
I think it depends on the prospective buyer as to $value$; for some it would (if the seller is honest).

For judging purposes, if the judges can't see it, hear it, or 'sense' it, then it isn't modified... :)    (The owner isn't obligated to 'tell all' when his car is being judged..  :)

Gary W / 09C 69Z28-RS, 72 B 720 cowl console rosewood tint
69 Corvette convertible, silver/black 350 hp,
60 Corvette white/red, 72 Corvette coupe, '70 Mach I 
90 ZR1 red/red #246, 90 ZR1 white/gray #2466
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Stingr69

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Re: 186 heads - rocker arm studs
« Reply #15 on: November 16, 2019, 10:57:23 PM »
Keep in mind...GM found the stock pressed in studs to be a weak point and redesigned the SHP heads to use the hex style screw-in studs and guide plates on even the lowly L-82 engines.  They did not add them just because they look sexy with the valve covers off.  GM required them on every HP engine from 1970 all the way thru the 1980's.    They also redesigned the inside of the finned aluminum valve covers at the same time to add the drippers to improve reliability. 

69Z28-RS

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Re: 186 heads - rocker arm studs
« Reply #16 on: November 17, 2019, 09:18:51 PM »
I suspect the problem (if it was a real problem) came about due to solid lifter,  higher revving engines, especially when higher pressure valve springs are used with higher lift cams.  Chevy used the press in studs for at least 15-20 yrs prior to 1970, including the SHP corvette engines of the mid sixties, and first gen Z28's.   I've never had one pull out, but the engine in my '69 Z28 had one that had pulled out prior to my obtaining the car, but it was also owned by a young teenage girl for a few years and her and her teenage bf raced it quite a bit... :)    You can just about guarantee that it was over revved when the stud let loose..    and Yes, you CAN over rev a 302... (You know you over revved it when something breaks!   ... and that's why so few early Z28's had their original engines more than a couple of years...
Gary W / 09C 69Z28-RS, 72 B 720 cowl console rosewood tint
69 Corvette convertible, silver/black 350 hp,
60 Corvette white/red, 72 Corvette coupe, '70 Mach I 
90 ZR1 red/red #246, 90 ZR1 white/gray #2466
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Stingr69

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Re: 186 heads - rocker arm studs
« Reply #17 on: November 18, 2019, 06:09:33 PM »
Why risk it?  There are other people having bad experience with this hexless rocker stud mod. 

https://chevroletforum.com/forum/engine-building-overhauling-43/cylinder-head-cracked-49940/
Also, If you need to fix an alignment issue caused by the new screw in stud being out of alignment with the valve stem you are "screwed" literally.  :D   

69Z28-RS

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Re: 186 heads - rocker arm studs
« Reply #18 on: November 18, 2019, 11:23:53 PM »
The example you cited appears to be a situation caused by the owner/installer - perhaps improperly drilled prior to the tapping of the threads.  There's also something 'sticking up' around the stud... was it not tapped deeply enough and he screwed the stud in too far stripping out the threads and cracking the boss?   Apparently he rebuilt the engine himself and the problem occured before it ever ran.   This sounds to be like an 'amateur' attempting to build his own engine without having the proper knowledge or experience...

I've never heard of the screw in studs having an issue personally...
Gary W / 09C 69Z28-RS, 72 B 720 cowl console rosewood tint
69 Corvette convertible, silver/black 350 hp,
60 Corvette white/red, 72 Corvette coupe, '70 Mach I 
90 ZR1 red/red #246, 90 ZR1 white/gray #2466
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Stingr69

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Re: 186 heads - rocker arm studs
« Reply #19 on: November 19, 2019, 07:11:00 PM »
The example you cited appears to be a situation caused by the owner/installer - perhaps improperly drilled prior to the tapping of the threads.  There's also something 'sticking up' around the stud... was it not tapped deeply enough and he screwed the stud in too far stripping out the threads and cracking the boss?   Apparently he rebuilt the engine himself and the problem occured before it ever ran.   This sounds to be like an 'amateur' attempting to build his own engine without having the proper knowledge or experience...

I've never heard of the screw in studs having an issue personally...


The stud needs something to tighten against or it will never torque to spec.  Rocker stud holes are not blind as they go into the water jacket. The raised area on the side of the stud will contact the top of the boss as a stop but the torque spec required to seat the threads is enough to crack the top of the boss. 

(Scroll down to the bottom)

http://forum.grumpysperformance.com/viewtopic.php?f=52&t=2746

Kelley W King

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Re: 186 heads - rocker arm studs
« Reply #20 on: November 19, 2019, 09:36:18 PM »
I have used hex less screw studs and had no issues but had the machine shop do it. And actually the only press in stud I have had pull out was with hydraulic lifters. A little wild on the cam but hydro lifters.
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z28z11

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Re: 186 heads - rocker arm studs
« Reply #21 on: November 20, 2019, 03:26:48 AM »
I forgot to add this earlier - GM used to sell oversized studs (.002", if I remember correctly) as a service part for those unfortunate enough to pull a stud out, or have one come loose. I've never had one pull, even with running an Erson hydraulic with pretty stout Erson springs, no give up ever. If you've ever replaced a damaged or broken stud (I have on a customer's small block a long time back), they are really tough to break loose. Pinning used to be common, but I never thought it was needed on a street engine - 

Regards,
Steve
1968 Z28 M21/U17 BRG/W
1969 Z28 X77/M20/VE3 LeMans/W
1969 L78 X66/N66 Cortez/BVT
1969 Z11 L48/M35/C60/C06  1949 3100 5wd 235/6

70z28lt1

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Re: 186 heads - rocker arm studs
« Reply #22 on: November 20, 2019, 01:19:07 PM »
Just like everything there were tolerances on the parts.  A few tenths small (still in spec) on the shank of the stud mated with a hole that is a few tenths on the high side and you have a marginal fit that may live forever on a low lift, low RPM engine but on a 302 or similar it would pull out eventually.  That's why as posted earlier they went to screw in studs on SHP applications.  There must have been enough warranty claims to make it worthwhile.  The General didn't do things like this without good reason.
My advice would be if you didn't care about appearance, machine them for screw in studs and guideplates.  If you cared a little about appearance pin them and if you are really hard core then pull all of them and select fit studs to bores for the maximum press fit along with a dab of red locktite.

69Z28-RS

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Re: 186 heads - rocker arm studs
« Reply #23 on: November 20, 2019, 02:19:22 PM »
It's good that you have a '70 with GM screw in studs....   :)
Gary W / 09C 69Z28-RS, 72 B 720 cowl console rosewood tint
69 Corvette convertible, silver/black 350 hp,
60 Corvette white/red, 72 Corvette coupe, '70 Mach I 
90 ZR1 red/red #246, 90 ZR1 white/gray #2466
72 El Camino, '55-'56-'57 Nomads, '55-'57 B/A Sedan

z28z11

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Re: 186 heads - rocker arm studs
« Reply #24 on: November 22, 2019, 02:36:19 AM »
I said .002", obviously I meant .003" (lol). These two are about 49 years old, used to have blue paint on the bottom of the studs denoting the oversize shanks. Trick to install them was to freeze them, heat the boss with a torch, drive into place using a lock nut to protect the threads. You had to be quick - once they came to temp, they were in for a good while. Danged tough material, too - if you ever adjusted valves frequently, you knew that the lock nuts would wear a lot faster than the stud threads. Using two piece locking retainers was better over time than taking a chance of lock nuts backing off.

My heads are still originals, never hot tanked or disturbed (other than Erson springs, PC seals). I had the occasion to change a couple of damaged studs out on a '68 291 head on my Bel Air around '70, had these left over. Amazing how things can accumulate -
1968 Z28 M21/U17 BRG/W
1969 Z28 X77/M20/VE3 LeMans/W
1969 L78 X66/N66 Cortez/BVT
1969 Z11 L48/M35/C60/C06  1949 3100 5wd 235/6

rszmjt

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Re: 186 heads - rocker arm studs
« Reply #25 on: November 27, 2019, 01:35:10 AM »
Iíve used the Mr Gasket 1076 screw in studs with the small shoulder with great success in the past, we use a small end mill and counterbore the head stud hole after tapping so the small shoulder is encapsulated and has a positive stop, being careful not to overtorque and also use copious amounts or red loctite. If you donít counterbore the head sometimes the rocker rubs on the small shoulder. My own Z has had these studs in for 35 yrs+ with no problems. Of course you could also use 70 style screw in studs and guide plates but I kinda like the stock type appearance.