• Hi Guest!
    If you appreciate British Car Forum and our 25 years of supporting British car enthusiasts with technical and anicdotal information, collected from our thousands of great members, please support us with a low-cost subscription. You can become a supporting member for less than the dues of most car clubs.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Balancing Connecting Rods

Share this page

D

DougF

Guest
Guest
Offline
I have a gram scale and a beam from which to freely suspend the rods. What I am trying to figure out is a good system for hanging the rods from from consistant fulcrum points to get accurate weighing.

I have tried tying string around it, but not a good method, too inaccurate. I have considering drilling two 3/32" holes and running safety wire through, but am not excited about boring holes in the rods.

Any ideas?
 
When I did this years ago on a TR4, I used a 1/4 inch rod, lubricated with a light machine oil, as the pivot point. The rod rested on uprights with a notches. To the piston end of the rod I added a temporary weight to help increase the rotational mass. To test the rig, I weighed the rod ends twice to insure I got the same readings each time.

It worked out well, just be careful not to take to much off, to fast, or you will be chasing your tail to get them all the same weight.
 
I just put mine on a digital postal-type scale. They were pretty close so I didn't have to take much off to balance them.
 
Doug,

This is how the machine shop did mine. Two separate measurements, both as stated above.
 

Attachments

  • 15320.jpg
    15320.jpg
    36.5 KB · Views: 782
  • 15321.jpg
    15321.jpg
    35 KB · Views: 735
Paul- Understand the bottom one, What's the top one measuring?
 
Big-end mass. It's pretty important to have big-ends, given they're rotating masses, match. Even if overall rod weights are the same, you'll have balance issues if you have one eith an extra-heavy or extra-light big end, say.
In the past I have added weight to a rod - maybe an extra washer under a big-end-nut, or occasionally a little solder inside the "H". Be careful with tinning though!
 
for the big end, use your piston pins (light weight oil)for your pivot, sitting between v-blocks. for the other end,use your crankshaft(on v-blocks) for the pivot. You want to use a SMALL amount of sewing machine oil on the pivot (no drips). make sure the centers of the rod make a perfectly horizontal line, ie. shim the pivot end to get horizontal. try to balance to 1/10 gram or grains if you have the time and patience.
R
 
also,
if you want to make it better, make ball bearing shafts.
 
Also, this was after all machining for bushings was completed The rod caps were machined for the ARP rod bolts and they were installed.
 
Thanks everyone for the information. It will all be a huge help.
 
No one has mentioned that after all this time and expense you still have 50 year old cast iron rods...that weigh nearly 200 grams more than the modern replacements....(that's .8KG of reciprocating mass your old crank has to move)I forget how many tons of strain this is at 5000RPM but some one out there is math wizard enough to enlighten us all
MD(mad dog)
 
Well for a minimum of $860 for 4 cyl or $1290 for 6 cyl, you can get Pauter rods https://www.pauter.com/exotica.htm that will handle high rpms. And Carillo rods are at least double that. But then you'll need more than new rods to make HP at that rpm so figure in more $$$ for that. But for a street or even fast road engine that touches 5K, you're right, all we have are originals.

I think that Joe A and Jack were playing around with Chevy 350 rods mod'ed to fit the TR for racing to keep costs down. But I don't know what other mod's were required for them to work.
 
actually,
the rods with bolts (stock original) weigh 940 grams! The pistons 525grams(AE brand).The pins i think are 145grams.
If you are building a new engine and want to mess with things a bit, there are a few connecting rod optins. Jag AJ6 connecting rods from the 1990's straight 6 can be made to work (6.5" lg), Scat modular ford connecting rods can be made to work(6.657" lg)(piston located rod). Both require custom pistons. I am drastically modifying my tr4 with Scat flathead Ford 7" rods,2" journals(reduced from stock 2.08xx"),custom pistons and pins using Buick 6 cylinder clevite 77 bearings(cb610p)(piston located rods).
rods=567grams, pistons=350grams, pins=72 grams. HUGE reduction in load on the bearings. 989grams total VS 1610 grams. 40% reduction in reciprocating weight. Remember that the load on the bearings is exponential to the weight. OK Scat rods ARE made in china, but better material 4340, much lighter, no hole in the side of the rod, and $80 each (flatehead rods)!
R
 
You might try my approach,hand one of the new Carrillo rods to your wife at dinner and ask her if she has ever seen anything so beautiful.....Her actual reply was "what the heck is it"....oddly enough she did'nt hit me with it.
True its not cheap,but IMHO do it right and you will do it once.The motor in the tr4 made 153HP at 7000rpm with the stock carbs and the point distributor,on 88 octane.I will put on a petronix this spring.
MD(mad dog)
 
very true,
Carrilo's will not let you down. If i was going to spin to 7000rpm, I would be using them also.
I plan to stay below the 3rd harmonic (6200rpm) with rev limiter set a 6100rpm. Longer rods improve the piston design so the piston can be lighter-shorter pin centerline to top of piston dimension. Longer than stock Carrilo's are about 1.5 times the price of off the shelf cost. longer rods also have additional benefits, thrust loading,rod angle,dwell time at tdc,bdc that helps port efficiency, etc. I wouldn't spin my motor to 7000 with a prepared,nitrided,stock crankshaft, so that would escalate my build price unnaceptably. what kind of cam duration do you have to spin to 7000? Is that a street engine?
R
 
PS-"she who must be obeyed" would not only hit me with it,she would shove it down my throat and tell me that there will be no dinner for 6 months-"eat the connecting rods". Our kids are here first and formost priority and my "toys" are LAST on the list. We all have our own issues. I've got it pretty good though. I am in to all kinds of things, not just the triumph.
R
 
The cam was by Integral cams,it was the stage 3.We used pinto intakes and VW exhausts.The chamber is my own design,and brought the over all compression to 9.0/1 even with the 87 mm venolia pistols.Kent did not let me get wild with the design of the pistons but if I had it to do over,I think I know where 20 more horses could be had.......
MD(mad dog)
 
MDCanaday
You need to get rid of that 4 and do all that stuff to the TR6 and tell us about it. That is a lot of power you are getting and on street gasoline as well. You no doubt have hit a very good design with the combustion chamber.
 
Back
Top