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Discussion Starter #1
I have what was a mostly original 1966 GT convertible "A" code that I took delivery on last June and just got done having a higher level restoration performed. In the process, we determined that the Borg Warner T10 was trashed and was replaced with a rebuilt period correct, not date code correct though, Top Loader. When we took delivery, we determined that the engine was running very rich, leaving heavy carbon deposits anywhere that it idled. After the rebuild of the carb, we found a loose, unrelated to the carb, ball bearing in the carb and also the secondaries were most likely hanging open, probably washing the cylinders in fuel. The engine oil actually smelled like raw fuel. We replaced that with a Holley Street Warrior and shelved the 4100 neatly away. The PO honed the cylinders, replaced the pistons, rings, honed the cylinders and replaced the bearings. Fast forward to now, we can see that the engine is consuming a lot of oil. About 3oz every 60 or so miles driven. I am sending it for a compression and leakdown test next week, but feel that we may have to replace or rework the short block once the shop reports back. So, most likely decision time in a week or so.

While we have done some period correct modifications with the transmission and saved the original carb, do we save the original block with 77k miles and bore it risking cooling problems or just move onto a crate short block for reliability? Has anyone else faced these decisions and what did you end up doing?

Just looking for some thoughts based on your personal experiences and how it worked out for you.
 

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I was in a similar situation with my 68's 289. Consuming tons of oil, smoking, issues staying cool. Had it looked at and with all the machine work it would need plus reassembly (not doing it myself) it was gonna be near the price of a crate long block, so I got the Blueprint Engines 302, 300 HP engine with warranty. I love it, it runs great, it hasn't had any issues or used a drop of oil in 4k miles.

I will note that the 289 was not original or date code correct in mine and I am not worried about those types of things with my car, just the visual appearance of period correct.
 

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OK, first the trans. It is extremely likely that the T10 is original to the car. Ford was running short of Ford Design transmissions, and many A code Mustangs with 4-speed and 3.00:1 axles were equipped with a unique 1966 Mustang T10. The Ford Design was a better transmission, though.

Have your 4100 rebuilt professionally. It is superior to the Holley in every way.

If the cylinders are beyond limits, bore to oversize and use the block you have. However, with only 77,000 miles on it, I'd be amazed if the cylinders were worn beyond limits. More likely the repair was not done well. Have you checked the valve stem seals? I've replaced them in numerous engines and virtually stopped oil consumption.

My 65 289HP has 400,000 miles on it, was rebuilt at about 200,000. So, it now has about 200,000 miles on a 30-over rebuild, and uses less than a quart of oil every 3,000 miles.
 

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WeGreg,

agree the t-10 could have been original to your car.
your oil burn rate is just under 1qt per tank of fuel.
no need to run leak down given that, it obviously will leak like a sieve.
compression check it a simple process, tool can be rented from most auto parts stores if need be.
1-remove all spark plugs
2-crank motor over to test and record each cylinders psi dry
3-squirt oil in each cylinder and retest and record each cylinders psi.

if all cylinder compression readings, wet vs dry, are similar your valves are the likely weak link.
if all the cylinder compression readinding improve, wet vs dry, your rings and or cylinder walls are the weak link(s).

77k is not a lot of miles for your block.
You seem to prefer keeping it as original as possible so no harm in pulling the motor and disassembling it to see what you have. If it is a standard bore and all cylinders would clean up with a 0.020 or 0.030 bore and hone I would vote for that.
Also gives you a chance to select a better piston/ring pack and compression ratio bump.
Just be certain the shop that does the block work:
1) checks your mains and if needed line hones them on the main cap side.
2) then squares the block off the mains.
3) USES a torque plate when boring your cylinders.
4) consider having them balance the bottom end, not that much additional money but makes for a much happier motor.

my .02.

good luck
 

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I agree with posts above, keep your original block and fix as needed. However if you do decide to change the original block, please contact me as I am looking for an original block. Please keep us posted with details as you go along.
Good luck!
 

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LS swap!!! :eek: :p
I’d try to keep every single core possible even if the cost exceeded buying a new crate. Unless the block is cracked it should be able to be rebuilt for less than the cost of a new crate. I second the call for rebuilding or having the 4100 rebuilt as well. If you’ve gone to the trouble of having a good restoration the value could potentially be compromised by a new engine if someone is looking for a collector piece.

A properly rebuilt short block vs. crate won’t be a reliability issue. Reliability on these is largely the ignition and fuel systems. That is fairly easily solved by new and rebuilt components and doing the old school maintenance/tune up procedures. With good parts and upkeep these are dead nuts reliable cars just like they were when everyone used them as daily drivers. The combination of old/worn parts and unfamiliarity of working on them is what causes the reliability and some drivability issues. These days points/carbs/timing is pretty much a lost art. Though it does give us old guys a bit of hope we’re still superior to these kids even if we can’t remember what day it is.
 

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Discussion Starter #8
The 4100 was professionally rebuilt by a very reputable Mustang restoration shop after I took delivery of the car. The Holley just runs better and I boxed the 4100 up to save. The T-10 was trashed beyond repair when I got the car and we had multiple Mustang speciality shops trying to track down the parts for it with no luck, so that door stop is gone. It was not a normal T10 according to several of the suppliers. Sadly, this car sat at a consignment shop for 6 months before I purchased it and who knows what they did to it while it was there. I do not believe that the PO with their excellent reputation would have placed a loose ball bearing in the carburetor body, nor would they have let this run that rich.

I am torn on swapping out the original block for the crate, but do have concerns over possible cooling issues once bored out. I am still thinking that may be the way as I would like to keep the engine as close to stock as possible.
 

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Discussion Starter #10
OK, first the trans. It is extremely likely that the T10 is original to the car. Ford was running short of Ford Design transmissions, and many A code Mustangs with 4-speed and 3.00:1 axles were equipped with a unique 1966 Mustang T10. The Ford Design was a better transmission, though.

Have your 4100 rebuilt professionally. It is superior to the Holley in every way.

If the cylinders are beyond limits, bore to oversize and use the block you have. However, with only 77,000 miles on it, I'd be amazed if the cylinders were worn beyond limits. More likely the repair was not done well. Have you checked the valve stem seals? I've replaced them in numerous engines and virtually stopped oil consumption.

My 65 289HP has 400,000 miles on it, was rebuilt at about 200,000. So, it now has about 200,000 miles on a 30-over rebuild, and uses less than a quart of oil every 3,000 miles.
I did just have the heads pulled, checked, valves lapped and new seals. Worth noting, I had the former leaks in the manifold and oil pan gaskets repaired also and confirmed PO former work was done. We did not mic the bores as we didn't realize that we had an issue. The only thought is that the rich carb was washing the cylinders in fuel. I am seeing so little smoke out of this that I am shaking my head and asking where the hell that amount of oil is going. And the antifreeze is a beautiful green, so it isn't heading through the head gaskets. The pipes are dry and no oil hitting my hands in the exhaust.

I am going to have the compression test and leak down done next Monday and listen to the shop's advice. They are a classic shop that does nice engine work. If the block can be saved, I am going to spend the time and money to do it at this point.
 

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Discussion Starter #11
Thank you all for your thoughts on this. I value the intelligent and knowledgeable feedback and opinions. Given the general consensus of the group to preserve the original block and my want to maintain as much of the originality of the car, I will run this through the leak down and compression test next week and, contingent upon the outcome of that test, will work with the original block and preserve the condition of the car. It isn't at all about the cost of one option over the other as I will happily and willingly spend more to preserve the original block and car when feasible. Thank you again for your feedback.
 

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Thank you all for your thoughts on this. I value the intelligent and knowledgeable feedback and opinions. Given the general consensus of the group to preserve the original block and my want to maintain as much of the originality of the car, I will run this through the leak down and compression test next week and, contingent upon the outcome of that test, will work with the original block and preserve the condition of the car. It isn't at all about the cost of one option over the other as I will happily and willingly spend more to preserve the original block and car when feasible. Thank you again for your feedback.
We Greg,

neat part of this hobby is we are all generally coming from similar point of reference. we don’t own these cars but rather are the ccaretakers of them for a time. I like many others I have learned a great deal from others and only giving forward as was done for me Over the years. So enjoy the rebuilding for your 289 and passing along what you learned.

good luck, have fun.

Pat
 

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The 4100 was professionally rebuilt by a very reputable Mustang restoration shop after I took delivery of the car.
Either there is something seriously wrong with it, or they botched it. Maybe someone messed with it after that. What carb number is it? Not the tag, the stamp on the flange.

I do not believe that the PO with their excellent reputation would have placed a loose ball bearing in the carburetor body, nor would they have let this run that rich.
I rest my case. Someone has royally screwed with the 4100. If you were to personally rebuild it straight out of the Ford manual, it should run perfectly. Assuming, of course, it isn't some Thunderbird reman that's all wrong for the car.

The Holley just runs better and I boxed the 4100 up to save.
I've spent a lot of time on Holleys. They lack the annular discharge and one piece main body of the 4100. Better fuel dispersion for more power and better economy are built into the design. The 4100 doesn't leak- In time, your Holley will. It's what they do. Your Holley runs better because it's new, and hasn't been screwed with by whomever messed up your Autolite. It's actually difficult to get a 4100 to run as rich as you describe, even on purpose. Mine gets over 21mpg highway, can't do that running rich. Best I ever got when I was using Holleys was 18mpg.

The T-10 was trashed beyond repair when I got the car and we had multiple Mustang specialty shops trying to track down the parts for it with no luck, so that door stop is gone. It was not a normal T10 according to several of the suppliers. Sadly, this car sat at a consignment shop for 6 months before I purchased it and who knows what they did to it while it was there.
The 1966 Mustang T10 was unique to 1966 Mustangs, in fact was only used on cars with the 3.00:1 rear axle. And yes, it is not a "normal" T10. A restoration shop near here just pulled the same trans out of a 66 GT. After tracking down innumerable NOS parts for it, they determined that it would cost more to fix than replacing it with a Ford Design 4-speed. The car was delivered to the owner with a Fresh Ford Design trans, and the T10 in a box. The Ford Design is superior, disposing of the T10 is no mistake on your part.

I am torn on swapping out the original block for the crate, but do have concerns over possible cooling issues once bored out. I am still thinking that may be the way as I would like to keep the engine as close to stock as possible.
My 289HP is bored 0.030" and I have air conditioning. When my original C6ZZ 3-row radiator gave up, I put in an aftermarket brass 3-row. I also have the factory 7-blade thermal clutch fan and shroud. No overheating problems at all.

"Crate Motors" are not new. They are somebody else's worn-out engine rebuilt, and almost always are bored out. I've never seen one that wasn't, boring is pretty standard for crates. You can do that to your engine, and end up with originality a "crate" could never have.
 

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Discussion Starter #14
Just a brief follow-up on this for all that took the time to comment and provide good feedback. Thank you, it was very helpful and contributed to me staying focused on preserving the original car. First, the car continues to go through oil at a higher rate, .14 ounces per mile now. We have had the compression and leakdown tests performed. We found that plugs #1- #7 were a perfect ash grey with a nice burn on them, no oil residue. Plug #8 had a very minor residual oil burn on it. I wasn't that concerned with it. The compression test netted an even 150 lbs of compression across all eight cylinders. My readings from a year ago with my test set and gauges netted 145 lbs across all 8 cylinders with no real variances. I think that both sets of readings are pretty consistent and in alignment with the thought that the block has good compression. The leakdown test netted a variance of between 15%-18% across all of the cylinders. Not great, but not horrible either. Soooo, we were kind of leaning into the heads and thinking that the oil consumption issue might be there, but look back at the clean plugs and shake our heads bewildered.

We pulled the PCV valve while the car was running and she looked like Thomas the Tank Engine at full steam, forcefully chugging and blowing blue oil smoke out of the PCV valve port on the PS valve cover. Hmmmmmmmmm, now we are wondering about the rings again and possible blowby into the crankcase. Again, those plugs leave me shaking my head.

We are in the process of maybe moving to a wet compression test just to confirm blow by, but are also at the point where we are weighing just pulling the engine and having it rebuilt while preserving the original factory long block on the car.

In any event, looks like this little red GT is back in the garage for at least a month. I will check back with you all when we have a little more guidance on our direction. Thank you again for all of your past comments and suggestions.
 

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The 1966 Mustang T10 was unique to 1966 Mustangs, in fact was only used on cars with the 3.00:1 rear axle. And yes, it is not a "normal" T10. A restoration shop near here just pulled the same trans out of a 66 GT. After tracking down innumerable NOS parts for it, they determined that it would cost more to fix than replacing it with a Ford Design 4-speed. The car was delivered to the owner with a Fresh Ford Design trans, and the T10 in a box. The Ford Design is superior, disposing of the T10 is no mistake on your part.
Really? While it has been decades, I was sure my buddy replaced an aluminum cased T10 in his '66 Shelby with a top loader. I believe they used that same transmission in the 65 Shelby as well. That said, no question the top loader is a better transmission.
 

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If you have to have it bored, PLEASE have it LINE Bored. (some people call it align bored). Short version, when the original engine block was bored, there may have been some slop in lining up the borers, ie, between 1 and 2 may be thinner than between 2 and 3 as borer number 2 was a little forward. If the rebore just follows the original holes, then the wall gets even thinner.

If the true centers are located and rebored to the true center, it lines up the holes, and eliminates SOME chance of the thin area. A 030 overbore true center may have less of a thin wall than a 020 rebore using the existing holes.
 

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The leakdown test netted a variance of between 15%-18% across all of the cylinders.
So the cylinders are pretty constant. That is good.

At what pressure was the leak down test done? In aviation circles the leak down is done at 80 PSI, hence when I do a LD test on my 289 I choose 80 PSI. The leak down test is reported as 79 (leak) over 80 (pressure) (79/80) for a great cylinder and 60/80 for a leaky cylinder. Where is the leak? that is the question. That is what a leak down test is for. To locate the leak!

The leak down test is done to pinpoint where the cylinder is leaking, hence the reason for a leak down test. If the cylinder is leaking past the rings you'll hear the leak via the oil filler tube. If it leaks by he intake valves you'll hear it at the intake. If the leak is at an exhaust valve, well you'll hear the leak in the exhaust.

Whomever performed the leak down test should have told you where the leaks were occurring. If not, they have no clue on how to perform or diagnose a leak down test. Find a different guy.

You also mention a lot of blow through the PCV valve port. Could be normal. How are the baffles in the valve covers? Are they present? No baffles and PCV can lead to an oil burner.
 

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Wegregory, before you consider doing anything to the shortblock, what are the valve covers like ? It could well be that you don't have the PCV baffles that you should. You may want to invest in an oil / air separator. Some of the imports and turbo cars have them. Note, you DO NOT want a 'catch can', you want a separator that drains oil back into the pan, and blowby into the PCV. Side question, what oil are you running ? My 69 302, with 171K miles, burned a quart per 50 miles of 10W-30. If filled with 20W-50, it burned a quart in about 200 miles, still too much, but what a difference ! LSG
 

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Discussion Starter #19
Really? While it has been decades, I was sure my buddy replaced an aluminum cased T10 in his '66 Shelby with a top loader. I believe they used that same transmission in the 65 Shelby as well. That said, no question the top loader is a better transmission.
So the cylinders are pretty constant. That is good.

At what pressure was the leak down test done? In aviation circles the leak down is done at 80 PSI, hence when I do a LD test on my 289 I choose 80 PSI. The leak down test is reported as 79 (leak) over 80 (pressure) (79/80) for a great cylinder and 60/80 for a leaky cylinder. Where is the leak? that is the question. That is what a leak down test is for. To locate the leak!

The leak down test is done to pinpoint where the cylinder is leaking, hence the reason for a leak down test. If the cylinder is leaking past the rings you'll hear the leak via the oil filler tube. If it leaks by he intake valves you'll hear it at the intake. If the leak is at an exhaust valve, well you'll hear the leak in the exhaust.

Whomever performed the leak down test should have told you where the leaks were occurring. If not, they have no clue on how to perform or diagnose a leak down test. Find a different guy.

You also mention a lot of blow through the PCV valve port. Could be normal. How are the baffles in the valve covers? Are they present? No baffles and PCV can lead to an oil burner.
The leak down test was done properly and the covers have baffles. It was a combination leak that I didn't feel like getting into here. We are performing the wet compression to confirm rings and waiting for that to figure next steps. At this point, it looks like we are leaking through the rings and into the crankcase. There is also some valve bypass, but the case seems to be getting the majority.
 
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