Maximum Bore on a Model A Cylinder

southfork

southfork

Senior Member
Many years ago when I was still wet behind the ears, I found a 1928 Model A engine at a Terry, Montana, service station. I bought it for $20 and decided to have it rebuilt. It was already bored to 80 over and had a slight ridge at the top of the cylinders, but I figured what-the-heck, just more displacement if I rebore it. The rebuilder I talked to was used to rebuilding Model A and T engines and he measured the bore and said it would have to go 120 over to clean up. I asked if that was too much and he said only that it could not be rebored after 120 over. Anyway I had him rebuild the short block and bore to 120 over. But today, I wonder if I am to put that short block into a Model A if it will get too hot all the time. I don't live in a real hot climate, but even so, I don't want to put the engine in a car if it will just overheat all the time. Can a 120 overbored Model A engine run cool enough? or if not, are their special fans to keep it cool enough under most circumstances?:confused:
 
Ford offered pistons up to .125" over. I have one engine in my collection at this size. After .125", the next action is to re-sleeve the block back to std. I also have one engine which has been re-sleeved and is now up to .080".
The piston sizes offered, at least in Australia, were std. .010", .020", .030",.040", .060", .080",.100" and .125".
 
About ten years ago I built an engine for my 28 RPU.It was .125 over,B camshaft,B dist.cam,original police head with the B on it,and an old JC Whitney two row radiator.On a 95 degree day I could leave that truck idling,with the spark retarded,for an hour with no problems.The older rebuilders around here told me the big bore was no problem as long as I had a good cooling system.The armchair experts told me I would always have problems.I had no idea what the anti-freeze mix was,I bring barrels of it home from the junkyard and use it in old machinery and old cars.I really had doubts about the cheapo radiator,but it was clean so I figured I would use it until I found a better one.It is still on there today.
 
I also built a .125 engine with Brumfield head, touring cam, full insert bearings, standard oil delivery....
It works fine...I have about 60,000 miles on it now and no problems. Runs cool at normal "A" speeds.
 
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Just for a FYI, I (-and many others too) have succussfully taken them out to 4.030" ...which equate to .155" overbore without heating issues. While off-the-shelf pistons generally stop at .125", a 4" piston is the same size as a .125", and so adding a .040" overbore to that 4.000" piston gives you something to work with if one desire to go that big. While an added expense, have your rebuilder install sleeves and start over again at Std bore size.
 
I too have built several engines with 4.030 bore.
I offset the bores when I do to keep the area betwen the bores wide as possible.
 
I have a set of the old "semi" pistons that could be turned down in a lathe to any size desired, I think down to 0.080 over. They measure 0.140 oversize out of the box. Has anybody heard of running these at such an oversize, assuming you can find some rings to fit?
 
I've run 4.030 and 4.040 with a champher on the top to keep from possibly hitting the
head gasket.
 
They used to race these engines at 4.060" with no problems caused by the big bore. You could also buy big-bore, big-valve head gaskets. Today 4" is the max that I recommend.
 
I have also bored them to 4.030. The over size bore cool better, because the heat does not have to travel so far through the metal to the water. The tail about getting hot came from plugged radiators. Any kind of a rebuild would make them get hot.
 
Tech question: what is the theory behind the notion that thinner walls cause overheating, and is this overheating of the oil or just a higher observed water/coolant temperature?

Just curious.
 
I've got a set of new old replacement semi pistons like Ursus mentioned above. I don't think that the skirts are suppose to be exactly round, I may be wrong. I have wondered how to size them. I remember older guys, back when I was young talk about rasping pistons down. I wish I had asked more questions back then. Does anybody know how its done ?
 
Rick Finsta said:
Tech question: what is the theory behind the notion that thinner walls cause overheating, and is this overheating of the oil or just a higher observed water/coolant temperature?

Just curious.
Click to expand...

Boyle's laws of heat transfer say thin walls are fine.
 
Purdy Swoft said:
I've got a set of new old replacement semi pistons like Ursus mentioned above. I don't think that the skirts are suppose to be exactly round, I may be wrong. I have wondered how to size them. I remember older guys, back when I was young talk about rasping pistons down. I wish I had asked more questions back then. Does anybody know how its done ?
Click to expand...

You need a piston grinder. It uses a master cam somewhat like a valve cam grinder. Piston cam grinds were denoted as "A", "B", "C" and so on depending on the ammount of cam.
 
I questioned that when I first started driving my 125 overbore engine, but after a few hundred miles it was broken in and now runs at a steady 160 with a thermostat.
 
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