piston with pin bottom

Balancing Stock 350 Engine


Balancing Stock 350 Engine


When balancing stock 350 engine seems that a lot of us these days have gotten caught up in the need to balance there engine. As for someone that has done it and not done it they can be close to the same. Just depends if your shop cares as much as you when doing the work.

What about cost

Rod and piston Weight are very important. One simple thing we can all do when building and engine that will make it run better and live longer that seems very simple but is not done by most shops when building a normal engine. Take all the engine parts and weigh them one by one and find the lightest of each part and make them weigh this much. Using sand paper and light grinders to adjust each part this should be done first by removing all the flashing from manufacturing and making the parts smooth.

For pistons only remove from under the head or pins in the recommended areas per the manufacturer and by polishing the heads. once they all weigh the same make the pin the same by trimming the inside edges of the pin adjust enough to remove some weight. Most of the pins should be pretty close.

Factory rods need some help

The rods can be a bit trickier. Since they are balanced front and back you will need to keep this balance the same. Rods carry more weight towards the crank pins than the piston pins. They can also be made stronger by removing all tooling marks and machining marks. This is very helpful to relieve the rods and makes them much much stronger. Cracks will start at the a point and rounded areas and areas that are flat are much less likely to allow a crack to start. So once these are removed the engine can rev higher and last longer. This is also where you go into removing metal to make all the rods weigh in at the proper weight.

Making them weight the same..

Now all the rods pistons and pins are the same weight. If you have also installed lighter pistons, pins and rods you can take some weight off the crank throws. Now you say how much well that is not an easy one to answer. But again we go to the old stand by of removing casting marks and rounding sharp corners. Once we have the entire rotating assembly a bit lighter but more importantly all the rods and pistons are putting force on the crank evenly. This allows the engine to live longer rev higher and be stronger over its life time.

Now if you ask an engine balancer guy he will tell you to bring it down when you get it done and he will make the balance dead on. But in truth the factory never puts there engines on any such machine to balance them. They also as you well know if you have weighed all your pistons, rods and pins that is way off from the factory. In the end the engines run great with out any types of computerized balance. So how is this possible. Well the crank is over weighted by two to one. So even if the crank throw are almost entirely cut off the engine will run smooth and maybe even smoother.

Over Balanced, Under Balanced

Over balanced engine such as the chevy v8 work great even if things are not dead on. This is also something that allows you to tune the system to run even better. Now if your not 100% right it will still work. The engine will still run and work better.

At some point the crank can be made too small or too light. Now with a small block chevy it takes a lot to get to this point. If you have read Vizard’s book on small block v8’s on a budget you can see he takes the crank down to about the same size as the crank pins this is a lot of weight off the throws. By putting them in the lathe he was able to turn the crank down so the throws are around the same height as the rod pins. This removed pounds of weight and improved performance. You can find a link to this book in my library section.

Making it light

Now with the crank lighter the rods and pistons can be made lighter if there is metal that can be removed that is not part of the support structure of the rods or pistons. With all this done you can make more power. You can also make an engine that last longer, runs smoother and make more power. With this in hand the camshaft choice can be driving up the rpm scale a bit for a larger boost in power knowing your engine is balanced it will last at that higher rpm level.

Now why dont we just take it to the shop and get it worked out with the computerized balancer. Here is one big reason most of those balancers work like a tire balancer and tell the person where to add weight. It does say where to remove it but given the choice between adding 3 grams and removing 4 it will tell you to add 3 grams because it does not understand where the crank throws are located.

It will also never tell you that removing 1 grams from the bob weight will solve the problem because it works with the crank balance only. Taking the weight of the rods into account but not adjusting them. It just assumes this cant be done. So in most choices it tells you to add weight. But this is not the goal of performance engine builders. We know that an over balanced engine will be just fine since the crank is setup right.

What about a regular build

Ok but im going to be running 8k rpm I need it balanced right. Well chevy ran engines to 7K with stock loosely matched parts. With in several grams of each other but not perfect. It works just fine. But if you do the procedures above and then get it balanced it will come out with the greatest outcome and benefits of the both.

Well what about the balancer and flywheel should they be super light. The short answer is NO! what you say. This is true because the heavier balancer does its job better. If your engine feel like it needs more balance then you should run the larger 8″ balancer. If its dead smooth and you want it to rev faster then the smaller one will do the job.

With all of the above we can all make our next build just a little bit better. It should make more power and run smoother. This is also something we can all do in our home garage and make the engine run just that much better. It may not be the most important part of the build but it needs to be done and it makes everything last longer.

Bob weights for typical SBC oem engines.

SBC 350 1,800 and 1,950 grams. With 1880-1900 grams being ideal.

Note that as above is accurate for most sbc engines. The crank is balanced for the target bob weight in the 1900 gram range pretty much a bit below is fine but would not want it to far off from that. Making the rods lighter or the pistons or the crank throws is all possible.

Actually telling how much to remove is the trick and not just drilling a deep large hole into the crank to even it up but to make the throws smaller when weight goes down. When weight goes up you have no choice but to drill and fill back with heavier Mallory metal. If you even are at a cross roads between I beam and H beam this is a very important thing to look into. I strong light I beam with longer rods using .866 pins or other lighter pins. With the longer rods the pistons pin height can get tighter and the ring packaged moved closer to the top of the pistons. Which is not a good thing but does a lot for weight. 6.125″, 6.135″, 6.2″, 6.3″ and longer in some short stroke cases.

There is always a lighter combo

Using the Honda big end size can also make for a much lighter rod over all. When used with smaller pins. With lighter parts it’s possible to build a much stronger engine that with a stock off the shelf crank go together and work properly.

A lot of us have purchased heavy dome pistons and H beam heavy rods with lighter cranks. That even being new can still require a lot of machine work to get in balance. When the correct crank for those parts is probably cheaper than the one extra light one purchased. So best to always get parts that not only are balanced but can be balanced.