Model A engine
- Thread starter old skool
- Start date
George Miller
Well-known member
The best running Model A engine I ever had. I bored out .155 over size, and ran it 30,000 miles before I sold it. Many will tell you it will get hot, bored that big not true.
MikeK
Well-known member
Here's the skinny, and I DO MEAN the skinny:
The model A block has a centerline spacing between bore 1&2, and 3&4 of 4.25". Stock bore is 3.875, leaving a theoretical web of 0.375 (3/8") between 1&2 amd 3&4 to compress and retain a gasket at pressures exceeding 10,000 psi. (25,000 psi with a HC head, hot cam, etc.). Subtract from that web width the chamfer put on top of the cylinders to permit the rings to get in during assembly. A very conservative chamfer is about 0.020", reducing that web 0.020 x 2 = 0.040". Most engine builders knock off a sloppy 0.030 to 0.045" when chamfering, leaving you with less. So, a stock web is at most 0.335", and possibly as little as .285". Add (subtract!!) to this the "tolerance" deviations from the non-obtainable perfectly machined block and you may have even less.
Now it's overbore time. At 0.080 over, you are down to a web of 0.255 to 0.205. The original gaskets no longer fit, the pistons hit the overhang. Gasket makers respond by reducing the web, making the gasket good to 0.125" overbore. An original Ford gasket, and an original Fel-Pro 7013 had a gasket web between Siamese cylinders of 5/16". The new 7013 R3, and gaskets from other manufacturers reduce the web to 3/16", a 40% loss!! Gasket blowout problems begin. On the positive side, most blocks still have sufficient "meat" to get by without sleeves.
Go for broke time (life beyond 0.125 over): At 0.156 over (4 1/32") you can fit stock pistons from Ford and other apps if you are prepared to deal with non-stock piston pin heights, requiring either longer rods or a crank with longer throws (stroked). SOME blocks, only those with cores that were dead on center during casting, and almost no rust scale loss in the jackets, will go 0.156 over, leaving a nail-biting 0.120 wall in places, measurable with an ultrasonic casting thickness meter. No self-respecting NASCAR $75,000 engine builder is without one! Diesel shops have this tool too. Everyone else, it's a crap shoot.
But wait! What about the web thickness that holds the gasket between siamese cylinders? At 0.156 over you are down to 0.179 (Less than 3/16") with a conservative chamfer, scrap iron or sleeve time if you take more. Every head gasket made will have some overhang, the pistons need a top chamfer to prevent striking the gasket, and NOBODY that goes 0.156 over is building a stocker with a conservative 10,000 psi at full throttle. They have a hot build up with way more cylinder pressure. Goodbye gasket?? Racers often went with solid copper (leaks everywhere!) or sealer and NO gasket (leaks everywhere!). They don't care, it just needs to run once, today, then we drain the water. Everyone else with a water tight multi-layer gasket on a street engine is living on borrowed time before blowout. Light foot? Maybe years. Hot motor, heavy foot? Maybe hours.
Does an overbore run hotter? At 0.156 over the cylinder wall surface area increases 4.8%, and with no other changes the displacement increase will be a bit less than 10%. The surface area is the major factor. Henry's 28-29 rad probably had a 25% over capacity when new, the 30-31 likely a 35% over capacity margin. Henry's truck rads were probably 50% over. If an overbore makes you run hot, your cooling system is crapped out, it's not the overbore to blame. Increased compression and better breathing cams decrease the waste heat into the block, they are not a problem either.
The model A block has a centerline spacing between bore 1&2, and 3&4 of 4.25". Stock bore is 3.875, leaving a theoretical web of 0.375 (3/8") between 1&2 amd 3&4 to compress and retain a gasket at pressures exceeding 10,000 psi. (25,000 psi with a HC head, hot cam, etc.). Subtract from that web width the chamfer put on top of the cylinders to permit the rings to get in during assembly. A very conservative chamfer is about 0.020", reducing that web 0.020 x 2 = 0.040". Most engine builders knock off a sloppy 0.030 to 0.045" when chamfering, leaving you with less. So, a stock web is at most 0.335", and possibly as little as .285". Add (subtract!!) to this the "tolerance" deviations from the non-obtainable perfectly machined block and you may have even less.
Now it's overbore time. At 0.080 over, you are down to a web of 0.255 to 0.205. The original gaskets no longer fit, the pistons hit the overhang. Gasket makers respond by reducing the web, making the gasket good to 0.125" overbore. An original Ford gasket, and an original Fel-Pro 7013 had a gasket web between Siamese cylinders of 5/16". The new 7013 R3, and gaskets from other manufacturers reduce the web to 3/16", a 40% loss!! Gasket blowout problems begin. On the positive side, most blocks still have sufficient "meat" to get by without sleeves.
Go for broke time (life beyond 0.125 over): At 0.156 over (4 1/32") you can fit stock pistons from Ford and other apps if you are prepared to deal with non-stock piston pin heights, requiring either longer rods or a crank with longer throws (stroked). SOME blocks, only those with cores that were dead on center during casting, and almost no rust scale loss in the jackets, will go 0.156 over, leaving a nail-biting 0.120 wall in places, measurable with an ultrasonic casting thickness meter. No self-respecting NASCAR $75,000 engine builder is without one! Diesel shops have this tool too. Everyone else, it's a crap shoot.
But wait! What about the web thickness that holds the gasket between siamese cylinders? At 0.156 over you are down to 0.179 (Less than 3/16") with a conservative chamfer, scrap iron or sleeve time if you take more. Every head gasket made will have some overhang, the pistons need a top chamfer to prevent striking the gasket, and NOBODY that goes 0.156 over is building a stocker with a conservative 10,000 psi at full throttle. They have a hot build up with way more cylinder pressure. Goodbye gasket?? Racers often went with solid copper (leaks everywhere!) or sealer and NO gasket (leaks everywhere!). They don't care, it just needs to run once, today, then we drain the water. Everyone else with a water tight multi-layer gasket on a street engine is living on borrowed time before blowout. Light foot? Maybe years. Hot motor, heavy foot? Maybe hours.
Does an overbore run hotter? At 0.156 over the cylinder wall surface area increases 4.8%, and with no other changes the displacement increase will be a bit less than 10%. The surface area is the major factor. Henry's 28-29 rad probably had a 25% over capacity when new, the 30-31 likely a 35% over capacity margin. Henry's truck rads were probably 50% over. If an overbore makes you run hot, your cooling system is crapped out, it's not the overbore to blame. Increased compression and better breathing cams decrease the waste heat into the block, they are not a problem either.
Last edited:
craneygartz
Junior Member
The walls are nominally .250" thick at standard bore looking at the block drawing. I.E., the foundry cores were made to create an outside diameter of 4 3/8".
As mentioned above, the siamesed wall is shared by two cylinders and is only 3/8" thick with stock bore. (4.250 - 3.875 = .375")
John Oder
As mentioned above, the siamesed wall is shared by two cylinders and is only 3/8" thick with stock bore. (4.250 - 3.875 = .375")
John Oder
Chris in CT
Senior Member
Hi Old skool, The one thing that is missing from all of the excellent and scholarly information above is the fact that there are often irregularities in sand castings and cavities in cores, etc. One can never be sure until one gets too close to one of these (unless one has a UCTM, above). Want to stay old school? Bore up to 0.060 over, then start over with new sleeves... Good Luck!
Pete
BANNED
All good info but you forgot one thing Mike...When we go big bore we offset the bar so we only take just enough to clean on the web side...That width between cylinders stays pretty much the same.
No, the offset is not enough to cause rod alignment problems.
MikeK
Well-known member
Yep, I've seen it done with 0.050 offset saving 0.100 on each web when going to 100 over for an OHV. Beyond that no stock gaskets work, and you are outta wall on the wet side with most blocks. Pete, can you do it for .155 over and not blow out the water jacket? What do you use for a head gasket? Not too many guys are skilled enough to accurately offset a small-shop portable bar like a Van-Norman that bolts to the deck and is normally indexed off the existing bores. They have enough trouble getting them on center! This is where a big vertical mill setup where you can index is a better option.
Pete
BANNED
I have done several B engines to .156 and used the copper head gasket
with no problems...We have a Kwikway bar and just offset it with 3 feeler gauges..
I have a friend with a Pratt & Whitney jig borrer but I hesitate to think what the hourly rate would be for that machine even for me.
I have another friend that has a CNC program now for O-ringing a flathead V8 block...I haven't talked to him about doing a program for a B but I might...That would sure cure the head gasket problem even on mediocre
back yard rebuilds.
Mike V. Florida
Well-known member
What size do they bore to fit sleeves?
dave in australia
Well-known member
The largest factory pistons I've seen are .125". After that, I've seen sleeved blocks. I have one engine in my collection that has been sleeved and then bored a few times out to .080" again due to normal wear. That's a lot of milage on one engine.
Chris in CT
Senior Member
Hi Mike. I think the sleeves are 3/32nds in wall thickness, and the bore would have to be slightly undersize to allow for the press-fit, of course. But that is not the point. If one were to hit a void or gas cavity in the original casting, the sleeve would clearly cover such a void (unless it were extraordinarily large). Clearly, the model A block will allow for a larger bore than 0.060, but does that make such an overbore a good choice?
MikeK
Well-known member
Standard sleeves have a 3/32 (0.094) wall. The O.D. is 4.0625. You bore to 4.059 - 4.060 and leave a step at the bottom of the bore (optional, keeps the sleeve from slipping down too far). Shrink with a -20 freezer, dry ice, CO2 extinguisher or liquid nitrogen and drop/hammer in, FAST. There are also 1/8 and 5/32 thick sleeves that you can order if the bore is ripped up from removing the last sleeve.
The one disadvantage is they supposedly create a slight bit of a thermal hiccup, but some engines come new with sleeves. I know one shop that likes to scrub down the bores and sleeve O.D. with a copper sulfate & acid mix, leaving a flash copper plate on the walls before dropping in the sleeves. One advantage to sleeves in an A is the sleeve iron is harder than Henry's and can easily stand rings with higher tension.
A standard "A" sleeve bored 0.080 over will have a theoretical 0.054 wall. Subtract misalignment, tolerance, piston clearance, and you have less than 50 thou in places. Yikes!!! I would be nervous at 60 over, leaving a 1/16" wall. The siamese web with new standard sleeves? 3/32 sleeve + 3/16 remaining Henry iron + 3/32 sleeve = 3/8 (back to standard). Remaining Henry iron water jacket thickness? Theoretical 5/32 minus core shift, defects, corrosion.
Larry Brumfield
BANNED
I know of one well known engine rebuilder who sleeves every block whether they need it or not.
He leaves a lip at the bottom for the sleeve to rest on.
Larry B.
He leaves a lip at the bottom for the sleeve to rest on.
Larry B.
Deluxe Coupe Chris
Member
I am running .125 pistons in my Model A.
No problems so far and the engine runs really cool with a stock radiator.
No problems so far and the engine runs really cool with a stock radiator.
MikeK
Well-known member
I imagine the best practice would depend on what equipment you have. The few sleeves I've done started with a portable boring bar and I finished to the correct undersize with my Sunnen 815 hone and 240 stones. I can't imagine either gambling on being dead-on 3 under with a light, portable bar, nor 100% round. The finest (fixed) feed on the boring bar leaves a definite tool pattern that would not give 100% contact to a sleeve.
The industrial diesel shop I saw do the copper flash plate does not hone. They use negative rake, zero side clearance carbides in a huge, rigid CNC, leaving a shiny, burnished finish that smears the grain structure closed. They then deglaze it a bit with a stiff nylon-emery flap brush and water soluble before the copper. Perhaps to add tooth so the sleeve doesn't slip? They're not big at answering questions and touch nothing automotive. I was there a few years ago with a group of tractor guys, and of about a hundred tech questions, they side-stepped about 98.
IronTite is a water based mix of sodium montmorillinite, a sticky polar clay often used as a foundry green sand binder, and an acid salt of sodium silicate that reacts on contact with metal to release hydrated silica and bind up the clay into a rudimentary ceramic material. If anything, IronTite would add a thermal barrier rather than helping. Also, trying to get a sub-zero sleeve down a wet hole might be a challenge.
My 2c, if you wanted to experiment and fill the minute voids with a thermal conductor to increase the thermal contact, would be to thoroughly rub and burnish into the bore and sleeve a rather wet mix amalgam of powdered silver and mercury. This would, of course drive the O.S.H.A. guys and environmentalists nuts. I suppose you could try silver based thermal heat sink compound, but this would leave you with lube between the sleeve and block, and you don't want it to slip.