4.6 Fix: Rebuilding Your Trashed Mod Motor

Horse Sense: As '99-'04 Mustang GTs ageand become affordable starting material for hot-rodders, they willsuffer more and more bottom-end failures. Standard production materialsare fine for making 300 rear-wheel horsepower, but once you add a blowerto these cars, you're on borrowed time.

Our repair of a broken 4.6 GT engine started with a call to ProPower.They shipped out an affordable rebuild kit that features forged rods,high-quality Mahle pistons (0.020-inch over), and all the necessarybearings and rings to get this thing back up and running. For thisstory, Paul's Automotive Engineering handled all machine work andassembly.

Major engine failure happens to the best of us--usually when we leastexpect it. Recently, as we began a technical feature on a supercharged'00 GT, the worst happened: major bottom-end failure in our stock 4.6short-block. We could have called one of our friends, popped in a crateengine (or a replacement short-block), and been on our way. But instead,we decided to let our readers learn from our mistake. Knowing that manya supercharged stock modular Ford has tossed its cookies duringbig-boost applications, we started hunting for an affordable repair thatwould allow us to get back in the game as quickly as possible.

Luckily, Paul's Automotive Engineering (a frequent stop on our testingand evaluation circuit) has a complete in-house machine shop facilitymanned by Steve Barker, an accomplished engine builder. Paul's cranksout all sorts of hot Ford iron, in addition to repairing 4.6 modulars ona regular basis, so this was a routine engine project for them.

Pop goes the piston! One too many rattles of detonation, over-revving the engine, or high cylinder pressure, and this is what you get- a major bottom-end failure from the Two-Valve 4.6 engine. our supercharged '00 GT lost two pistons when the rings broke and the ring lands let go. But we're lucky we got it before it ate up the cylinder wall of the block or let a flame front rip through a rod.

Evaluating the damage, we found terminal piston failure due to elevatedcombustion pressures and perhaps a slight over-rev on the owner's part.The old 5.0 engines could take much more than the modular engines, whichare saddled with hypereutectic (or hyper-pathetic as they've becomeknown) pistons and cracked-cap, powdered-metal rods. The new stuff cango at any time. In all, we lost two piston rings, with the ring landsgetting shattered shortly after. Surprisingly, the car ran OK until wegot aggressive with the throttle (and boost), then we knew something waswrong. So, at a minimum, we needed new pistons and stronger rods.

Enter Dale Metlika from ProPower and his many different modular-enginekits. He's seen dozens of engine failures just like ours, and hiscompany has designed kits for all sorts of applications. If you want tobuild something that will take 1,000 hp, Dale has you covered. If, onthe other hand, you want to simply get your modular Mustang backtogether with some stronger parts for the least amount of money, you'rereading the right story.

With our parts in hand, our test car's owner began removing the engine.This is a labor-intensive task, but it can save a bunch of money. Dropoff the engine at your chosen machine shop, then pick it up after thework has been done. If you feel adventurous, you can strip theaccessories, cams, and heads from the engine. But, unless you know yourway around a modular engine, we'd leave that to the experts like thoseat Paul's Automotive Engineering. This is an account of what it took toget this once-damaged short-block back up and running.

ProPower supplied us a set of forged Mahle pistons in 0.020-inch over(3.572 inch) size. This necessitated honing the block, but it ensuredgood ring seal.

Check out this dramatic comparison shot of the old rod and the newforged-steel units we sourced from ProPower. With a good tune and solidpistons up top, theses rods will swing happily up to 650 hp. Remember,the Two-Valve 4.6 engines have a strong, cast-steel crank and a heartyblock, so once you fix the weaker rods and pistons you'll be in goodshape. We also ordered a complete bearing set and other miscellaneousparts through ProPower, which included Mahle pins and spiral locks,Sealed Power rod bearings, and Federal-Mogul main bearings. It's a wiseidea to get these parts at the same time so your engine parts supplierknows what you're putting together--it will save you a lot of grief andconfusion down the road.

Once the broken motor was pulled from the Mustang and taken apart, wegot to work on the machining. Paul's Automotive Engineering has acomplete engine-building facility at its massive Mustang super shop.Head Machinist Steve Barker began the project by honing the block. Thisprocess was mandatory to ensure a smooth cylinder wall. Using deckplates, Steve torqued them to the appropriate specifications to emulatethe stress placed on the block by bolting down the heads. Once the deckplates were in position, Steve started the honing process of cuttingeach cylinder wall out to the prescribed 3.570 inches.