1G 1g 7 bolt head on a 6 bolt block

Smilinjay · Sep 4, 2015

Hi everyone, so i am going through my 1g 7 bolt head and i have a question about the head stud hole diameter. I took this head off of a 6 bolt block that had been run for quite some time to put on my block (my stock head had bent valves)c, and ran it on my stock block for about 800 miles. Since then, my motor has suffered main bearing failure. Anyway, i am trying to verify if my head bolt holes have been drilled out for 6 bolt arp head studs or not. I read somewhere that a 1/2" drill bit should pass through the head bolt holes on a 6 bolt head. Using this logic, i assume the same should be true if the 7 bolt holes have been drilled out to 13mm. A 1/2" drill bit would NOT fit through all of the holes except the one closest to the firewall on the timing belt side. This baffles me as the head was pulled off of a running and relatively healthy block. If it matters, my arp studs pass through the holes with quite a bit of play. If anyone could verify what i have described, that would be a huge help.

Cruciatus111 · Sep 4, 2015

If the head came off a 6 bolt block that means the head has been drilled for 6 bolt studs already. 6 bolts use 12mm bolts and 7 bolts use 11mm bolts and the head must be drilled out to fit the larger 6 bolt head bolts.

If your head studs can fit through the head already then you don't need to worry about anything.

Smilinjay · Sep 4, 2015

Cruciatus111 said:
If the head came off a 6 bolt block that means the head has been drilled for 6 bolt studs already. 6 bolts use 12mm bolts and 7 bolts use 11mm bolts and the head must be drilled out to fit the larger 6 bolt head bolts.

If your head studs can fit through the head already then you don't need to worry about anything.

From what I have read, 6 bolt head studs will fit through a 7 bolt head. The Issue is that there isn't enough clearance for oil to pass through to through the holes and properly oil the head without drilling out the holes. I do agree though, it is probably fine since it has been ran. But i'd like to be 100% sure before a turn the key on a freshly rebuilt motor.

tmoney20g · Sep 4, 2015

Smilinjay said:
From what I have read, 6 bolt head studs will fit through a 7 bolt head. The Issue is that there isn't enough clearance for oil to pass through to through the holes and properly oil the head without drilling out the holes. I do agree though, it is probably fine since it has been ran. But i'd like to be 100% sure before a turn the key on a freshly rebuilt motor.

^ this is correct. verify you have a 12mm hole for all the studs. go buy a caliper and see for yourself.

actually i just went outside and checked a 6 bolt head, the teardrop headstud hole, from the hg side, measures 13.95mm diameter. so did a couple others that weren't dowel pin holes. looked like the center 2 stud holes are 12.95mm diameter.

Smilinjay · Sep 5, 2015

tmoney20g said:
actually i just went outside and checked a 6 bolt head, the teardrop headstud hole, from the hg side, measures 13.95mm diameter. so did a couple others that weren't dowel pin holes. looked like the center 2 stud holes are 12.95mm diameter.

Awesome thank you :thumb:

that's exactly what i needed

Smilinjay · Oct 1, 2015

Ok guys so I finally got around to buying a set of calipers and taking some measurements.
First are the exhaust side head stud hole measurements. Starting with the tear drop hole and working my way across to the cam gear side.
1.(tear drop) 13.43mm
2.13.42mm
3.(center)12.17
4.13.20mm
5.13.52mm
Now the intake side, once again starting with the hole farthest from the cam gear.
1.13.45mm
2.12.21mm
3.(center)12.10mm
4.13.61mm
5.13.75mm

By the looks of it, they have not been drilled out. Although the sizes are all over the place. This is interesting because the head was run for a pretty long time on a 6 bolt block with no signs of abnormal wear. Although it does make me wonder if this played some part in the bearing failure on my block.

BogusSVO · Oct 1, 2015

Doubtful it had had anything to do with the bearing failure in the block, the crank gets oil before the head.

The only headbolt hole that matters for clearance is the oil transfer one, the one you call the tear drop.

The main reason to ream out the other 9 is so the head dose not get stuck on the block when ARP studs are used with the 6 bolt block and 7 bolt head

If you dig around a bit you will find some cases guys had issues removing the head for one reason or another.

Most of the time it can be chalked up to spec stack from the factory machining.

Smilinjay · Oct 1, 2015

BogusSVO said:
Doubtful it had had anything to do with the bearing failure in the block, the crank gets oil before the head.

The only headbolt hole that matters for clearance is the oil transfer one, the one you call the tear drop.

The main reason to ream out the other 9 is so the head dose not get stuck on the block when ARP studs are used with the 6 bolt block and 7 bolt head

If you dig around a bit you will find some cases guys had issues removing the head for one reason or another.

Most of the time it can be chalked up to spec stack from the factory machining.

Thanks, learn something new everyday.. The teardrop hole was one of few that seemed even close to spec. I've never had any issues getting the head off the block but I guess that makes sense. I assume it would still not be wise to run this head as it is? The head studs were not a particularly tight fit but the measurements don't lie.

19Eclipse90 · Oct 1, 2015

The 6-bolt head bolts/studs are a nominal 12mm in shank diameter. As Dale said, the only one of any concern is the teardrop hole. If you've never had trouble with the head coming off before, it will be fine as it is; there is plenty of clearance around that specific hole per your measurements to be trouble free.

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