UPDATE: Bobweight Calcs finally....Piston swap and assembly balance

[Pentroof]

My recent fiasco of splitting a cylinder is forcing me to swap to a whole new set of pistons (and a block  ). I've gone cheap, since this is a very mild street motor in my F100 that my daughter will be using as daily transportation while she's in school. Also, this motor will likely get revisited with a stroker package when she's done with it.

I'm swapping from OE rebuilder style Silvolites (+.020") to a set of L-2291F (+.030").

My question is, how much should I expect the bob weight to change? When the new pistons and rings arrive, I'll be measuring both and calculating the difference. Just curious what to expect from anyone that may have done similar. I'm assuming some weight is coming off.

My real concern is finding a shop near me that I can trust to perform the balance.

[My427stang]

Likely not much, but post the numbers here and we can calculate the bobweight difference. 

It depends on the weight of other parts but with Ford parts it's generally bobweight increase or decrease is pretty close to change in piston weight.  However, small changes may not need any change in balancing if it has already been done.

[gt350hr]

  I am comfortable with +10 or -20 on gram weight before I rebalance. "I" feel the importance is all 8 assemblies weighing within a gram of each other. Others will have different opinions I'm sure. Reciprocating weight is a percentage applied to the bob weight usually 50%. There are varying opinions as to what percentage is best.
   Randy

[Pentroof]

Well, the new pistons are over 50g lighter than the old cast slugs. I borrowed my buddy's balance jig and got the rods within 1g on each end. After taking very little weight off a couple pistons and matching them up with appropriate pins, the piston assemblies are also within 1g.

I threw a spreadsheet together to document and calculate the bobweight, which turned out to be 2,273g.

Before I seek a shop to balance my crank, I thought I'd post my data to let you guys point out anything I missed.

Does anyone know what the target bobweight is for a stock 390?

[My427stang]

My last "stock" .030 390 was 2315 gram bobweight.  I don't have details on the part numbers of that build, it was a long time ago, but was over the counter parts.  The added weight was in the recip, everything else looked similar to yours

Only thing difference I see is I have always calculated with 4-5 grams of oil on the recip weight.  Otherwise looks pretty normal to me

I am not sure what you mean by a target bobweight though, match balance then calculate, it is what it is.  I assume Ford used some standard bobweight based on expected component weight, but pretty useless number for anything other than mass production

[Pentroof]

Thanks Ross, the target bobweight question was about what a factory setup would be. You essentially answered that. I was just curious how different it is from what my new setup is, which really isn't all that much different.
Now I just need to find someone within 100 miles that can balance the crank. The shop that is boring the block doesn't even have a sonic tester so had to send it to another shop to check the walls at my request. The shop that has the equipment doesn't know how to use it and resulted in bogus numbers. Not comfortable with either guy balancing a crank.

[mbrunson427]

Now I just need to find someone within 100 miles that can balance the crank.

Where are you from?

[Pentroof]

Now I just need to find someone within 100 miles that can balance the crank.

Where are you from?

I'm in the Richmond, VA area.

[My427stang]

Richmond has a ton of machine shops, there has got to be some capable of doing what you want.  Spinning a crank that is already likely within tolerances shouldn't be tough

However, some options

http://www.candcmotorsports.com/about.html  If you want to drive to Manassas  these guys have been doing big Ford stuff for a long time, maybe a bit further than you want though

In Richmond

http://citizensmachineshop.com/services/  This place had high reviews on line, don't know them at all though

However, if you really think you need to travel, these guys are real good in Newport News, VA.  I used them when I lived there, I had about 6 different shops that were closer, including one within walking distance, but they were the only ones that could hit the target every time.  I had my own 489 balanced there as well as a few others, and I have had my hands on their Stewart Warner balancer.  They know how to use it too.  75 miles from Richmond, might be an option if you can't find someone you trust. Nice guys too

https://dunkums.com/

[WerbyFord]

Well, the new pistons are over 50g lighter than the old cast slugs. I borrowed my buddy's balance jig and got the rods within 1g on each end. After taking very little weight off a couple pistons and matching them up with appropriate pins, the piston assemblies are also within 1g.

I threw a spreadsheet together to document and calculate the bobweight, which turned out to be 2,273g.

Before I seek a shop to balance my crank, I thought I'd post my data to let you guys point out anything I missed.

Does anyone know what the target bobweight is for a stock 390?

Nice table! That conditional formatting comes in handy.

Did I miss something or is a weight DIFFERENCE of 50gm per piston a huge amount?
I mean, 50gm, that's like a whole McDonalds hamburger!

Don't pistons have to be within a weight spec or they're not really a "replacement"?

[gt350hr]

Now I just need to find someone within 100 miles that can balance the crank.

Where are you from?

I'm in the Richmond, VA area.



      Kevin Blanks @ Blanks Performance is a high quality engine shop owner. Nascar quality stuff.

[Joe-JDC]

If you do use the shop recommended in Newport News, make sure they don't add shotshell and weld a cup in place.  My crankshaft rattled like a carnival ride after they balanced my 428 engine parts.  In all fairness, that was back in 1981ish time frame.  Joe-JDC

[My427stang]

If you do use the shop recommended in Newport News, make sure they don't add shotshell and weld a cup in place.  My crankshaft rattled like a carnival ride after they balanced my 428 engine parts.  In all fairness, that was back in 1981ish time frame.  Joe-JDC

That's funny, no shot used in mine, however the guy was likely 45 years old, running the shop with his likely, 40 year old brother in 2006.  So they would have been 15 and 20 years old then, my guess is it was the dad back then.

They make their big money off the shipyards and even got out of the performance stuff for a few years I heard, but I was real happy with them in the 2006-2008 timeframe.  Apples Machine up the road in Yorktown, a different story

Were you stationed at the 1 FW Joe?  I was there for a staff tour at Air Combat Command (TAC in your time), loved that area, figured we'd retire there but momma loves Nebraska.....

[Joe-JDC]

Wife worked for HQ, IG in Admin Support.  I was recovering from 70% Service Connected Disability being recently medically retired.  We bought a  house there, expecting to retire there, also, but 9 years at HQ DO at KAFB my wife and I just decided to stay in SA TX.  She retired in '95.  Joe-JDC

[My427stang]

Cool Joe, I was a Branch Chief and Deputy Div Chief in DOYR Reconnaisance Ops from 2005 to 2008, I liked Langley a lot