Slap Happy Pistons

I had a really good 440 built by a shop, and I spent a bundle on this build. We used custom forged pistons at the recommendation of the builder. I have since installed the engine in the car, and when the engine is first started in the morning, I get some kind of tapping sound, which seems to be coming from the bottom end of the engine.

I was worried that the bearings or pistons might be going away, and I don’t want to have to rebuild the engine. When the engine is warmed up, everything sounds normal. One of my friends is a very experienced Mopar racer and he told me that the problem is piston slap in the cylinders. He said that it was normal in the old days, but not with modern forged pistons. I want to avoid having the engine fail completely. Like they say, an ounce of prevention is worth a pound of cure. What do you think?

Ed Mays - Via moparmusclemagazine.com

Ed, there was indeed an audible piston slap that was fairly common with some of the earlier aftermarket forged pistons, especially when cold. In those days, piston to wall clearances of .010-inch or more were common. If you have audible piston slap with modern forged pistons, chances are the piston-to-wall clearance is a little wider than it should be. With minor piston slap, I have found that the longevity of the engine is generally not affected. That being said, there is quite a variation in the degree or severity of piston slap. If the condition is relatively mild, as in a low, barely audible sound when dead cold, I would not be overly concerned. If, however, the noise resembles a diesel, and is accompanied with high oil consumption, I’d prepare for a rebuild.

There is really no cure for the piston slap, other than rebuilding or replacing the pistons. Unless the problem is pretty severe, or coincides with excessive oil consumption, consider it nothing but a minor annoyance.



To Stroke Or Not

Recently, I purchased a complete 1971 383 Magnum core, which is actually a nice used engine in good condition. My intent was to build this one, so that I would have a spare bullet ready for the Challenger when the time comes. I’m not in a huge hurry to get this engine done, and plan on working on it for the better part of a year.

I’m just now starting to formulate my plan, and as you know, this is when the key decisions need to be made. I’m pretty sure which way I want to go as far as balancing the ultimate power output, and the day-to-day drivability. What I’m not sure of is whether to build this engine as a stroker, or retain the factory 383-cube displacement. I know with the stroker I can potentially make more power, but what are some of the pros and cons? I would love to find 550 horsepower on the engine dyno when this one comes together. I’m not really too concerned with the budget, since for me the pleasure of the build is a real value.

Joel Manton - Via moparmusclemagazine.com

It’s pretty hard to find fault with the notion of building a moderate stroker. For any given peak horsepower level, the stroker will reduce rpm and provide a gain in torque that you will really feel. This characteristic allows a higher output engine that is more flexible in a true dual-purpose performance application. Arguably, the smaller engine will provide better fuel efficiency, simply because it is pumping less displacement at any given rpm.

Obviously, more horsepower and torque are a plus, but consider the effect of all that extra power on the drivetrain. The higher torque loads are definitely more strain on the driveshaft, rear end, clutch, and transmission. At the power level you are targeting, the rear, u-joints, and clutch will likely need upgrading to keep up, especially if you are considering track time. This is probably not a bad idea with either engine displacement, but undeniably there will be more strain with the larger engine’s torque.

Obviously, more horsepower and torque are a plus, but consider the effect of all that extra power on the drivetrain.

Whole Lotta Shakin’

I have a drivetrain vibration that is noticeable at speeds over 50 and is pulsating in nature (about two pulses per second) and it increases with increasing speed. It stops if I let off the gas pedal. I don’t notice it if I rev the engine with the transmission in neutral. The car is a ’71 Challenger and has a 360 LA engine with a 4-inch stroke crankshaft. I had the engine parts and damper balanced before assembly.

The torque converter for the 727 supposedly has the correct counterweight. The driveshaft was checked for balance and straightness several times. I plan to replace the transmission with a rebuilt one, and recheck the torque converter counterweight. Is it possible that, with the 4-inch crankshaft, I need a different harmonic balancer and different counterweights on the torque converter than for a standard 360 LA engine? Could the problem be in the differential, which is 83⁄4 with a Sure Grip and 3.23 gears?

Ira Jacobson - Via moparmusclemagazine.com

Ira, it is possible that the engine takes a balance factor other than the original 360, depending upon how it was balanced. I have a couple stroker small-blocks that were internally balanced so they take no balance weight at all. However, if the out of balance was in the engine and converter, the vibration will be evident just free revving the engine in neutral. Since you’re telling us it doesn’t vibrate in neutral, your vibration is likely somewhere further down the line.

The number one culprit for vibration is in the driveshaft. I’m not sure about the severity of the vibration, but these cars were not as smooth as a modern car, especially when driveshaft speeds are high. Chrysler actually added all kinds of band aids to try and make various carlines smoother riding. They tried everything from hanging damping-weights on the transmission, to adding inertia rings to the driveshaft, and even later, the full “Isoclamp” rubber isolation system. Frankly, the only way I’ve ever gotten one dead smooth is with a custom-built aluminum driveshaft.

If the vibration is greater than what would be considered “normal,” I would suggest starting with a factory service manual and go through the driveline angle checks and adjustment as required. Driveline angle will have a noticeable influence on vibration. I’d isolate the drivetrain components as much as possible to find the point in the chain where the vibration is coming from. For example, start by just revving the engine in neutral to check up to the engine and converter. Next, remove the driveshaft, plug the transmission output with a yoke, and run it through the gears to check the transmission. Then add the driveshaft with the axle shafts pulled. Add the shafts, then the brake drum, then the tires and wheels while testing to see at what point the vibration comes into play.


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