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Old 10-22-2018, 03:26 PM   #1
Cape Codder
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Default Changing Heads Original / High Compression

For those of you that have changed Heads can you tell me if changing from original to high compression requires longer bolts? If so how much difference in thickness is the two?

Thanks for your time.
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Old 10-22-2018, 03:46 PM   #2
Dave in MN
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Default Re: Changing Heads Original / High Compression

They should be the same.
Good Day!
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Old 10-22-2018, 03:50 PM   #3
Synchro909
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Default Re: Changing Heads Original / High Compression

When I changed, I replaced the original head studs with high tensile ones. They are the same length.
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Old 10-22-2018, 03:52 PM   #4
Cape Codder
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Default Re: Changing Heads Original / High Compression

Dave in MN & Synchro909 Thanks to you both for the information!
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Old 10-22-2018, 03:52 PM   #5
Charlie Stephens
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Default Re: Changing Heads Original / High Compression

In general the previous comments are correct but what type of high compression head are you using?

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Old 10-22-2018, 05:20 PM   #6
Cape Codder
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Default Re: Changing Heads Original / High Compression

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5.5:1 recommended by one of the parts dealers over the 6:1.
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Old 10-22-2018, 06:40 PM   #7
Russ/40
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Default Re: Changing Heads Original / High Compression

Did he say why the 5.5 over the 6?
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Old 10-22-2018, 10:22 PM   #8
Cape Codder
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Default Re: Changing Heads Original / High Compression

Easier on the Babbit.
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Old 10-22-2018, 10:52 PM   #9
Chuck Sea/Tac
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Default Re: Changing Heads Original / High Compression

What vender was that, just out of curiosity?
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Old 10-23-2018, 01:09 AM   #10
DJ S
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Default Re: Changing Heads Original / High Compression

Quote:
Originally Posted by Cape Codder View Post
Easier on the Babbit.
I would check the credibility of this source before making your purchase... It seems foolish to purchase a lower compression head when a higher compression can be purchased at the same price. If the babbitt is of concern, a higher compression of any sort shouldn't be installed; the engine should be rebuilt at that point.

Engine rebuilders such as Antique Engine Rebuilding don't even offer the 5.5:1 head anymore for their rebuilds because it doesn't make sense to install the 5.5:1 over the 6:1.
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Old 10-23-2018, 07:46 AM   #11
ryanheacox
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Default Re: Changing Heads Original / High Compression

If you have a stock distributor with no electronic or centrifugal advance the 5.5 *might* give you a little more wiggle room with where you run your spark lever.

I pay more attention to mine after going to the 5.5 but it still seems forgiving and it tells me when it's too far advanced (usually runs halfway down).

I can only assume that going to 6:1 would mean you have to pay even more attention to the spark advance or go to an automatic advance setup.
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Old 10-23-2018, 09:12 AM   #12
old31
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Default Re: Changing Heads Original / High Compression

Cape, I had 5.5 and then went to 6.1. It made a big difference. If i were you I would go directly to the 6.1, have it checked for straightness and go with grade 8 bolts.

With the 6.1 over a 4.2 and 5.5 you really have to play more with the spark lever especially on hills, no matter how small.
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Old 10-23-2018, 12:46 PM   #13
Ernie Vitucci
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Default Re: Changing Heads Original / High Compression

Good Morning all...I agree with old31, however if the engine is not 'a fresh rebuild', it is a good idea to drop the pan and check the clearances on the mains and rods as you are going to ask them to work a bit harder. Definitely install an oil filter, I use the one from Snyder's that attaches to the valve cover. Even if you run your oil over 500 miles, don't forget to lube all 31 or so zurk fittings each 500 miles to keep down the wear on all the moving parts...Ernie in Arizona
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Old 10-23-2018, 02:56 PM   #14
wensum
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Default Re: Changing Heads Original / High Compression

When I fitted a Winfield 7:1 alloy head, the original studs proved too short and I tried to get a set the length of the stud that holds the distributor cable. That proved impossible so I had to get a new high tensile set made. Unfortunately, only metric stock is available, but the end result was great as they fitted more accurately and the head slipped effortlessly on.
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Old 10-24-2018, 07:28 AM   #15
chap52
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Default Re: Changing Heads Original / High Compression

Duh!

Last edited by chap52; 10-24-2018 at 07:34 AM. Reason: double post, again
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Old 10-24-2018, 07:33 AM   #16
chap52
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Default Re: Changing Heads Original / High Compression

Most of the vendors suggest changing out the head bolts to Grade 8 when installing a high compression head because they are less likely to stretch out of torque specs. Makes sense, more compression=more pressure on the hardware.
Worth spending the $40 for the "hopefully" easy replacement. Even if you have to wait on your next Social Security check to cover the cost. Chap
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Old 10-26-2018, 02:38 PM   #17
john in illinois
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Default Re: Changing Heads Original / High Compression

The grade 8 studs also have tighter tolerance threads which I think means more than the added strength. I have had some grade 5 s that were very loose fitting in the block.

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