Good morning class! Today we are going to learn about the how’s and why’s about stroker kits. My goal is to debunk some myth’s and common misconceptions about stroking a big or small block Chrysler. As someone who has built National Championship quality racing engines for over 20 years I will share specific information about bearing clearances, bore notching and much more information. This is really required for properly building a stroker engine. I will also ”in clear terms and detail” explain what questions you need to ask so you can purchase a stroker kit that is truly “ready to go” when you receive it.
We built our first big block stroker engine in the fall of 1988. “Back in the day” the only way to build a stroker engine for a Chrysler was to buy a billet crank or weld and offset grind a stock crankshaft. Since billet cranks are machined out a solid piece of steel they are very expensive ($2800+ in 88’) and took over 12 weeks to make. This was not a practical option for us. That left us to use welded strokers. Welded strokers evolved out of weld repairing damaged steel crankshafts. Someone figure out: instead of welding the journal to repair it, one could just weld on the top half of the rod journal and then offset grind it to increase the stroke. People have been doing this since the late 40’s!! While this worked well, you still are starting with a 1053 steel crankshaft that was never intended to make serious power. Not to mention any factory 440 steel crank is now 40+ year old and most likely has seen a pretty hard life.
Fast forward to1992. Mopar Performance releases the 3.75 & 4.15 stroke crankshafts. While they are a little bit more money than a welded stroker, they are brand new and can be ordered off the shelf and into you hot little hands in days not weeks! Mopar decided on a 4.15 stroker crankshaft because it makes exactly 500” in a 440 block that is bored + .060. These stroker crankshafts were made by Norton Foundries. This was a company that was in Fostoria OH that made OEM production cranks for the big 3. These cranks could be purchased for around $1200 and once balanced could be dropped right in to a properly prepared 440 block . Due to Norton’s experience with the OEM’s they held very consistent bearing sizing (clearances). These consistent bearing tolerances made them very friendly to the end user.
Fast Forward again to 1998 when Eagle manufacturing released a 4.15 stoke crankshaft for Chryslers. Eagle was the first company in the performance aftermarket to sell economical forged steel engine components (read: made overseas) Up until this point Eagle had focused mostly on making cranks and rods for the GM and Ford markets and had only made a few connecting rods for Mopars. When Eagle stepped up and delivered a crank for Mopars it retailed for about $800. This was a really big deal. $800 was cheaper that the Mopar Performance crank and cheaper than the cost of welding and offset grinding a stock crank. This made the price of our kits come DOWN!
Fast forward to the new millennium. China’s manufacturing might has exploded. It sometimes feels that everything is now being made in China. The main reason is that China has an almost endless work force (around 1 billion people) and their labor rate is very cheap (around $.50 an hour). This all translates into products made very cheaply. One of the major downfalls in this process is that American companies go to China and want products made “as cheaply as possible!” Well the old saying still holds true: “you get what you pay for!” Now this is not to say that the Chinese are not capable of building high quality products. The Chinese have some of the world’s tallest sky scrapers, they have built nuclear reactors, and are working on the world largest dam. As a country, they can build just about anything!!! Again, the problem lies in the fact that foreign companies go over to china with the thought “how cheaply can we have this made!” I believe the quality of the cranks and rods coming out of China has gotten exponentially better in the past 10 years. I have never seen a Chinese rod fail due to material quality. I have honestly never seen one “just break”. Over 95% of the failures I see are directly attributed to sizing issues. Same can be said for crankshafts. As long as you stay within a crankshafts HP ability they work very well.
Connecting rods are very easy to machine properly. Sunnen (a company that has been a leader in honing since the 1924) actually makes a machine for production use that uses diamond stones (very hard, wear very evenly) and it sizes rods to what ever number you specify. Basically an automated rod hone. Just place rods on the machine a push “play”! Consistent, properly sized connecting rods are a snap!
Crankshafts tend to have more issues. Nowadays 95% of the machining done on a crankshaft is done by CNC mills. Counterweights, flanges and bearing journals are all machined automatically. This makes for a very nice, consistent component. Where most of the Asian cranks “drop the ball” is in the bearing journal finishing. This is still a process done with a large spinning abrasive wheel. Due to the size of the grinding wheel required for final finishing of crankshaft journals, (usually about 3’ in diameter) the wheels are made out of compressed abrasive. Ideally a wheel made with “diamond stones” would be more consistent but due to the size of the wheel required and the cost to make a wheel that big is not practical. Abrasive stones have done a great job for centuries in the finishing of a metal surface. The only area where you can have issues is that the wheel or abrasive stone does not wear evenly. This is a constant situation when grinding a crank. One must repeatedly “dress” the wheel. To “dress” a grinding wheel is a process in where the operator makes sure the wear surface of the grinding wheel is square to the surface it is machining. When a crankshaft is ground too quickly or the wheel is not constantly checked for straightness the grinding wheel will wear more on the edges of the wheel. Because the wheel moves back and forth across the journal the bearing surface is still consistent and flat. The problem is at the edges or radius of the crank. This is where the wheel is not square and the journal surface has taper or “ramps up” just before the radius of the crank. Most people miss this because they measure in the center of the journal (assuming it’s flat all the way across). The journal measures within a tolerance and you are good to go, WRONG! If the journal has taper you will have a MAJOR failure if not addressed.
Chrysler had “backcut radiuses” from the factory. What this means is where the bearing surface ends at the edges, Mother Mopar had a narrow plunge cut on the journal. This allowed for a wide bearing that went right to the edge of the journal. While this is great for production engines, it does create a weak spot or stress riser where cracks will start. Performance cranks have what are called ”full radius” journals. This means there is a continuous radiused transition at the edge of a journal. These journals, with a full radius require a chamfered bearing to properly clear the transition. NOTE: This is where less expensive cranks drop the ball!
Think about it like this: remember the first time you ever used a can of spray paint. You just held the spray nozzle down and moved your arm back and fourth spraying continuously. You then realized you had way too much paint on the edges when your armed stopped and started moving the other way. A similar thing happens with a grinding wheel. When the grinding wheel gets to the edge, it has to grind the radius and the cheek of the journal. This causes the edge of the grinding wheel to wear faster than the rest of the wheel. If not constantly “dressed” or resquared, the wheel will now grind the edges of a journal unevenly. This is called taper.
Since most all overseas cranks are made as quickly/cheaply as possible, getting the radisues correct is a constant issue. Like I said earlier, they are getting better and bearing sizing as a whole is much better. However, getting the radius issue correct has not happened yet. My lawyer says I can’t name names but just about ANY crankshaft made overseas has some sort of taper issue. People will try to cover up this situation by saying: “sometimes you have to carve on the bearings”. That’s BULL!! My grandfather taught me “don’t cure the symptom, cure the problem” meaning if you fix the real issue, everything is better! While carving on bearings will resolve a radius issue it is a cobble fix. If you have a crank that has a taper issue, FIX IT. Then there is no guessing, no cobbling
K1 and the Callies “Comstar” line of cranks are made overseas and DO NOT have these issues. I was just visiting Callies a few weeks ago and saw their QC area. They had a 3’x5’ scrap container being filled with cranks that were out of spec. These cranks looked PERFECT to the human eye but had issues like “.001 underize #5 main”, “taper on 3rd rod journal” etc. Instead of regrinding them Callies just back charged the overseas supplier. They said this has made drastic improvements in QC. Not all companies do this, but we can still get a good crank if the proper steps are taken.
The only way to fix a problem is to know about it. That is why when you buy a Muscle Motors stroker engine kit, you get a kit that has had all of the components checked by an experienced machinist. When we balance a stroker kit we start with the connecting rods. The big end sizing is verified and corrected if necessary. Nowadays 98% of the big end rod sizing is dead on but once and a while you get a few that need to be “touched up”. This occurs most often when people switch from traditional ARP 8740 bolts to ARP 2000 bolts. The 2000 bolts, have a higher yield strength (220,000 vs 185,000 for the 8740’s) and therefore must be torqued higher. Raising the torque valve from 63 ft/lbs in moly to 75 ft/lbs in moly will distort the bearing housing. Since we are a machine shop and check the sizing, our kits do not have a sizing issue.
The small end of the rods are a different story. While it is better to be too small (easier to make a hole bigger than smallerJ) most every small end on an overseas connecting rod are too tight. This is mostly because no one can seem to agree what size a .990 wrist pin should be! Some are .9905, some .9900. Small ends that have .0003-.0005 clearance (that’s 3 ten thousands of an inch!) are very common. Here again some may say “it’s within factory spec”. While that may get the seller off the hook, it doesn’t help you assemble your engine without a trip across town to the local machine shop to get your wrist pins to fit correctly. What is your time worth? Not to mention a lost Saturday afternoon because you’ll have to wait until Monday when the machine shop is open to get the situation addressed! This is not acceptable! Factory clearance spec for a wrist pin to connecting rod (straight from a factory service manual) is .0002-.0006 (that is two, ten thousands of an inch). As someone who built PERFORMANCE engines almost everyday of my life for over 20 years, let me tell you this: If you ATTEMPT to run a PERFORMANCE engine with .0002-.0003 clearance on the wrist pin, it WILL seize in the rod! That is the difference in ordering a stroker kit from someone who sells parts vs someone who sells parts and is a professional engine builder. A naturally aspirated performance engine with .990 wrist pins should have .0008-.001 clearance.
You must remember: you are building a performance engine. You are not going to drive this stroker engine back and forth to church once a week. While you MAY drive your car to church, I would bet a very large stack of $100 bills that the Saturday night before church you will be leaving rubber from here to the next county. WHY NOT, that’s what you build a stroker engine for. To be faster than your buddy or to whip up on that camero across town! Forged pistons, 4340 heat treated steel crank and rods are all designed to take a lot of abuse and its up to you to abuse themJ !!! BUT if you attempt to build your engine like a 2 barrel 318, you may get 2 barrel 318 performance!!
Same goes for bearing clearance. The Clevite bearing books says .001-0035 is and acceptable range for rod and main bearing clearance, MORE BULL! If you attempt to run a stroker engine with .0015 rod bearing clearance you will have a problem! At least if you are building a 550+ HP performance engine! For rod bearings I recommend you run .0025 bearing clearance. For main bearings we recommend .003. You can be + or - .0002 on either side of these numbers. Some may argue this is a little on the loose side but let me tell you this: it is ALWAYS better to error to the loose side when dealing with bearing clearances! That is why we always use high volume oil pumps in any performance engine build. You can always shim the spring for more oil pressure. If your bearing clearances are too tight and you engine “grabs a bearing” it will ruin you day, and wallet!
OK, now back to the taper issue. You have established your wrist pin, rod and main bearing clearance but what do you do if the crank has taper? Carve on the bearings? Yes that will work but let’s cure the problem not the symptom! Fixing taper: if a crank has taper it means that the journal ramps up at the edges. You can not simply “polish” it out with a traditional crankshaft polisher. A traditional polishing belt just follows the surface that was made by the grinding wheel. UNLESS of course you have what is called a micropolisher. Not all micropolishers are the same. Ours is designed with what is called a shoe to support the back of the polishing belt. The shoe is mirror shape of what the journal should look like. A perfectly flat bearing surface that transitions in to a .125 radius. The abrasive belt (very fine grit) runs between the journal and the shoe. Because of the support of the shoe, the polishing belt will remove more material from higher pressure points (tapper) and leave us with a mirror like > 1 RA finish. So now we have a superior journal finish AND have removed the tapper from the journal AND maintained our standard size with NO carving on bearings!!!
Now that we know what to look for, how does Muscle Motors process their stroker engine kits?
First an order is taken: A stroker combination that is correct for the customer’s application is determined. Stroke, rod length, bore size and compression ratio are established by asking what cylinder heads the customer will use and how much compression is desired
Next the order is pulled. We stock cranks, rods, and pistons. Certain rods and pistons work in many different applications. We now match up the correct components and a work order is established.
The next step is to verify sizing. This is first done on connecting rods (big and small ends) and then on the pistons (bore size and wrist pin sizing) once these are verified and corrected as necessary these parts are moved to balancing.
First, all of the big ends of the connecting rods are equalized. On modern connecting rods this is usually between 2 and 3 grams. This may not seem like a lot, but if you are paying to get your kit balanced, you should get it balance correctly. Big ends are now all within .2 of a gram, and the small ends are now equalized. After the rods are completed the pistons are all equalized to the same >.2 gm tolerance. I have seen some companies advertise the fact that they establish bob weights down to the tenth of a gram. They also only measure one rod and one piston to obtain these numbers. Unless all of the rods and all of the pistons are then equalized to the same tolerance (not the factory + or - 2 to 3 gram tolerance) this “accuracy” is lost. I was taught when I first started machining engine components “it takes just as much effort to do something wrong as it does to do it right. SO DO IT RIGHT!” Having a bob weight measured down to a tenth of a gram and then accepting a companies + or – spec of 2-3 grams never made any sense to me?
In the balancing process there is reciprocating weight and rotating weight. Just like the names sound like, reciprocating weight are things that go up and down. These are parts like: the small end of the rod, piston, wrist pin, sprio locks, and piston rings. Rotating weight components are the big end of the rod and the rod bearing. This is the formula for determining the balance bob weight (these are the weights bolted to the crank. This assimilates the forces on the crank during crankshaft balancing) using big end and small end weights separately hence the need for balancing each end of the rod independently. The bob weights are then set to a specific weight and bolted to the crank. The crank is now spun to 500 RPM’s and our Hines Computerized crank balancer tells the operator where to remove weight. This process is repeated until the same >.2 of a gram tolerance is established.
Now that the crank is balanced we measure the journals for sizing and taper. We mark where the taper is most pronounced and the crank is then micro polished. We do this as the last step because very minor marking of the journals can occur during the balancing process. This additional step ensures you not only a premium rod and main journal finish, but also journals that do not have taper that will not carving of bearings to “make it work”
Things to know when ordering a stroker kit:
Am I getting the right kit for my application? Biggest may not always be the best!
Is the kit balanced and if so how accurate is the process?
Are the people doing machine work to you engine components actually machinist?
If so do they measure things like rod bearing, wrist pin and crank journal sizing and correct them if necessary? Is this service offered?
There are a lot of people out there selling engine components these days, we suggest that if you are being sold a kit that is ready to go, are you truly getting a “ready to install” (when you receive it) rotating assembly. I am in no way suggesting that there is a substitute for having an experienced engine assembler be responsible for measuring a double/triple checking everything as your engine is final assembled. I am just trying to help the less experienced person understand what they are purchasing. So many times projects go over budget. This is because of hidden cost, unexpected expenditures and simply not understanding the true cost of having a ready to assemble stroker kit. I hope this information will help you get the most for you money but if you have to spend a little more up front to get a better end result, remember this: Quality is remembered long after price is forgotten!