276 stroker from 35 years of parts

[38 coupe]

I used four balls of tin foil stuck to each piston (center, far left, far right, bottom, didn't worry about the top with the clearance in the factory head) with a dab of grease, bolted the heads down using four head bolts, and spun the crank over a couple times. Pulled the heads and measured the tin foil. The tin foil has a bit of spring, but you can tell well enough what your clearances are. The modeling clay may be a little bit more precise, but I find tin foil easier to grab in short notice.
My friend got a bit enthusiastic on surfacing the heads on this 1950 Mercury engine (.035 clearance) and ended up using a flap disk to carefully add clearance over a couple pistons. This was also not a super precise method, but clearance was verified by a second round of foil balls and dang that engine runs well.

[GB SISSON]

Ok perfect you guys. So, you do one cylinder at a time? I think I'll get that party started in the morning. When my wife gets home from work I'll ask if we have any clay for the grandkids in her toy stash. Thank you to all who have shared your flathead knowledge with me. Like I said before, I feel like I'm getting a college course from the panel of experts. I've got a friend I accuse of wearing a tin foil hat. Wait til I tell him I have tin foil balls.

[38 coupe]

I did one bank (four cylinders) at one time. Had a piece of scratch paper (maybe cardboard?) to make notes on, and measured each ball after spinning the crank a couple times with a dial caliper. The first time I did this I was worried about jamming up a cylinder, but got over it when a friend pointed out I would be the person working the wrench turning the crank.

[51504bat]

Originally Posted by 38 coupe

I did one bank (four cylinders) at one time. Had a piece of scratch paper (maybe cardboard?) to make notes on, and measured each ball after spinning the crank a couple times with a dial caliper. The first time I did this I was worried about jamming up a cylinder, but got over it when a friend pointed out I would be the person working the wrench turning the crank.

Or in GB's case a scrap piece of 1 by?

[tubman]

I seriously doubt that you could hit the head with a piston with anywhere near the force of a power stroke no matter how fast and hard you turned it over.

[GB SISSON]

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Well this is not at all what I expected to find. This is with a used head gasket that measures .062 everywhere on it. I took one of the new Best composition headgaskets and it it unused, uncompressed and it also measures .062. I ran the test on #5 and #8 and both had a good portion at .025 squish. I have no interest running on the ragged edge of valve and piston clearance. I got out my other set of EAB heads that were more crusty. I didn't want to set them on my pristine short block so I did some measuring with my dial indicator. These seem to be heavily surfaced too. (Both from the same source and orange paint). When placed upside down, all 4 EAB heads show a space almost 3/8" between a new gasket and the factory milled dome. In other words, 3/8" milled flat 'eyebrow' showing. I then grabbed an 8ba head and flipped it and tried the head gasket. The combustion chamber area lined up perfectly with the domed area in head. No eyebrow.I have a nice looking pair of 8cm heads and various 8ba's as well. I think I saw that the copper clad head gaskets were maybe .070, but when squished we still don't gain enough. Perhaps I have made some newbie error, but those foil balls sure looked flat to me.

[petehoovie]

Originally Posted by GB SISSON

Well this is not at all what I expected to find. This is with a used head gasket that measures .062 everywhere on it. I took one of the new Best composition headgaskets and it it unused, uncompressed and it also measures .062. I ran the test on #5 and #8 and both had a good portion at .025 squish. I have no interest running on the ragged edge of valve and piston clearance. I got out my other set of EAB heads that were more crusty. I didn't want to set them on my pristine short block so I did some measuring with my dial indicator. These seem to be heavily surfaced too. (Both from the same source and orange paint). When placed upside down, all 4 EAB heads show a space almost 3/8" between a new gasket and the factory milled dome. In other words, 3/8" milled flat 'eyebrow' showing. I then grabbed an 8ba head and flipped it and tried the head gasket. The combustion chamber area lined up perfectly with the domed area in head. No eyebrow.I have a nice looking pair of 8cm heads and various 8ba's as well. I think I saw that the copper clad head gaskets were maybe .070, but when squished we still don't gain enough. Perhaps I have made some newbie error, but those foil balls sure looked flat to me.

[Mart]

GB, make sure you check in the middle of the piston dome also. The pistons can sometimes be a little pointy and that can be a problem area.

[GB SISSON]

Originally Posted by Mart

GB, make sure you check in the middle of the piston dome also. The pistons can sometimes be a little pointy and that can be a problem area.

Thanks Mart. I will check the center this morning, but do we not find the entire head to be a problem area so far? If we are looking for .040 we need .015 added. An .070 copper clad gasket would add .008, making the squish at .033, but that is before the gasket is compressed. So how much does a gasket compress and what may be the ramifications of too close? Most engines I take apart have carbon etc build up on the valves and pistons. Do some people here think I am in a good situation with this? I was hoping to be more conservative like .045 if I could choose. Thanks and sorry about all this whining and Pete, you've done it again.

[Ziggster]

I’m going to have to go through this entire thread now, as I’m interested in perhaps doing a 276 stroker if my new block checks out. Lol !

[GB SISSON]

Originally Posted by Ziggster

I’m going to have to go through this entire thread now, as I’m interested in perhaps doing a 276 stroker if my new block checks out. Lol !

It's a roller coaster ride for sure!

[GB SISSON]

Best Gasket website says their big bore copper gasket measures .080 and compressed thickness is .052. Are my concerns valid, or am I just obsessing needlessly? Also when it was suggested that the EAB is a better head than the 8BA or 8CM, is this based on the squish? If so, wouldn't milling 8BA/8CM to 40-50 accomplish the same thing?

[Bored&Stroked]

It "appears" as if the clearance is getting closer toward the crown of the piston . . . so as Mart noted, make sure you measure the clearance on the crown area as well.

This is why I like the modeling clay. When I cut it with an XActo, I can really see the thickness and how consistent (or not) it is. There are two basic styles of piston crowns - one is a consistent radius (like Ross forged pistons), the other is comprised of multiple radius values (like an ellipse) - starting from different origins.

The second type tends to have a more pointy crown - which can be an issue on certain heads. Some head chambers have a consistent radius like Ross' pistons (Edelbrock, Navarro, etc) - while others and some of the stock heads have a chamber with more of an elliptical type of shape (multiple radius values).

Recently I had to have the head chambers reshaped on a CNC machine to use a set of older cast Jahn's 4-ring pistons with new Edelbrock 49-53 heads. I wanted the piston domes to match the heads - such that I could achieve a consistent squish dimension of .040.

This is why it is very important to measure the squish across the whole top of the piston and see how the shapes correlate or not.

If you find you need more clearance, another option is to use a set of Cometic MLS gaskets - they are supplied in thicknesses from about .018 to .080 or so (maybe even thicker). They are expensive - about $280 a set . . . but sometimes yah gotta do what yah gotta do!

One thing to note - the deck/head surface finish needed for MLS gaskets is important - they like smoother surfaces than you can get away with composite gaskets.

Attached are some cross-section views and a couple images I grabbed from my CAD models - you can get a better idea of what I'm talking about - maybe! LOL

Note: I use my 3D models to generate CNC toolpaths to modify chamber shapes.


Egge Cast Piston Shape - Pointy domes
3DSectionView-Ford-Egge-Dome.jpg

Ross Piston Shape - Consistent Radius
3DSectionView-RossEdelbrock-Dome.jpg

Dimensioned View - Elliptical Shape - Egge and Cast Ford
Shows one side dimensioned.
Sketch-Ford-Egge-Dome.jpg

Dimensions View - Ross Style
Shows one side dimensioned
Sketch-RossEdelbrock-Dome.jpg

Note, the "dimensions" are obviously changed/tweaked - based on bore diameter. The above sketches were done for a 3 5/16 bore (exactly what you have!)

Important: Notice the DEPTH difference between the two shapes (center dim). Approximately .265 for the elliptical chamber vs .187 or so for a typical consistent radius chamber. You can use a depth gauge to measure your heads - but the shape does matter!

If you run the Ross style piston on an elliptical style head and achieve enough clearance on the outer edges of the piston, you'll have a BIG gap above the crown - the shapes just don't match up well. If you run the "pointy" style cast pistons on a consistent radius head (like an Edelbrock), the center of the piston crown will hit the head - so you're screwed again. This is why I had to machine the Edelbrock 49-53 heads for a build.

[tubman]

It sounds to me like you need some different heads. I've never had a problem of too little quench. It may be because I've always had access to "virgin" heads. Do you know the history of the heads you are using? It could be they are discards from being resurfaced too many times. It could be a problem in the crank, rod, piston area, but "when you hear hoofbeats, think horses, not zebras".

[GB SISSON]

WELL...... It appears I got lucky. I cleaned up one of the other EABs enough to test, but found they looked about the same area inside of a test gasket and had a serious rust pit in a bad place. I then went for the pair of nice red merc EAC (I had thought they were 8CM) heads. Buffed one of em out with the cup brush. I could see from the headgasket 'template' that this one had more like a 3/16" flat inside the head gasket. Encouraging, evidence of milling, but less so. And voila! I think the pics tell the story. And now I get the added bonus of 'Mercury' logo. I found that at the crown had a measurement of .042. I only did cyls 5 and 8 again. I will clean up the other head before lunch and start a test on the valves. I am hoping for good numbers there as my cam is mild. Geekers. I think I 'done good' this morning.

[petehoovie]

Originally Posted by GB SISSON

WELL...... It appears I got lucky. I cleaned up one of the other EABs enough to test, but found they looked about the same area inside of a test gasket and had a serious rust pit in a bad olace. I then went for the pair of nice red merc EAC (I had thought they were 8CM) heads. Buffed one of em out with the cup brush. I could see from the headgasket 'template' that this one had more like a 3/16" flat inside the head gasket. Encouraging, evidence of milling, but less so. And voila! I think the pics tell the story. And now I get the added bonus of 'Mercury' logo. I found that at the crown had a measurement of .042. I only did cyls 5 and 8 again. I will clean up the other head before lunch and start a test on the valves. I am hoping for good numbers there as my cam is mild. Geekers. I think I 'done good' this morning.

[Bored&Stroked]

Great News! Is it good that you have a bunch of parts to choose from and that you're taking the time to get this all figured out. You definitely don't want .025 over the crown as once that fills up with carbon, then you get some "knocking" from the carbon whacking the head! LOL

[cadillac512]

GB, the EAC heads look like they'll fix the problem (never would have expected that problem!). The EAC's do have a bit less compression but you'll be just fine...you're not building a racer. Good job with the checking.


p.s. The MERC script alone is worth an easy 10 HP.

[Bored&Stroked]

Plus, because you've increased the cubic inches to 276, you'll pick compression back up due to the increased cylinder volume.

[GB SISSON]

Thanks for the kind words all! I've managed to stretch this into a 5 day weekend now, and counting. I did find some tight quench areas on the merc heads, but a couple spots at .030 and some at .035 got me to doing some grinding. Once started, I decided to take everything to .040 or a bit over as a few areas were more like 50. When I was in an area that showed .038, I blended it into an area of .042. I wanted to err on the high side of .040, not the low side. I find a dremel or die grinder is rather hard to contol so I used a worn down 4" grinding disc and traced the outline inside a standard size head gasket with a sharpie. I took out the sharpie line and a little over in the low spots for the overbore. I musta smashed a hundred foil balls today, and lifted those darn heads at least 40 times on and off. Consensus here says it will be worth the effort.
Since I hate sitting around doing nothing, I reassembled and primed my truck pan and made some main cap hard washers that fit in the milled areas on the caps. The grade 8 washers at our hardware store are too large ID. Plenty of room on front cap so I made six. Stacked them on a long 1/2" bolt and angled them against my beltsander spinning and scuffing. While much of my shop work may be a bit too'Gilligan's Island' for a lot of folks, I'm thinking these washers with their neatly beveled edge would please even the most discerning 'barners.