The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: Truckedup on 02.10. 2020 16:16

Title: Connecing rods and balance factor
Post by: Truckedup on 02.10. 2020 16:16
 I have 5 connecting rods for my  650 SR project. None of the big ends measure the same....I hate to spend $450 on new rods but  a thrown rod is more expensive. My Triumph race bikes have R&R alloy and MAP steel rods so I am familiar with them
 I searched and found the aftermarket rods are  6.5 inches center to center just like Triumph 650. But the original A10 items are about .035?  shorter.. I have a 1149 alloy head with 1.5 inch intakes and IMD 7.25 pistons, 357 cam..Has anyone here used the longer rods and the slightly extra length was no issue ?  I want to avoid raising the cylinder if possible. Slighly more compression isn't an issue...I can cut deeper piston valve reliefs if the piston crown has enough  material.
  The new rods will require rebalancing the engine....what balance factor has been used here that worked out ok for normal semi aggressive street riding? Thanks
  Oh, the stock rods average 382 grams on my triple beam scale. I believe the MAP are about 440 grams, the R&R a little less...
Title: Re: Connecing rods and balance factor
Post by: KiwiGF on 02.10. 2020 20:52
Some weight info. here. I’m pretty sure the rods from thunder engineering are std length, its generally the steel rods that aren’t, maybe cos they involve tooling unlike the cnc manufacturing method?

https://www.a7a10.net/forum/index.php?topic=6086.msg41598#msg41598
Title: Re: Connecing rods and balance factor
Post by: muskrat on 02.10. 2020 21:04
G'day Trucky.
I have RR rods in my Cafe. Sorry I lost all my data in the fire (think my memory went too  *smile*). She is up over 10.5 : 1 and still had clearance in the valve pockets (357 cam, SR valves in 67-1102 head).
By my calcs at 5:30am the longer rods increase comp from 7.25 to 7.75.
Cheers
Title: Re: Connecing rods and balance factor
Post by: Truckedup on 02.10. 2020 23:08
 Thanks, the information it's helpfull.  I searched online and found my notations for the 650 Triumph race bike on Brit Bike....Yeah, I don't keep track of stuff. Anyways, Triumph 650 has a 6.5 inch rod with a big end similar in size to a LG A10. R&R Triumph rod is 415 grams, Map says their A10 rod is 440 grams..I know additional weight can be compensated with proper balancing, but I believe less weight is always better for vibes....I hate decisions,  lol