Aluminum L98 heads and drilling depth allowance
01-16-2019, 09:55 PM
#1
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Car: 1989 ASC GTA
Aluminum L98 heads and drilling depth allowance
Howdy..
As some of you may already know, the L98 in my 89 T/A was showing symptoms of a cracked block so on my quest to find a solid replacement I came across a 1991 Corvette that was wrecked with 19,000 original miles. I’m getting close to being able to bolt my T/A accessory drive on however the taps on the head are in different locations from the cast iron head and what I’d like to know is if anyone here has drilled and tapped the aluminum head before and what was the “comfortable” depth of drilling? I’d like to guess 3/8”-1/2” but don’t want to drill into a water passage.
01-16-2019, 11:06 PM
#2
Re: Aluminum L98 heads and drilling depth allowance
The hole where the a/c bracket stud is fairly thick . To drill the hole mount the bracket and use the hole as a guide. As for the depth use tape on your drill bit. Make it the same depth as the other holes.
01-17-2019, 06:19 AM
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Car: 1989-92 FORMULA350 305 92 Hawkclone
Engine: 4++,350 & 305 CIs
Transmission: 700R4 4800 vig 18th700R4 t56 ZF6 T5
Axle/Gears: 3.70 9"ford alum chunk,dana44,9bolt
Re: Aluminum L98 heads and drilling depth allowance
The 128 heads I have drilled before didn't have a boss for the Fcar bracket. (Have to use sealant on the stud there) ZZ4 heads did, I've also done some 113 (almost the same as ZZ4s) but I can't remember if it had the extra material for the boss.
I can tell you that you can see the extra boss through in the coolant port. If you have the deep boss, bolt on your pass accessory bracket, use a bigger drill bit that just fits in the bracket to locate the center and then give the cyl head a shallow centering mark. Remove bracket, stick correct (for tap) size drill bit in another accessory hole that's close, use tape on bit for the depth, drill and then tap it. Also use a bottom tap to make the threads clean all the way to the bottom.
Takes maybe 10 mins
I can tell you that you can see the extra boss through in the coolant port. If you have the deep boss, bolt on your pass accessory bracket, use a bigger drill bit that just fits in the bracket to locate the center and then give the cyl head a shallow centering mark. Remove bracket, stick correct (for tap) size drill bit in another accessory hole that's close, use tape on bit for the depth, drill and then tap it. Also use a bottom tap to make the threads clean all the way to the bottom.
Takes maybe 10 mins
Last edited by TTOP350; 01-17-2019 at 06:35 AM.
01-17-2019, 06:29 AM
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Car: 1989-92 FORMULA350 305 92 Hawkclone
Engine: 4++,350 & 305 CIs
Transmission: 700R4 4800 vig 18th700R4 t56 ZF6 T5
Axle/Gears: 3.70 9"ford alum chunk,dana44,9bolt
Re: Aluminum L98 heads and drilling depth allowance
Drilled ZZ4 on left engine, non drilled 128 on right but you can see circle where the bracket hits it.
01-17-2019, 10:56 AM
#6
Re: Aluminum L98 heads and drilling depth allowance
FWIW - I drilled my LB9 head for the serpentine brackets and (as expected) hit the water passage. You just use some thread sealer on it. Just like a head bolt. It's not an issue really. Honestly everything I've read indicates it's expected that the accessory bracket holes may hit water jackets in that location.
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