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  1/2" 289/302 Head Bolt Conversion

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Author Topic:   1/2" 289/302 Head Bolt Conversion

Posts: 3058
From: Spencer, WV
Registered: Jan 2001

posted 08-05-2001 05:09 PM     Click Here to See the Profile for n2oMike        Reply w/Quote
This is what's involved in converting from the little 7/16" stock head bolts to beefy 1/2" 351W units.

Just a little something I came up with when having head gasket problems when the nitrous was first installed.

Thought you guys might get a kick out of it.

1. Install 7/16" heads on the block as a guide and drill out the stock threads with a 27/64" drill bit.

2. Clean out chips and use a head with 1/2" holes as a guide to tap it out for the larger bolts. (1/2" coarse thread) The extra long tap is available through Travers Tool company. Bottoming taps can be used to tap all the way to the bottom on earlier blocks with blind holes.

3. Since I was using 289 heads, I had to drill out the head bolt holes for use with the larger bolts. 1/2" seemed a bit tight for the ARP bolts, so I went back to Travers and purchased a 33/64" bit to loosen them up a bit. a 17/32" would have worked as well, and could have been purchased locally. You MUST use a drill press that can keep the head level and straight for this step.

4. I came up with this procedure myself, so I can't provide any real references. The bolts are rated for 129 ft-lbs with 30W oil, but I feel nervous torquing them that tight. I tighten the top bolts to 125, and the bottom ones 115.

5. These bolts and the 1011-1 gaskets completely eliminated gasket problems. However, those torque figures are pretty hard on the thinwall block, and can significantly distort the bore. I consider a torqure plate bore/hone job mandatory for this operation. Loc-Wire gaskets might be a better idea as far a ring seal is concerned. I had the block power-honed at the machine shop at the latest teardown, and he said the bores were out around a thousanth or so.... until the plate was torqued down.

6. I used an early model block, and have had zero problems with stripped threads or cracking... and the heads have been on and off several times. The next engine is going to be built with a Mexican block with the 1/2" conversion AND Loc-Wire gaskets.

Good Luck!

Mike Burch
66 mustang real street
302 4-speed 289 heads
10.63 @ 129.3


Posts: 9835
From: Sonoma,CA,USA
Registered: Mar 2000

posted 08-05-2001 10:09 PM     Click Here to See the Profile for steve'66        Reply w/Quote

That's good info, thanks for sharing it with us!



Posts: 29200
From: Lyons, IL, USA
Registered: May 99

posted 08-07-2001 01:48 PM     Click Here to See the Profile for Moneymaker        Reply w/Quote
Too much torque on the bolts. You are stretching the block. 105 long, 100 short is plenty.

Alex Denysenko
Co-Administrator and Moderator
NHRA and IHRA SS/LA National Record Holder '00 & '01
Fleet of FoMoCo products
Moneymaker Bio
US Class Nationals link


Posts: 3058
From: Spencer, WV
Registered: Jan 2001

posted 08-11-2001 06:52 PM     Click Here to See the Profile for n2oMike        Reply w/Quote
I like this conversion... As long as the engine is bored/honed with stress plates! With that done, the higher torque figures seem to work just fine.... but I know what you are saying. No sense in stressing it any more than necessary.

I came up with this cure when I first started experimenting with nitrous... and was tired of changing head gaskets! The ARP instructions recommended not going -below- a given torque value. The instructions recommended 129ft-lbs! It was nerve racking that first time I pulled them down to 115 bottom, and 125ft-lbs top! It felt like WAY more than the old 80ft-lb setting! I guess a person could use a set of stock 351W bolts that just torque to 100ft-lbs.

Head gasket problems were a thing of the past after this mod... and I was a happy guy!

All this went down before the Loc-Wire gaskets came out. Now they might have worked with the smaller 7/16" studs... but weren't an option at the time. I didn't want to fight conventional "O ringing" the block because of concerns of leaking water. (I knew some chevy boys who were having throuble with this, and didn't want to fool with it.)

I'm going to go ahead and use the Boss 302 block that's been laying around in my basement for the next engine. It came apart way back when and busted a cylinder. I had a thick wall (1/8") sleeve installed recently, and need to get with it. (I bought the whole engine 10-15 years ago for $500... and got pulled over going 84 in a 55mph zone on the way there!)

I still need to install the oil restrictors and set it up for the larger head bolts. After that, it will go to the machine shop for boring, etc...

It will be filled with a Boss 302 crank, bushed Boss 302 rods, and Probe pistons.

I was also thinking of filling the bottom part of the block before it goes to the machine shop. I plan to torque the mains, and heads in place as it cures.

Any comments on the short block or its preparation? I haven't used block filler before. I've heard it will make the water temp go up and down more slowly, but won't necessarily make the coolant temp much hotter if it isn't filled too high. However, oil temperature is supposed to increase. Any good info out there?

Thanks, and Good Luck!

Mike Burch
66 mustang real street
302 4-speed 289 heads
10.63 @ 129.3

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