Flathead Overbore/Size Feedback
- Thread starter GOSFAST
- Start date
One of my current engines broke a piston pin retainer while I was flogging it on the dyno. The tiny little broken piece got jammed on the side of the piston and created a perfect groove in the cylinder bore. I was VERY happy I had only bored to +.060 during the original build so I could clean it up at +.125.
The power value difference between +.060 and other sizes will never be noticed on the street. In fact there is usually more potential in professional, time consuming, tuning than the favored overbore.
The power value difference between +.060 and other sizes will never be noticed on the street. In fact there is usually more potential in professional, time consuming, tuning than the favored overbore.
GOSFAST
Well-known member
We generally establish the ring availability before ordering ANY custom size pistons. We then order the piston sizes to correspond with the rings!
I do agree here, this is my take on it also. There will very little (if any) HP difference from .080" over to .125". Generally there WILL be from a "standard" bore build to let's say an .080" over.
Would make very little sense to use an Eagle crank, H-beam rods, and ANY cast pistons whatsoever, we wouldn't do a build like this here! I'm going to talk Eagle cranks here since we don't use any Scat FH cranks at all, but the Eagle cranks, Scat FH rods (which we ONLY use, no Eagle rods here) and most aftermarket pistons will work together at any o'sizes!
Today we are working on a 3.385" (finished) bore/metric ring pack on a "59" block with a 4.250" arm which will and up being a 306" unit!
Thanks, Gary in N.Y.
P.S. I will have more info about this post in the coming days concerning the o'bore sizes we will be putting on the shelf. Some of the o'sizes we are considering today will be .070", .080", .100", and we always have the .125" on hand. All of these will be Ross pistons for the 4.250" stroke only since the general consensus here appears to be all about the most "cubic-inches". To me it seems just more appealing to have .080" or .100" over with a 4.250" arm, either will still yield over a 280" build!
Ol' Ron
Well-known member
Egge makes 3 5/16 + .020,.030 . Pistons for just this reason. I'm using a set of the 20 over pistons in my truck. I saved a 276 , and now have a 279.4 ci engine which i call the 280 for short. I put the engine back to stock. EAB cam and heads with 2GC carb on Bored out Merc intake and a SBC dist. Engine torque is unbelievable, climbed the Weston/Andover hill in overdrive. Went over the top at 43 mph, engine was turning 1300 rpm. Not many engines of today can do that.
GOSFAST
Well-known member
I brought this back up to let everyone here know what we decided to do about the piston sizes to put into inventory!
We chose to go with .070" and .080" overs, this will allow many blocks to be finished to an "in-between" size so to speak. Not to have to initially go to the popular .125" over.
We will also still stay with this combo however, the 3.312" (bore) x 4.250" (stroke), Eagle cranks, Scat h-beam rods, and Ross pistons!
Thanks, Gary in N.Y.
P.S. These will ALL be Ross (forged) pistons and have only the "metric" ring packs, 1.5, 1.5, 3.0. They will also be for various stroke combinations, mostly the OEM 4.000" and the also popular 4.250". There will be a "handful" for the not-as-popular 4.125".
We chose to go with .070" and .080" overs, this will allow many blocks to be finished to an "in-between" size so to speak. Not to have to initially go to the popular .125" over.
We will also still stay with this combo however, the 3.312" (bore) x 4.250" (stroke), Eagle cranks, Scat h-beam rods, and Ross pistons!
Thanks, Gary in N.Y.
P.S. These will ALL be Ross (forged) pistons and have only the "metric" ring packs, 1.5, 1.5, 3.0. They will also be for various stroke combinations, mostly the OEM 4.000" and the also popular 4.250". There will be a "handful" for the not-as-popular 4.125".
Interesting. Is the 4.125" stroke down on a 4" Merc or is this a aftermarket crank?
GOSFAST
Well-known member
Hi Tim, makes absolutely no difference how you get to the 4.125" stroke, the piston doesn't "know" how it's done, only that it is 4.125!
These pistons will work with all the factory components if that's what the customer decides. They will go with a 4.000" conventional Merc crank or an "up-stroked" one as well, same with the OEM 8BA style conn rods, doesn't matter, they'll work here also!!
Thanks, Gary in N.Y.
P.S. What we're trying to accomplish is "saving' the blocks for future builds w/o any hassles (sleeves, etc). If you "wear-out" (highly unlikely) an .080" o/b you still have the .125" over option open to you? We can also generally go anywhere between the .080" and the .125" if necessary, as long as the rings are available?
Bored&Stroked
Well-known member
Hey Gary - a bit late to this party . . . but here are my thoughts. I have used the 3 5/16 bore (3.3125) for most everything that I've built - and in a good block, I can get a few more over-bores at that size (so I've not worried about it). With that said, if you can get good 1.5, 1.5, 3.0 mm ring packs at smaller bores - why not go to .080 or .100 for most situations? The last few cubic inches will not make any difference for about 99.9 % of the folks who want a "hot" flathead. Hell - most are more concerned with how it sounds and how it looks - versus how it actually performs. I've sonic tested quite a few blocks and when you get out to 3 3/8 bore, then I tend to see cylinder wall thicknesses in the .090 to .110 range. Obviously it depends on the individual casting, amount of rust over the years, core locations, etc.. I see no real reason to bore the crap out of a good block these days . . . unless you're going to Bonneville . . .
I'm not a big fan of putting 8 sleeves in a block that was previously bored to 3 3/8 - to get it back to 3 3/8. It tends to get quite expensive and also a lot of structural strength was removed (bored out of it for the sleeve thickness). This is more of a concern for high-end race engines - which is about the only place you see it. Not as much of a concern for NA engines, more of a concern for supercharged engines where sealing the deck surface is already hard (because the decks are thin). Also, most race engines are relieved - so the transfer areas are already thin . . . so sometimes issues are caused in these areas as well.
I'm not a big fan of putting 8 sleeves in a block that was previously bored to 3 3/8 - to get it back to 3 3/8. It tends to get quite expensive and also a lot of structural strength was removed (bored out of it for the sleeve thickness). This is more of a concern for high-end race engines - which is about the only place you see it. Not as much of a concern for NA engines, more of a concern for supercharged engines where sealing the deck surface is already hard (because the decks are thin). Also, most race engines are relieved - so the transfer areas are already thin . . . so sometimes issues are caused in these areas as well.
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