Low Budget 350 Chev - Page 9
Out of the box the crank was balanced as a replacement crank. On a standard rebuild using flat top pistons .030 oversize and standard weight Chevy rods, I believe the crank would drop straight in without any need to rebalance it. That is assuming the RPM limit was kept reasonable. A lot of guys get swept up with the hype of pro stockers running 9000+ RPM engines. Whilst this is necessary if you are racing in the Pro Stock classes, the average racer does not need to rev his engine this hard. Exceeding safe limits in relation to piston speed will add substantial costs to your rebuild. Ask a pro stocker how often they pull down their engines, change bearings, replace valve springs, replace rods and pistons and unless you are prepared to shell out lots of dollars, settle for a maximum RPM that balances within your budget.
To save time and money I checked all of my components that form part of the balance job myself with the aid of a reasonable set of scales and a few homemade parts.
There is not much you can do about the weight of circlips, rings and bearings, but I weighed and recorded each set.
Next were the pistons, each weighed on a set of digital scales down to .1gram. One point to remember is that the scales may not be exact to true mass but using the same scales for each piece will give you a reference as to which is heaviest and which is lightest. I then weighed each piston pin and with a bit of juggleing put the heaviest pin with the lightest piston. This ended up with a range of pistons that wwere rather close, only a small amount of grinding on a few pistons got them all to within +- .9 of a gram.
The conrods were next, all were weighed to get the total for each and recorded. I then made up a bush out of alloy to fit into the small end and one to fit into the big end, with a hole bored in the centre the same size as the extension arms on my magnetic base of the dial indicator. Then by setting up the scales and the magnetic base on a steel benchtop, I could use the adjustments on the extension arms of the mag base to hold each end of the rod to get the other to sit level on the scales and get a fairly true weight of each end of the rods.
All of the small ends were faily close and a bit of material was needed to be removed from some of the large ends on the balance pads.
With this done, at home even if not exactly right, it saved a whole heap of work at the engine balance shop (and some cash).
There is not much choice as far as getting an engine balanced in our area (within 300klms) only one place I know of, and most high performance rebuilds are sent off to the big smoke about 1200klms away. These guys have the newer fancy computer balancers that I had intentions of using, but a young local racer who works at the closer machine shop assured me he would do the job for me and would do a good job.
A good balance in my opinion is as important in a crap, low budget engine as it is in a 100 000 $ pro stocker if you want the engine to live. And even though I was building at low budget I did not want to skimp on getting a good balance job.
After some thought I opted for the local guy (Mike Durso @ Repco in Townsville) as he has a keen interest in drag racing and I found him pleasant to deal with. If his work was not up to scratch his reputaion would be on the line..
The work was done efficiently and I was more than pleased with the result. As far as me knowing if it was balanced properly or not, I will never know other than if it was real bad I may feel vibrations.. (The engine is running now and is smooth right through it's rev range so I would believe he did a good job - no harmonics felt)
Maybe if I was building a pro stocker I would have gone for the newer balancing machines - then you have to wonder what sort of job are they going to do on an el-cheapo cast crank ! I dare say they would not be as fussy with it comared to if you sent them a high dollar top of the range billet crank!
Next - all the prep work done - Time to assemble the bottom end ..
|Some pics coming.|