Ford 302 to 331 - on a Rock Bottom Budget!

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Ford Mustang 302 to 331 – On a Rock Bottom Budget!

Text, photo’s and drawings

by

David Vizard

Here’s how to save money while building a stroker 331 from a 302 small block Ford – the right way!

If you want to build a motor like the one below then let it be understood there is more to it than just finding the cheapest parts that add inches. Let’s not mince words here – be aware there are cheap parts that work and cheap parts that break. A 5.0 block is already a weak part but to compound the situation by adding a weak stroker crank and a set of possibly dubious rods is not where we want to go.

It may look like an expensive piece of hardware but in reality a 500 hp 331 incher like this can be built for a very moderate cost.

A little over two years ago I teamed up with some guys who had a good hooking chassis but no motor. The plan was to test Pro Power’s low cost crank and rods. At this point I had already been on the breaking end of another companies stroker crank and so was leery about recommending something that had not been personally put through the ringer.

Look down the middle of this block and you will see the 1/16 wide crack which runs virtually the entire length of the block. This happens when the power tops the 550 hp mark. A good selected block though, may well hold out until about 650 hp.

It all started when it was decided to build a low cost 331 specifically to test Propowers’s almost unbelievably low cost crank and rods. The plan was to push the power level of the stock block to the limit. What you see here is the end of our tests as the block, after several hundred passes at the 550 hp plus mark, finally split. The crank and rods are still good so, in my estimation, have passed the power holding capability and the value for money test.

What You Get.


Buy a rotating assembly from Pro Power and you will get a crank, a set of longer than stock rods (up from 5.09 to 5.4 inches center to center), pistons, rings and bearings. All this comes for as little as $699. That price applies to a 331 or a 347 kit. And if you are wondering why we are building 331 instead of 347 it’s because of the higher rpm potential of the shorter stroke.

Because it is externally balanced at 28 oz/in (instead of the more common 50 oz/in) and has a longer stroke the Pro Power cast steel crank is, at 40 lbs overall, a few pounds heavier than a stock 50 ounce-inch balance OE crank. Note the aero counter weights to cut windage.

It’s already pretty much a given that the crank is well up to taking punishment so now lets look at it’s accuracy and material specs. First move was to run a micrometer over the journals. These revealed tolerances well within original equipment specs so Pro Power were good there. Just for the record we have had cranks that were so far off that they really needed a regrinding before installing them first time around. As for cost the crank alone from Pro Power is a skinny $169!

All the principle dimensions such as size, roundness, taper etc were checked and the Pro Power crank found to at least as good as Fords OE tolerances.

Although not the cheapest of dampers because it is a race spec item Pro Power does have their one Prime One brand of SFI approved at $199 to go with this crank. Whether you use this or a less expensive one if the application is street only, be aware that you do need a damper in perfect working order.

As for the rods these also looked real good. I did have an issue with the pin sizing though. Pro Power sizes the pins for quiet operation for street use (no pin knock). I feel that though this is fine for more normal street use it is a little too tight for high performance use even on the street and talked to Dale about this.

Made from 5140 forgings these 5.4 inch long Pro Power rods feature a lightweight full floating pin design and 8740 Chrome Moly twelve point cap bolts (50 lbs-ft tightening torque). A lot of rod for only $139!

His point, and rightly so, is that the pin can be honed to suit the end users need and taking a little metal out is easy but putting it back is not. For race use, especially if nitrous is going to be used I would recommend the pin bushings be honed to give 0.0008 to 0.001 clearance on the pins. Honing the pin bores, and this is an op which many of you will not need to do, is cheap enough to do so does not end up adding much to the already frugal $139 cost of these rods. As far as all else goes the rods are of a much sounder design than the stock items and are about the same weight (nominally 365 grams on the big end and 175 on the pin end for 540 total) even though they are, at 5.4 inches, somewhat longer than the stock ones.

Piston Options from Pro Power.

OK – so much for the crank and rods. The entry level pistons recommended by Pro Powers boss, Dale Metlika, are the KB hypereutectic ones under part #
KB333. I have had a lot of experience with this type of piston and they are a pretty tough piece that can be used with up to 150 nitrous hp so long as the system is set up with the appropriate mixture. If you are building something in the 400 to 425 hp range these pistons will work fine and last forever. The cost of the pistons (less rings) from Pro Power is $305.

The KB 333 piston features 5/64 rings which are much less money than the 1/16 wide rings used for typical aftermarket performance pistons. These wider rings, though a little more draggy than skinnier ones, are good with nitrous use as the wider platform is less prone to wear from increased cylinder wall loading.

If you want to upgrade to forged pistons then KB also makes some basic but stout forged items under part # KB 720. These, from Pro Power, will run your wallet down by $424 but will pretty much handle anything you can throw at them in the way of boost or nitrous.

We have tested these low cost KB pistons and they are tough pieces for sure. Although they are devoid of any fancy bells and whistles they are well designed and you can expect good results from their use. At ProPower’s price of $424 they represent a cost effective choice.

Rings and Bearings.

Both the cast and forged piston 331 rotating assemblies from Pro Power are equipped with file fit Plasma Moly ring sets. At $99 these are not the cheapest you will find by a long shot. They are however top quality items that you can expect to last at least 200,000 miles. If you are hell bent on saving money at the build stage it is possible to get 5/64ths ring sets (which the cast piston takes) for less than $30 a pop. These rings are OK at a pinch but it’s often hard to be specific as the material and quality control varies so much. Some of the rings we have seen might crack under anything but the mildest of nitrous shots. One the other hand if the engine is going to be no more than say 325 hp and no nitrous these cheap rings when used with top quality oils and filters (air and oil) will often still look good after 100,000 miles.

The Pro Power kit, even with the least cost options, comes with a top quality Plasma Moly file fit ring set.

For bearings Pro Power normally supplies the King brand. I have to admit this is not a brand that I have had a lot of experience with. For the 331 we built to test the Pro Power crank and rods ACL bearings were used. The reason here is that when it came time to test the Pro Power crank and rods just the crank and rods were obtained not a rotating assembly as you see here hence the use if different bearings. The limited experience I have had with the King bearings has not shown up any problems but there again I would not expect them to on the lower powered engines I have used them in. For a high output situation this leaves me to suggest that since Pro Power’s kits are used to some pretty high output levels the bearings must be OK or we would have surly heard otherwise.

The Pro Power rotating assembly stroker kit features these King rod and main bearings.

So Where are We at Now.

In terms of a stroker rotating assembly purchase we can now come to a conclusion – namely that the Pro Power kit has, in our experience, played it’s part toward delivering tiring burning power in a very cost effective manner. If our crank and rods continue to allow the delivery of the 550 hp plus that is currently being seen from our test combo for the next two years without showing any signs of wear or distress (as it so far has done) then I would have to say that the Pro Power crank and rods would be worth a GFN 5 star rating for cost effective performance parts. Although I am confident they will go the distance the fact of the matter is that we are only two years into this test not four. That being the case I am backing off and going for a GFN 4.5 star rating which is still plenty good.

So much for the rotating hardware to stretch a 5.0 to 331 or 347 inches without depleting the bank balance too badly. Now if you think that all you need do is make more cubes and you are all set for big torque and hp numbers then it’s time for a reality check. First if such things as the cam events were right for a 5.0 then, if the displacement is increased while all else remains the same (heads, intake etc) the valve events won’t be correct. There is more to building the best that it can be from a stroker kit than simply installing it and we are not talking minor differences here. Figure about 40 hp and 30 lbs ft difference between knowing what to do and not.

If you intend to build a stroker 5.0 then the next step toward success will be the acquisition of a good block. There are various levels of ‘good’ and ‘cost effective’. The 5.0 block has some peculiarities that often allow it to be restored to ‘as new’ for as little as about $50 but the hoops you need to jump through may not be where you want to go. However it is a starting point. In our next installment we will look at how to get a block performance ready for as little as possible with various levels of builder (that’s you) involvement. Whether you are into super low cost or just low cost I guarantee you will like the results. As they say – ‘Watch this space’.

In the GFN shop we have restored blocks for budget race motors similar to this for as little as $50. Next installment low cost do-it-yourself procedures and options will be discussed in detail.

Stroker Kits - 331 Versus 347

Sure you have heard that there is ‘no replacement for displacement’. Like any catch phrase it does simplify things to the point it makes the point. The real world is somewhat different. The big problem when stroking a 5.0 is the blocks short deck height (distance from the crank centerline to the cylinder head mounting faces). To get a 331 inch displacement from a 5.0 means having a crank that is up from the stock 3.00 inch stroke to 3.25 inches stroke. For a 347 the crank needs to move those pistons though 3.4 inches. Any increase in stroke means that the pin has to go higher up in the piston. Each move up crowds the rings into an ever decreasing space. In addition to this an increase in stroke means greater rod angularity which results in more friction generating piston side loads. The only way to offset this is to increase the length of the rod used. This further crowds the rings toward the top of the piston and there is a limit as to how much you can do this. With a 3.25 stroke the increased rod length can be accommodated such that there are minimal compromises in terms of piston pin height and piston side loads. This allows the engine to still make high rpm without undue problems. This makes the 331 a great road race/track day engine where it may spend a long period of time above 5500 rpm. A 3.4 stroke for a 347 pushes the envelope to the limit. The advantage of the extra cubes shows up if the car is to be driven at low rpm in a high gear as the extra torque seen low down will allow such. Great for lazy street driving but the price paid is higher piston friction such that additional top end power over a 331 is rarely seen but increased bore wear is always a factor. Bottom line here is that if you are going to use the rpm potential of your stroker motor a lot then a 331 would probably be your best bet. If most of your driving is done at a street normal rpm (1500 – 3500) then the extra low speed output of a 347 could well be what you want. Having a 347 doesn’t limit you to 3500 though – figure that for those times you go to the strip 6500 rpm is just fine.

David Vizard