The Ford Barn

southfork · 10-19-2014, 06:22 PM

I had a Model A engine rebuilt about 15 years ago, and today I dug it out of storage. It appears to be in great shape after resting under a protective cover on my work bench all these years. I'd forgotten that when I had it rebuilt, the machinist went from the then 80-over cylinders to the maximum 125 overbore. I'm planning to put a head, oil pump, oil pan, carb etc., etc. on it and get it running this winter, and am now wondering about how best to accommodate that large overbore: (1) will the regular copper type head gasket be big enough, or must I order a specialty head gasket from some source? and
(2) would a high compression head be advisable or not?

I probably ought to try to take advantage of the engine's larger displacement and not negate it with the head that I choose to put on it. Given that objective, your suggestions are invited, along with any pros or cons. Thanks!
-Jack

700rpm · 10-19-2014, 06:32 PM

Check your pistons against your head gasket before you button it up, and make sure the pistons don't hit the gasket. Many pistons today are chamfered to avoid this.

glenn in camino · 10-19-2014, 07:55 PM

Most new repo head gaskets will fit. My most powerfull stock Model A is 125 over bore.

ctlikon0712 · 10-19-2014, 07:59 PM

Quote:

I probably ought to try to take advantage of the engine's larger displacement and not negate it with the head that I choose to put on it. Given that objective, your suggestions are invited, along with any pros or cons. Thanks!

Even just over boring an engine gives a slightly higher compression with a stock head. If you think about it, you are squeezing more volume into the same sized combustion chamber. Ergo the compression ratio. However I know what you mean, the 5-6 CR heads do help out allot. I don't know if the copper gaskets of today will handle a 0.125" overbore.
http://www.fordbarn.com/forum/showthread.php?t=86855

Purdy Swoft · 10-19-2014, 08:01 PM

The Felpro 7013C copper gasket is a big bore gasket that can be used with bores even slightly larger than 125 .This is the gasket that was recommended for the Brumfield heads. I use this gasket on most of my engines with good results. I would go ahead and use the high compression head, every model A really needs one!!! I would much rather have a large over bore than a sleeved engine any day and if the rebuild was properly done there should be no problem .

Tom Wesenberg · 10-19-2014, 09:16 PM

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I don't want to take time to do the math now, but what is the volume per cent increase for one cylinder. That same % is the compression ratio increase, which isn't enough to stop you from also adding the 5.5 head.

Barry B./ Ma. · 10-20-2014, 07:22 AM

The bore on my 30 roadster is .125 oversize and the pistons were hitting the gasket. From reading an old manual I learned that any bore .080 and above require a chamfer on the piston tops for clearance, mine were not chamfered so I pulled the pistons and had a machinist chamfer them equally at .093x45 deg.

Tom Wesenberg · 10-20-2014, 07:48 AM

The .125" overbore will up your compression ratio to about 4.5 and if you install the 5.5 head the compression ratio will be about 5.9.

southfork · 10-20-2014, 10:25 AM

Good input form everybody. Thanks!

Bob C · 10-20-2014, 10:45 AM

Vince has some information about the head gaskets on his site.
http://www.fordgarage.com/pages/felproAheadgasket.htm

Bob

Purdy Swoft · 10-20-2014, 11:08 AM

Most oversize pistons that are .060 or more have a chamfered edge. The Felpro 7013C gasket will clear an overbore of .125 or slightly more even if the pistons have no chamfer at all. A lot depends on which high compression head that you plan to use. The Snyder 5.5 & 6.0 heads require that the model B type gasket must be used. The Best gasket co. makes a model B gasket that can be used with the model B style gasket with the large overbore. This gasket is metal clad. I think that Berts and Brattons handle the Best brand gaskets . If you use the Snyder heads be sure and ask about brand and large overbore style gasket. In other words if you use the standard composition model B gasket you will have problems with pistons with a bore larger than .060 striking the edge of the gasket .

southfork · 10-20-2014, 04:24 PM

Good to know that, Purdy.

Do you know why the Snyder 5.5 & 6.0 heads require that a Model B type gasket be used?

10-19-2014, 06:22 PM	#1
southfork Senior Member Join Date: May 2010 Location: Idaho Posts: 416	Rebuilt Short block with a Large Overbore I had a Model A engine rebuilt about 15 years ago, and today I dug it out of storage. It appears to be in great shape after resting under a protective cover on my work bench all these years. I'd forgotten that when I had it rebuilt, the machinist went from the then 80-over cylinders to the maximum 125 overbore. I'm planning to put a head, oil pump, oil pan, carb etc., etc. on it and get it running this winter, and am now wondering about how best to accommodate that large overbore: (1) will the regular copper type head gasket be big enough, or must I order a specialty head gasket from some source? and (2) would a high compression head be advisable or not? I probably ought to try to take advantage of the engine's larger displacement and not negate it with the head that I choose to put on it. Given that objective, your suggestions are invited, along with any pros or cons. Thanks! -Jack

10-19-2014, 06:32 PM	#2
700rpm Senior Member Join Date: May 2010 Location: Portland, Oregon Posts: 5,913	Re: Rebuilt Short block with a Large Overbore Check your pistons against your head gasket before you button it up, and make sure the pistons don't hit the gasket. Many pistons today are chamfered to avoid this. __________________ Ray Horton, Portland, OR As you go through life, keep your eye on the donut, not the hole.

10-19-2014, 07:55 PM	#3
glenn in camino Senior Member Join Date: May 2010 Location: Camino, CA. Posts: 3,086	Re: Rebuilt Short block with a Large Overbore Most new repo head gaskets will fit. My most powerfull stock Model A is 125 over bore.

10-19-2014, 08:01 PM	#5
Purdy Swoft Senior Member Join Date: May 2010 Location: Alabama Posts: 8,099	Re: Rebuilt Short block with a Large Overbore The Felpro 7013C copper gasket is a big bore gasket that can be used with bores even slightly larger than 125 .This is the gasket that was recommended for the Brumfield heads. I use this gasket on most of my engines with good results. I would go ahead and use the high compression head, every model A really needs one!!! I would much rather have a large over bore than a sleeved engine any day and if the rebuild was properly done there should be no problem .

10-19-2014, 09:16 PM	#6
Tom Wesenberg Senior Member Join Date: May 2010 Location: Mpls, MN Posts: 27,582	Re: Rebuilt Short block with a Large Overbore Sponsored Links (Register now to hide all advertisements) I don't want to take time to do the math now, but what is the volume per cent increase for one cylinder. That same % is the compression ratio increase, which isn't enough to stop you from also adding the 5.5 head.

10-20-2014, 07:22 AM	#7
Barry B./ Ma. Senior Member Join Date: May 2010 Location: Southbridge, Ma. Posts: 1,614	Re: Rebuilt Short block with a Large Overbore The bore on my 30 roadster is .125 oversize and the pistons were hitting the gasket. From reading an old manual I learned that any bore .080 and above require a chamfer on the piston tops for clearance, mine were not chamfered so I pulled the pistons and had a machinist chamfer them equally at .093x45 deg.

10-20-2014, 07:48 AM	#8
Tom Wesenberg Senior Member Join Date: May 2010 Location: Mpls, MN Posts: 27,582	Re: Rebuilt Short block with a Large Overbore The .125" overbore will up your compression ratio to about 4.5 and if you install the 5.5 head the compression ratio will be about 5.9.

10-20-2014, 10:25 AM	#9
southfork Senior Member Join Date: May 2010 Location: Idaho Posts: 416	Re: Rebuilt Short block with a Large Overbore Good input form everybody. Thanks!

10-20-2014, 10:45 AM	#10
Bob C Senior Member Join Date: May 2010 Location: So Cal Posts: 8,767	Re: Rebuilt Short block with a Large Overbore Vince has some information about the head gaskets on his site. http://www.fordgarage.com/pages/felproAheadgasket.htm Bob

10-20-2014, 11:08 AM	#11
Purdy Swoft Senior Member Join Date: May 2010 Location: Alabama Posts: 8,099	Re: Rebuilt Short block with a Large Overbore Most oversize pistons that are .060 or more have a chamfered edge. The Felpro 7013C gasket will clear an overbore of .125 or slightly more even if the pistons have no chamfer at all. A lot depends on which high compression head that you plan to use. The Snyder 5.5 & 6.0 heads require that the model B type gasket must be used. The Best gasket co. makes a model B gasket that can be used with the model B style gasket with the large overbore. This gasket is metal clad. I think that Berts and Brattons handle the Best brand gaskets . If you use the Snyder heads be sure and ask about brand and large overbore style gasket. In other words if you use the standard composition model B gasket you will have problems with pistons with a bore larger than .060 striking the edge of the gasket .

10-20-2014, 04:24 PM	#12
southfork Senior Member Join Date: May 2010 Location: Idaho Posts: 416	Re: Rebuilt Short block with a Large Overbore Good to know that, Purdy. Do you know why the Snyder 5.5 & 6.0 heads require that a Model B type gasket be used?