Need a damper for my engine I'm building and need to know the appropriate size to get. It is going on a 350 which will replace the 305 in my 92 RS. This is the only piece I need to get the internals balanced at a machine shop.

 

2 most common sizes for the SBC are 6-3/4" (same size as used on your original 305 and any of the 3rd gen engines) and 8" typically used on older cars, truck engines and various other applications.

You can use either size but you want to make sure it's going to work with your timing tab on the timing chain cover.

If your timing tab is welded onto the front of the cover about in the middle then you MUST use the 6-3/4" unit WITH THE TIMING MARK DESIGNED FOR THE STRAIGHT-UP 12 O'CLOCK POSITION. Application example- 1992 305 TBI engine application.

If your timing tab is on the side of the timing chain cover and is held in place by 2 of the timing cover bolts then you just need to determine whether you need a 6-3/4" or an 8" so that it lines up correctly (mesure the distance from the center of the crank to the tab and double it to determine which diameter you need).

If it's 6-3/4" diameter you need then just make sure you get a balancer in that diameter but with the timing mark in the 1:30 postion. Application example: 1987 L-98 350 TPI engine.

If it's an 8" that you need then it's easy- all 8" units have the timing mark in the 1:30 position.

NOTE: Do NOT use the 8" balancer designed for an earlier 400ci SBC. It'll bolt on and the timing mark will line up BUT the balance is totally wrong for a 350 or smaller engine.

 

A word of advice:

DON'T give them the balancer and let them balance the engine with it. What will happen, is you will end up with a custom one-off rotating assembly, that you can't ever swap any one piece out of without re-balancing. They will literally match the crank to any unbalance in that individual balancer.

Same for the flywheel / flex plate.

Specify that they neutral-balance it, to the stock spec; then take them the other parts later (after you pick up the crank) if you really want them balanced, and have them balanced to their stock specs as well. That way, any flywheel, any flex plate, or any balancer, will match your stuff; just like a new factory motor and its accessories.

Don't let them tell you they "need" the other parts for something. What they're really doing, is CYA on your dime, for taking the shortcut of balancing stuff to no spec in particualr, just whatever is the easiest way. Insist that they do it right instead.

Use your stock 305 balancer and stock timing cover; or a replacement for the stock balancer, that retains the same timing mark location; or a balancer and timing pointer purchased AS A SET (not a balncer from one mfr and a timing pointer from somebody else, no matter how shiny the chrome). Otherwise you'll be posting here next about "why is my timing 30° off" like everybody else that tries to mix and match balancers and timing covers.

 

However a larger 8" damper will quench crank harmonics better, giving a small power gain.

 

Damon, RB83L69, and ME Leigh
Thanks for the help you provided. Will be going with the 6 3/4 fluid damper. I will transfer the 305 timing chain cover to the 350.

 

The aftermarket 6-3/4" fluid dampener probably does NOT use the 12 o'clock timing pointer position (with the timing tab welded to the middle of the timing cover). I don't know that for sure, but you better ask the manufacturer. If you read my post again carefully, you'll see that 6-3/4" balancers can be EITHER 12 o'clock style or 1:30 style (but rarely double marked for both) so you have to be careful when you play mix-n-match with factory and aftermarket parts.

If I was in your position I would buy a new stock replacement 6-3/4" balancer just like your TBI 305 came with (don't reuse the original- buy a new one). That way you know the timing pointer will line up correctly with the stock timing chain cover.

You want a new one for many reasons, but engine balance is one of them. Your original balancer may have been drilled by the factory since they balance the whole bottom end as an assembly. Some of the balance weight sometimes gets taken off the balancer. That means that if you use it and then have to replace it later you may actually throw your engine slightly out of balance becuase the replacement one out of the box may not be balanced EXACTLY like the original one. Basically, what RB said. This is really a fine point and is not in any way catastrophic, but if you're going to lay down the money for the balancing you probably want to know that you can replace the balancer separately at some point in the future without altering the engine's balance.