The Ford Barn

Randall · 10-16-2023, 07:24 AM

Rebuilding a 239 mercury what’s the most I can bore and be safe ? just want a good street motor with a good cam

Tim Ayers · 10-16-2023, 09:22 AM

It would ill-advised to answer this question without more information. I would suggest getting the bores sonic tested so you know how much material you have to start with.

Once this information is known, you can make a judgement on the new bore size. Some may say, just bore enough to clean it up, some may suggest bore it .125 over to make a 276 with a 4" crank and others may say go as big as possible.

Until you know the thickness and condition of the bores, I would not do any boring.

In the meantime, try to get the block's water jackets as clean as possible to aid in the sonic testing. You want to avoid a false reading by having a thick mass packed in around the cylinder wall.

There are numerous threads on how to do this and all of them are messy jobs that even someone with little mechanical ability can take on at home.

19Fordy · 10-16-2023, 09:35 AM

Do as Tim suggested as good blocks are getting rare. Then bore it as little as possible and install the Merc crank and correct pistons as needed. You won't notice any big increase in performance by going with a BIG bore and leaving everything else the same. Plus, heat dissapation is better with a smaller bore.

Ol' Ron · 10-16-2023, 12:21 PM

The application of the car/engine is important here. The use of a 4" crank and say a .060 bore would increase your CR a point or more this will incrase you low end torque. A mild cam will increase where the torque is in the RPM range. most of this can be done with a single 1 1/16 carb. Unfortunally, This is an expensive rebuild in todays market any cam alone will cost over 500 bucks with lifters and the crank and pistonsd another grand or more. Sooooooo Think of what it;s for
Gramps

19Fordy · 10-16-2023, 01:10 PM

Ron makes some great points. The more you modify, the more it costs - a lot more.
One change entails something else changing.

Tim Ayers · 10-16-2023, 01:24 PM

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I'm going to offer a counter point. You only get one time around, so if you are able to, make it the best motor your wallet can afford.

here is some more counter point. A brand new cam is indeed $400-500 depending. A reground stock cam can be done all day long for $125-$150. Find some used Johnson's and get them resurfaced for another $100 for the cam grinder.

Quality hi-flow, stainless Chevy 1.5-1.6 valves can be purchased for approximately 1/2 of what the flathead versions cost.

Keep your eyes open on good deals and jump on them when you see them.

Few things draw as much attention in a crowd than a well built and great sounding flathead.

Not sure what car this motor will be going in to, but I would not change a thing on my big bore, big cammed motor that was built for my roadster. If anything, I may add a 4X2 intake to replace the 2X2. It's a blast to drive.

Others may disagree, but you need to build the motor the way it will make you happy.

Get some more info on the condition of the block and we can help you start to narrow down some options for your build.

tubman · 10-16-2023, 02:04 PM

I'm a big believer in a 1/8" overbore on a flathead that will take it. I believe the early Mercury's are in that crowd. For the cost of a set of pistons, you get 258 ci (3 more than a Merc crank). If Ford thought a 255 was enough for a heavier car, 258 should be a good improvement for a stocker. I have done 3 engines (all 8BA's) with the 3 5/16" bore, and all had plenty of cylinder wall left. Let's face it, with the way these cars are used these days, only an infinitesimally small number of them will ever need a rebore. Sure, a few may need rings down the road, but that can be done that with just a hone job.

If you need pistons anyway, why not get the extra 19 cubes for essentially nothing?

Pete · 10-16-2023, 02:52 PM

Get the block either wet or powder magged FIRST.
If it checks ok, then have it chemical stripped even if you have to ship it somewhere and back.
That is the ONLY thing that will get all of the SCALE and rust out of the water jackets.
As Tim said, you can NOT get an accurate ultrasonic test unless there is bare metal in the water jacket.
The final bore size should be determined by the available cylinder wall thickness. You should not go any thinner than .100.
No matter what size you determine, the cost should be the same in a reputable shop.
It is a 2 cut job with a boring bar. If they use a power hone like a CK10, they might charge a bit more but they shouldn't because the after honing is not required.
You do NOT have to go with standard round number bore sizes. Pistons can be made in any size. It only takes .010 to clean for a 75K mile rebuild. The more cubic inches the more pleasant the engine is to drive.
Be careful not to "overcam" for a street engine. Engine size will fairly well determine the cam specs. Try and stay away from ANY adjustable lifters these days. Use the stock STEEL ones even if you have to spend some time looking for them.

Randall · 10-16-2023, 05:26 PM

Thanks for the info , Its 040 over now and the walls look good so probably 060 will clean,I will be doing all machine work except grinding crank, lots of experience with A abd T engines but not a lot with v8 flatheads , when you say chemical clean are you talking hot tank or some other chemical? Its going in a 46 coupe, what cam do you all recommend for a driver?
I've read some use the 8n lifters are they no good?

Bored&Stroked · 10-16-2023, 08:07 PM

Pete is talking about a chemical stripper that will remove the rust/scale from the water jackets. I used to go to RediStrip - but they seem to be out of business in the places I used them before. I recently had a block backed, shot-peened, and tumbled . . . this turned out really good - but you have to make DANG SURE there is not shot-peen material left in the block. Are you going to be putting more carb than stock on the engine? What other performance mods? Without knowing more, I'd probably run an Isky 1007B grind on it. Pete can regrind the stock cam for you - he does excellent work.

The best way to do the lifters is to use stock lifters and do the valve job the way that Ford did it (non-adjustable). I'd probably use SBC length valves - will probably give you enough material to grind the stems to set the clearance.

tubman · 10-16-2023, 08:36 PM

The cam you run depends a lot on the car you are running the engine in. When I put the '51 Merc in my '51 Ford coupe, I had an ISKY MAX-1 that I was going to use. After research and some conversations here and on the H.A.M.B., I decided to keep the 8CM cam that was in the engine, mainly because of the weight of the car and it's intended use. I certainly wouldn't have done that if it was going in an 1800 lb roadster. (The MAX-1 is now in the 258 ci engine going into my "T" tub.)

10-16-2023, 07:24 AM	#1
Randall Senior Member Join Date: Nov 2010 Location: Lambsburg Va Posts: 296	239 Mercury Rebuilding a 239 mercury what’s the most I can bore and be safe ? just want a good street motor with a good cam

10-16-2023, 09:22 AM	#2
Tim Ayers Senior Member Join Date: May 2010 Location: NJ Posts: 6,181	Re: 239 Mercury It would ill-advised to answer this question without more information. I would suggest getting the bores sonic tested so you know how much material you have to start with. Once this information is known, you can make a judgement on the new bore size. Some may say, just bore enough to clean it up, some may suggest bore it .125 over to make a 276 with a 4" crank and others may say go as big as possible. Until you know the thickness and condition of the bores, I would not do any boring. In the meantime, try to get the block's water jackets as clean as possible to aid in the sonic testing. You want to avoid a false reading by having a thick mass packed in around the cylinder wall. There are numerous threads on how to do this and all of them are messy jobs that even someone with little mechanical ability can take on at home. Last edited by Tim Ayers; 10-16-2023 at 09:56 AM.

10-16-2023, 09:35 AM	#3
19Fordy Senior Member Join Date: May 2010 Location: Coral Springs FL Posts: 10,950	Re: 239 Mercury Do as Tim suggested as good blocks are getting rare. Then bore it as little as possible and install the Merc crank and correct pistons as needed. You won't notice any big increase in performance by going with a BIG bore and leaving everything else the same. Plus, heat dissapation is better with a smaller bore.

10-16-2023, 12:21 PM	#4
Ol' Ron Senior Member Join Date: May 2010 Location: Chester Vt Posts: 8,861	Re: 239 Mercury The application of the car/engine is important here. The use of a 4" crank and say a .060 bore would increase your CR a point or more this will incrase you low end torque. A mild cam will increase where the torque is in the RPM range. most of this can be done with a single 1 1/16 carb. Unfortunally, This is an expensive rebuild in todays market any cam alone will cost over 500 bucks with lifters and the crank and pistonsd another grand or more. Sooooooo Think of what it;s for Gramps

10-16-2023, 01:10 PM	#5
19Fordy Senior Member Join Date: May 2010 Location: Coral Springs FL Posts: 10,950	Re: 239 Mercury Ron makes some great points. The more you modify, the more it costs - a lot more. One change entails something else changing.

10-16-2023, 01:24 PM	#6
Tim Ayers Senior Member Join Date: May 2010 Location: NJ Posts: 6,181	Re: 239 Mercury Sponsored Links (Register now to hide all advertisements) I'm going to offer a counter point. You only get one time around, so if you are able to, make it the best motor your wallet can afford. here is some more counter point. A brand new cam is indeed $400-500 depending. A reground stock cam can be done all day long for $125-$150. Find some used Johnson's and get them resurfaced for another $100 for the cam grinder. Quality hi-flow, stainless Chevy 1.5-1.6 valves can be purchased for approximately 1/2 of what the flathead versions cost. Keep your eyes open on good deals and jump on them when you see them. Few things draw as much attention in a crowd than a well built and great sounding flathead. Not sure what car this motor will be going in to, but I would not change a thing on my big bore, big cammed motor that was built for my roadster. If anything, I may add a 4X2 intake to replace the 2X2. It's a blast to drive. Others may disagree, but you need to build the motor the way it will make you happy. Get some more info on the condition of the block and we can help you start to narrow down some options for your build. Last edited by Tim Ayers; 10-16-2023 at 01:48 PM.

10-16-2023, 02:04 PM	#7
tubman Senior Member Join Date: Dec 2013 Location: Minnesota, Florida Keys Posts: 10,320	Re: 239 Mercury I'm a big believer in a 1/8" overbore on a flathead that will take it. I believe the early Mercury's are in that crowd. For the cost of a set of pistons, you get 258 ci (3 more than a Merc crank). If Ford thought a 255 was enough for a heavier car, 258 should be a good improvement for a stocker. I have done 3 engines (all 8BA's) with the 3 5/16" bore, and all had plenty of cylinder wall left. Let's face it, with the way these cars are used these days, only an infinitesimally small number of them will ever need a rebore. Sure, a few may need rings down the road, but that can be done that with just a hone job. If you need pistons anyway, why not get the extra 19 cubes for essentially nothing?

10-16-2023, 02:52 PM	#8
Pete Senior Member Join Date: May 2010 Location: Wa. Posts: 5,409	Re: 239 Mercury Get the block either wet or powder magged FIRST. If it checks ok, then have it chemical stripped even if you have to ship it somewhere and back. That is the ONLY thing that will get all of the SCALE and rust out of the water jackets. As Tim said, you can NOT get an accurate ultrasonic test unless there is bare metal in the water jacket. The final bore size should be determined by the available cylinder wall thickness. You should not go any thinner than .100. No matter what size you determine, the cost should be the same in a reputable shop. It is a 2 cut job with a boring bar. If they use a power hone like a CK10, they might charge a bit more but they shouldn't because the after honing is not required. You do NOT have to go with standard round number bore sizes. Pistons can be made in any size. It only takes .010 to clean for a 75K mile rebuild. The more cubic inches the more pleasant the engine is to drive. Be careful not to "overcam" for a street engine. Engine size will fairly well determine the cam specs. Try and stay away from ANY adjustable lifters these days. Use the stock STEEL ones even if you have to spend some time looking for them.

10-16-2023, 05:26 PM	#9
Randall Senior Member Join Date: Nov 2010 Location: Lambsburg Va Posts: 296	Re: 239 Mercury Thanks for the info , Its 040 over now and the walls look good so probably 060 will clean,I will be doing all machine work except grinding crank, lots of experience with A abd T engines but not a lot with v8 flatheads , when you say chemical clean are you talking hot tank or some other chemical? Its going in a 46 coupe, what cam do you all recommend for a driver? I've read some use the 8n lifters are they no good?

10-16-2023, 08:07 PM	#10
Bored&Stroked Senior Member Join Date: Jan 2011 Location: Central Ohio Posts: 5,070	Re: 239 Mercury Pete is talking about a chemical stripper that will remove the rust/scale from the water jackets. I used to go to RediStrip - but they seem to be out of business in the places I used them before. I recently had a block backed, shot-peened, and tumbled . . . this turned out really good - but you have to make DANG SURE there is not shot-peen material left in the block. Are you going to be putting more carb than stock on the engine? What other performance mods? Without knowing more, I'd probably run an Isky 1007B grind on it. Pete can regrind the stock cam for you - he does excellent work. The best way to do the lifters is to use stock lifters and do the valve job the way that Ford did it (non-adjustable). I'd probably use SBC length valves - will probably give you enough material to grind the stems to set the clearance.

10-16-2023, 08:36 PM	#11
tubman Senior Member Join Date: Dec 2013 Location: Minnesota, Florida Keys Posts: 10,320	Re: 239 Mercury The cam you run depends a lot on the car you are running the engine in. When I put the '51 Merc in my '51 Ford coupe, I had an ISKY MAX-1 that I was going to use. After research and some conversations here and on the H.A.M.B., I decided to keep the 8CM cam that was in the engine, mainly because of the weight of the car and it's intended use. I certainly wouldn't have done that if it was going in an 1800 lb roadster. (The MAX-1 is now in the 258 ci engine going into my "T" tub.)