Merc 8CM 296 cu. in. cooling issues

[Curt in AZ]

I have a line on a fresh 8CM to replace the tired 8RT in my 53 F100. I'm wondering if this large bore - 3 5/16" - typically leads to overheating issues? Truck is stock including radiator.

[19Fordy]

It depends on so many factors, but in general thinner cylinder walls makes heat dissipation an issue depending on how you built the engine. Ambient temperature also is a factor. Although some folks say they have no cooling issues with 296'ers.

[svm99]

Did you mean 276cu in?

[Curt in AZ]

It is reportedly 296. I'm going to see it Thursday. We'll pull the heads and inspect things. I've not been around flatheads since the 1960s so any advice on what to look for will be appreciated.

[rotorwrench]

Cooling issues are affected by radiator condition and condition of the engine cylinder block & head castings in the water jacket areas. The engine can corrode from the inside out if someone didn't take care to add some corrosion protection to the coolant mix. Those old cylinder walls can get some deep pitting in the wet areas which can make them a lot thinner than they would be normally. Core shift anomalies from the original casting process can also give you some pretty thin cylinder walls. A 3 & 5/16" bore job can get to be too much in corroded or offset core blocks. A person should do some sonic testing of the cylinder walls before boring to the larger bore dimensions like 3 & 3/8" or 3 & 5/16".

[flatjack9]

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I've been running my engine with a 3 5/16" bore for over 20 years without a problem. This is with a stock 39 radiator and fan.

[Pete]

Originally Posted by carbonarc

I have a line on a fresh 8CM to replace the tired 8RT in my 53 F100. I'm wondering if this large bore - 3 5/16" - typically leads to overheating issues? Truck is stock including radiator.

If someone told you the engine you are looking at is 296 ci, you better do some closer research on it.
It would need a slightly less than 4 5/16 stroke for that and that is a very unlikely.
Aside from that, a big inch engine will cool fine if you start with an acid stripped block so there is NO rust or scale in the water jackets. Most of the engines we build are over 315 ci for vintage circle track racing and we have never had one that didn't cool.
Very few rebuilders have blocks stripped. The usual hot tank cleaning job will NOT remove scale and 60 year old rust.

[texas webb]

That's good info,Pete.Thanks.

[rotorwrench]

You can get to 296 cid by using a 4.125" stroker crank and 3 3/8" pistons so every part in there has to be close to the max. A 3 5/16" X 4" would be near 276 CID.

[Pete]

"You can get to 296 cid by using a 4.125" stroker crank and 3 3/8" pistons so every part in there has to be close to the max."

296 is nowhere near max. It is a VERY conservative size.
I am doing a 345 right now. Stock block configuration.

[RalphM]

Holy Cow! A 345 cubic inch flathead! I want to see this. What all machine work is involved? Details, details!

Just checked my CI calculator, a V8 engine that size would require a 3.5" bore and a 4.5" stroke! How are you doing this?

[19Fordy]

345 will bring it alive
6,7,8 will make it great
9,10,11 and it will be heaven
So, what is your magic formula for that monster?
Inquiring calulators want to know.

[rotorwrench]

Originally Posted by Pete

"You can get to 296 cid by using a 4.125" stroker crank and 3 3/8" pistons so every part in there has to be close to the max."

296 is nowhere near max. It is a VERY conservative size.
I am doing a 345 right now. Stock block configuration.

I fully well realize you can gut the block out with big cranks & overbored cylinder sleeves if you have the money. You can cast a new 350 cid block if you have enough money. BUT, for the average hot rodder back in the day, 3 3/8 pistons and an offset ground stroker crank was about the max he could afford and is still considered to be Max for the average hot rodder.

[40 Deluxe]

Originally Posted by Pete

"You can get to 296 cid by using a 4.125" stroker crank and 3 3/8" pistons so every part in there has to be close to the max."

296 is nowhere near max. It is a VERY conservative size.
I am doing a 345 right now. Stock block configuration.

Pete, I would think intake port size would limit peak RPM on a 345 cu. in. engine. Care to divulge the peak power (torque and HP) of this engine?
Thanks!

[Tim Ayers]

Originally Posted by Pete

"You can get to 296 cid by using a 4.125" stroker crank and 3 3/8" pistons so every part in there has to be close to the max."

296 is nowhere near max. It is a VERY conservative size.
I am doing a 345 right now. Stock block configuration.

That thing sounds like a beast.

I'm becoming really intrigued by big cubic inch small package motors.

Next car I do will get one of the those 427 aluminum SBC's. Yep, I said it. Those mills are friggin' out of this world....

[Pete]

3.405 bore.
4.75 stroke.
The customer asked me to not release any info on the engine, other than size.

[V8COOPMAN]

[QUOTE=Pete;1206829]3.405 bore.
4.75 stroke.
QUOTE]


346.026 inches, in a "stock block config"......interesting! DD

[RalphM]

Your building the ultimate flathead and you can't show us pictures?
You know your killing me!

[40 Deluxe]

Originally Posted by Pete

3.405 bore.
4.75 stroke.
The customer asked me to not release any info on the engine, other than size.

OK, thanks!

[Flathead]

4.75 stroke and the rods don't whack the cam?? Must be more to the story but I can understand if you don't care to share.