8mm or 10mm bolt holes on heads - does it matter?


OnABroncoRide

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Hi, we are replacing heads on the '86 Bronco II 2.9L engine with the upgraded non-oval rocker pedestals. New heads come with either 8mm or 10mm bolt holes. What are these bolts for and if we get heads with one or the other does it matter?
Thanks for the help.
 


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ericbphoto

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Are you talking about the holes for the bolts that mount the heads to the block? If so, they should match the size mounting bolts that your block uses.
 

franklin2

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What heads are you getting? I bought the new heads off ebay for my 86 ranger with the 2.9. I don't remember that being a issue. I just bolted the new heads in place, rocker assemblies in place, adjusted the valves and drove on.

I did have oil leaking problems with the new heads. I think I needed to get the little bit later valve covers, they may have sealed better than the original ones I was trying to use.
 

OnABroncoRide

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Are you talking about the holes for the bolts that mount the heads to the block? If so, they should match the size mounting bolts that your block uses.
Yes I believe that is them. I did not think of those bolts because I was pretty sure they mentioned that they are on the front of the heads and I took that as some other place. But it makes sense that they are the head bolts. Duh. Thanks for the reply.
 

OnABroncoRide

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What heads are you getting? I bought the new heads off ebay for my 86 ranger with the 2.9. I don't remember that being a issue. I just bolted the new heads in place, rocker assemblies in place, adjusted the valves and drove on.
Eric do you know if the '86 came with 8mm or 10mm bolt holes as a rule?
 

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PetroleumJunkie412

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Hey now. Rusty is the only real Don.

Get rid of your skinny rocker pedistals. They split and break over time.

86 has 8mm. 90+ and most aftermarket have 10mm
 

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Sometimes options is a bad thing lol...interesting they would offer different sizes. I think the only benefit to the smaller size would be if you were working in tight conditions, like the motor is still in the truck, and minimal stuff is removed around it, where smaller tools would fit better. Otherwise bigger would probably be less prone to stripping and easier to torque.
 

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Has to do with your brackets IIRC. Have a pile here, and some have different sized holes.

My original heads were 8mm, put 10mm back on for added strength. Had to drill one. I think. Maybe not.

Either way, 89tm heads are way to go. They have the upgraded square ish rocker mount points
 

PetroleumJunkie412

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Clean the crap out of your new heads btw. Mine had crap all through em. Double check oil passage running to rockers. Nothing like having shavings kill rockers.
 

OnABroncoRide

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Has to do with your brackets IIRC. Have a pile here, and some have different sized holes.

My original heads were 8mm, put 10mm back on for added strength. Had to drill one. I think. Maybe not.

Either way, 89tm heads are way to go. They have the upgraded square ish rocker mount points
The idea is get those rectangular pedestal heads. So if the bolt holes in the new heads are 10mm and the block is 8mm, is there any way to use those 10mm heads? Or are we in a position to only use 86-89 heads?

Thanks for the tip on cleaning them out.
 

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These are the heads I bought. The bolt size thing is not mentioned,, and it was never a problem for me. They went right on. I had a 1986 2.9.

 

OnABroncoRide

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These are the heads I bought. The bolt size thing is not mentioned,, and it was never a problem for me. They went right on. I had a 1986 2.9.

That is exactly the same thing we were going to buy.
I'm pretty sure, based on comments, that we will have the 8mm bolts. So I'll message and see that these have for bolt holes. I called them. I called them. They are 8mm. Now to check the truck to make sure they have not been upgraded already...
 
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PetroleumJunkie412

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The biggest thing as well is to clean out your coolant passages in your block.

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_20200402_080959.JPG
_20200402_080942.JPG




This was in mine.

Graph paper is 1/4"

As far as I can tell, this is why they blew heads.
 

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That's why I tore my engine down in the first place. But I think it was more neglect. The radiator would not even drain it was so full of rusty sludge. Heater core kept clogging up. I finally got it cleaned out and almost every block core plug was leaking. Pulled it down to replace them and found a crack in one of the heads.

Cleaned out the block as best I could, new heads new gaskets new freeze plugs, ran great after that.
 


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