Ok, some of you may have read my other post where I ended up having too little radial clearance around my 2.02" intake valves in the valve relief (eyebrow) of my flat top forged pistons. I struggled with getting this fixed for a couple weeks. I had called around to a few machine shops and couldn't get anybody that was willing to fly-cut the pistons for me to give more radial clearance.

So, I called up Isky and got a "piston notcher" that was supposed to fix the problem. Well, that tool is easy enough to use but for some reason, not matter how I used it, it would not cut the valve relief in the spot that needed more clearance. After a week of tinkering with that and getting nowhere, I finally broke down and took the Dremel and a carbide bit to the valve reliefs.

Believe it or not, to my amazement, this worked really well and cleaned up nicely. Now I'm trying to decide if I should have the rotating assembly balanced or not.

My pistons are already connected to my stock rods. So, I weighed each assembly separately on a postage scale that measures in 2 gram increments. The majority of the assemblies weighed in right around 1230 grams. Some were 1232, one was 1228; pretty much all within 2 grams though (remembering the scale won't do single grams). The only problematic ones were that one was 1236 and one was 1220 grams. I have no clue why one came in at 1220; it had very little material removed.

Anyway, I can't decide whether or not to have the whole thing professionally balanced at this point. I have a guy that can do it for about $200 locally but he can't get to it for 4 - 6 weeks. Then I have a guy about an hour away that can do it for $380 but can do it in a few days. They $200 guy wanted to get the pistons, rods, bearings, rings, and crank and make sure everything was right. The $380 guy just wanted the pistons, rods, and crank, nothing else.

A couple guys I have talked with say that for a street engine that isn't going to see 6000 RPM often and certainly not sustained that I should not be worried about it and just assemble the engine as is. Other people say it is a big deal and that I need to get it all balanced.

So, my question is for you guys that deal with this stuff more than I do, is what would you do?

A quick recap of the engine specs:

350 factory '81 block bored .030 over
stock crank ground .010 under
stock rods that have been checked at the machine shop
Clevite bearings all the way around
Trick Flow 23* aluminum heads
Performer RPM air gap intake
TRW Forged flat top pistons
Holley 600 CFM carb
Comp Cams XE274H cam
About 10:1 Static Compression Ratio
About 8.3:1 Dynamic Compression Ratio
I'm guessing somewhere around 400HP from the combo but that could be high or low; I won't know until it hits the dyno but have seen similar combos push 420HP.

Again, this is being built for the street and I'm just looking to do the right thing at this point. I'm willing to spend money where it needs to be spent but don't want to throw money and time away without good reason.

Thanks guys for any help with this decision.

 

You are good to go as is, don't waste your money on 2 grams. Mr Vizard will tell you in building a budget performance engine, a maximum delta of 50 grams/piston is OK.

BTW, $380 is ridiculous for that job. It is right at $200 locally as well.

 

I had my 540 rotating assembly balanced by a real good professional shop back around Christmas time, and it came out to between 1/2 and 1 gram from perfect balance (I'm told that most shops usually only go to the trouble to get them within maybe 2 grams, if that). I stayed with my parts as they did the work, and watched and photographed all the proceedings. So, I know how it really came out since I stood right there and watched the balancer.

Your's sounds to be rather far off. You are showing a 16 gram delta, which is enormous. So if you want to do the right thing, then you should have the rotating assembly professionally balanced. Funny thing is, it sounds like the cheap guy is trying to do a better job than the more expensive guy. However, it does seem a concern that neither one asked for the wrist pins (or spiral locks or wire clips, if you have them), since without them, you can't balance it correctly at all. Maybe they assumed by saying they wanted the pistons, that the pins were included, if so then OK. But just be sure they are included with the parts you take in.

Scott, he is not within 2 grams, he is showing at least 16 grams off, just on what he weighed. So who knows how far off things are overall. And GEEZ, VIZARD NEVER EVER SAID YOU COULD BE OFF 50 GRAMS PER PISTON. THAT WOULD SHAKE YOUR FILLINGS OUT. He may have said, 5 grams per piston for a budget build, but not what you claim. You should have your facts straight before you give advice, you could really screw someone up if they believed you.

 

I'd be much more concerned about the weight difference (if much) between new forged & OE old pistons.

• Do you have a old OE piston & pin you could weigh separately & post both here?
• What is P/N of new forged TRW? 2VR?, 4VR?, Trough? & what is oversize if any? Post it details here.

 

They are TRW-L2256F30 from Summit. They are .030 over, flat tops with 4 valve reliefs built in. Forged aluminum. The full specs are here: http://store.summitracing.com/partde...0&autoview=sku

I had the original machine shop work done about 5 years ago and did not receive the original pistons back. At the time I figured that was fine because I didn't need them anyway. Now I wish I had them for comparison (lesson learned). I'm going to try and find info about the OEM ones online now to see what I can find.

Is Delta calculated by total difference or is it in relation to where the pistons sit on the crank. In other words, is it calculated between pistons 1 & 2, 3 & 4, 5 & 6 etc. or can be be any difference including say between 1 & 3? I'm just curious if their position changes this calculation.

I'll try and find the original specs and post them here.

 

If anybody knows the factory piston weight that would be helpful. These replacements are 610 grams according to Summit.

 

The delta I was referring to, was simply the difference between your lightest 1220 gm and your heaviest 1236 gm.

When the proper balancing methodology is used, it makes no difference WHERE a particular piston/rod assembly sits in the engine, in relation to the calculation. The calculation used for the balancer bob weight, assumes the parts are weight matched. Which is part of the whole point of balancing in the first place.

Sure, I suppose someone could fudge things by juggling parts around, along with different bob weights on various journals, but Geez, why would anyone go to so much trouble to do something the wrong way. It would be less work to just do it right in the first place.

The bob weight on each rod journal is 50% of the reciprocating weight of 2 sets of piston/rod assemblies, plus 100% of the rotating weight of 2 sets.

There's an old saying, " There's never enough time to do it right the first time, but there is always enough time to do it over again".

If you don't balance the motor now, you'll almost certainly regret it, and things may be bad enough that you have to tear it down and start over again anyway. So, just do yourself a favor and do it right, while you're at it.

 

I guess the real problem here is that I'm not real thrilled currently with my two choices for having the balancing work done. On top of that I have two guys I know that have been building race engines for years telling me to not worry about it in a street engine.

When I have as much money as I do in this project the $200 - $380 isn't a big deal; I guess I just don't have confidence that I will be getting my money's worth out of the deal and I don't want to leave my parts at the other guys shop for 6 weeks if I don't have to.

It looks like the pistons and pins are going to be about the same weight as the stock pistons and pins (probably just a little lighter) and I am using the stock rods and crank. I wish I could find the stock weight for '81 pistons. I saw somebody had posted '76 piston and pin weight and it looks close but I don't know.

After reading more in the archives, it looks like with a stock crank, stock rods, and pistons/pins just slightly lighter weight than stock that I should be fine but my concern then is the weight difference between the rod/piston assemblies with one being 10g lighter and one being 4g heavier. I could always weight match them by removing material from the pistons but don't know if that's a good idea either or where to remove the material from now.

I'm just trying to figure it all out. I want to feel like I'd get what I was paying for if I took it for balancing and I don't want to leave my stuff for 6 weeks.

 

I just finished reading the section on balancing in David Vizard's "How To Build Max Performance Chevy Small Blocks On A Budget". What Scott stated is "close" to what Vizard says but not exact.

Vizard says "About 50 grams of overbalance can be used before there is a detectable loss of smoothness [in the engine]". What he is referring to is starting with balanced pistons and rods, checking the weight of the rod little end, measuring the overall weight of a rod and subtracting the little end weight. Then add together the wieght of the piston assembly, little end and twice the weight of the big end including bearings and 2 grams for oil. Then, assuming a crank is balanced to the stock weight of 1870 - 1900 grams and your calculation above is between 1850 and 1900, then the crank doesn't need to be rebalanced. This is where the 50 grams comes from. This is strictly speaking to the calculation above being up to 50 grams lighter than what the stock crank is balanced to.

He goes on to say that "Foregoing balancing does not mean that there will be more destructive internal engine loads present. What it does mean is that whatever forces are generated at one journal may not be fully countered at another. The result is an out-of-balance engine. But the engine is no more likely to fail than if it were perfectly balanced." It would be wrong of me to not point out that he also says "If the budget is available, get the crank balanced." In my case, it's not the budget that's stopping me but I do feel there are roadblocks in my way.

So, I think that if my pistons were all the same weight, and weighed less than the stock pistons and pins, then my situation would fall into what Vizard says doesn't need a crank rebalancing since it is stock and the rods are stock. However, this doesn't answer my question of whether the 10g under and 4g over are going to make a real difference in my application. I feel somewhat confident at this point that I would be fine without a balance job if all my pistons weighed the same but it would definitely be nice to continue to hear other people's views and thoughts on the matter. As a collective group I have always gotten the right advice here on the forum and I appreciate you guys a lot.

 

Well ... we're pretty sure your new flattop forged piston weighs about 610 without pin (that matches TRW hardcopy catalog) ... but we dunno what your old OE dished cast piston weighed. I suggest new is probably about 70-90 grams MORE than old. Suggest you should have the motor balanced by the $200 guy ... his list of needed parts are inline with what's truly needed. Be prepared to pay extra ... need for heavy mallory metal is likely.

BTW ... as pistons go ... your new ones are heavy.

IMHO ... vizard's pretty good ... but he ain't God.

 

Balancing....years ago there was an old man with a machine shop off US1 in Laurel Md.....he built for NASCAR....he worked his balancing at 3am.....why?? because there was NO TRUCK TRAFFIC back then...the ground vibrations from a passing truck would upset his balance machine, he was VERY finicky about his work...VERY...

upon hearing that, I am not totally amazed upon reflection....so you need know under what conditions your engine was balanced....
and I would not trust the readings off a typical machine shop....

there IS a shop near me, that is way the hell out in the stix, and I suppose that is when they do the work, there is allways work in progress, i'ts a father son team, they not cheep, and build race engines for all sorts of folks....pros down the list....but with their location at 3am....there ain't no ground vibrations...eh what??

so everyone that criticizes a stock factory balance job, you gotta think about what conditiions they are facing/working with....not as easy cut/dried as one would think....

 

Not to add to your woes, but you'll have to have the pistons removed from the rods in order to do a decent balance job. The rods have to be weighed big end/small end for rotating/reciprocating weight calculations. Take your time, have it done. Your bearings will thank you, plus you'll know it's been done right.

 

L88plus is exactly correct but for a mild street engine on a budget I would not do it for a 16 gram difference if that's what it is. If the replacement pistons are heavier than stock it does need to be balanced, however, but I would find lighter pistons before I balanced for that. Stock pistons are so heavy it would be amazing to me if an aftermarket forged piston were actually heavier. It is possible to find very good replacement pistons in the 400 gram range for your application and I would recommend spending money there first and then have it balanced since you have the engine apart anyway. You can safely go to 1.5% overbalance but you do not want underbalance. 1.5% overbalance would be pistons weighing 1.5% less than stock with factory crankshaft balance. If you just want to balance a street engine for peace of mind get everything within 2 grams but in the real world the engine would not care about a 16 gram delta. A stock engine with factory balance should be about in this range. The guy in Maryland must have been using an old Stewart Warner with a strobe light. They were sensitive but finicky; much better stuff out there now. Find a shop that uses a Hines.

 

Thanks everybody for taking the time to post and consider my problem(s). Some of you were gracious enough to let me know about other machine shops in this general area. I ended up finding a shop that does it for $200 for the basics and if I drop the stuff off on Tuesday he said it would probably be done by Friday. Even if it goes a little longer it will still be faster and cheaper than everybody else I was considering yesterday. So, there's no question now -- balanced it is.

I knew you guys wouldn't let me do the wrong thing! I appreciate everybody's input and I definitely learned a few things from this thread as I generally do everytime I post.