Hopefully Sluggo will come in here. Generally speaking, a forged rod is superior to a billet rod. However, alloy rods don't last forever and we have no idea just how much of an old rod's fatigue life has been used up. 25%, 50%, 99%? I still have original rods in my A7 and A10 but very rarely exceed 5,000 rpm. Having said that, my Norton 99 had a rod fail while bumbling along at a modest 4,000 rpm. On my way to start a new job just to make it more annoying.
<Flattered>
But I tend to sound like a broken record on this........ Blacksheep is correct though about a stock rod. Alloy rods are springy when new but have a service life, Stan Shentons "Triumph Speed tuning" Bible quotes the Triumph factory engineers as saying the service life of a Alloy rod is one 500 mile race. American top fuel dragsters get one pass down the strip. You can buy used NASCAR racing rods cheap on ebay as they have usually a one race life.
Our old bikes dont see those kinds of stresses but largely unknown with a used bike WHAT stresses its seen so poor economy to reuse. That being said... I DO sometimes. But when I do it gets a full crack inspection, checked for straightness and then resized. ( A little off the cap joint surfaces and then remachined to true round again, a used rod is NOT round once removed). But that being said, I will only do that if it looks, tastes,smells good to begin with and then severely RPM limited. (Same with the rod bolts)
For a Sunday plodder with low comp and mild cam I would say roll the dice after a full rebuild with a competent machinist. But you mention a perf cam and higher compression so my answer is I would suggest the steel rods (H beam). I agree with Trevor that Billet rods are kinda iffy.,, If it was for a Small Block chevy from a well known vendor and price was right I would consider it, but not for the BSA Application
I have seen some decently priced rods of the steel variety and similar to the Carrilos or were made by Carillo? (I dont recall) But I know MAP cycles and the owner is Marino and they have been a consistently awesome vendor for decades. I have used their belt drives and some of their other products and always been very happy. They support the purchaser with tech and are nice people to deal with. I was a dealer acct for them for years and often ordered from them for customer builds. Plus, they race, and they support racers so,,, I do like MAP..
The problem in the past was that steel rods were really heavy. Re-balancing is a must but it was a real problem 15-20 years back. I have not done a survey of rod weights and specs but my *UNDERSTANDING* is that the newer generation of the steel H beam Carrillo type rods are now made with much lighter weights while retaining great strength. So,, not as much of a problem.
I will add this,, (Because I am pedantic and a bit OCD) the following:
A) I cant say for preunit BSA Twins for sure, but I CAN say that BSA Unit twins and BSA Singles in my experience tend to have wildly varying piston weights and specs. The current crop of Asian made pistons out there for BSA tend to be VERY Heavy. That is something I am looking into for my 2 keeper A65s, It was a problem on my last build. I have some OEM Hepolight/Wellworthy A10 pistons and plan to do some weights and measures. The Asian made pistons are overall excellent quality and consistent standards as opposed to some really weird things with the OEM pistons. But for whatever reasons last 10 years the repops weighed way more than std, something to take into acct. (Measure twice-cut once)
B) While a static balancing job is better than nothing, in my opinion its next to worthless and a waste of time. DYNAMIC Balancing is the only way to go. You cannot fix rocking couple any other way and a Dynamic balancing job for your bottom end is well worth doing, If you cant find a good shop to do it, then find one who will accept the parts being shipped to them. My guy is 2 hrs south of me and I often shipped him a specially lined box with crank, rods, pistons and any other bit that rotates. I also do alternator rotors for crank mounted applications. (A lot of weight flopping around on the end of that bean stock). Local vintage Harley guys use him as well and I can point to Panheads, Knuckles and shovels that are so smooth you would think its a sewing machine. Nortons are the worst for this and have had Not-runs that shook so bad parts fell off L&R and Ernie transformed those motors. I have built Atlas's that people swore was a Honda they were so smooth.
C) Careful blue printing of the cases and crank will reward you greatly. Again I cant say for certainty on a Preunit BSA Twin, but I can say from experience that BSA Unit Singles, Preunit singles, and Unit twins all seem to suffer from core shift and dimensional changes. (Or they were buggered from day 1) My friend Sir Eddy used to blue print engine cases for me and others locally and while some were pretty good, We saw a LOT of them that were way out of spec. A local guy who used to race Goldstars had repeated seizure and blow ups with his Goldies. Eddy showed him how they were out of whack. Eddy tended to value his skills highly monetarily and this guy who had the Goldies,, well lets just say is quite focused on economy and said it was too much $$$$. Promptly seized up his motor next race. I will know more in the next 6 months about preunit cases for BSA as I have several cases to check, Might write a tech article on how to do it and my results, But it would NEVER hurt a dang thing to check your cases and parts carefully.
SRM parts sure look nice, But I am not sure they all are worth what they are charging, Maybe they are. I have no experience with those Thunder rods,, But I do have a BSA B50 Carillo rod sitting on the bench I can post pictures of. I think they are worth every penny and its well reported that the BSA B50 rods stock had issues so was well worth the dosh in my opinion to buy them. I worked in manufacturing (Aerospace) and my wife currently does in machine and manufacturing at a large volume shop and I can say the Carillo brand products are EXCELLENT and I would not hesitate to recommend them as good value. (35 years experience with them)
I have never regretted erring on the side of quality parts and replacing parts I was not certain of.
"Dammit to hell I wish I hadnt spent so much on new bearings said no one ever".
