Britbike forum Forums Thread Like Summary - Crankshaft Decisions (now with pics)

Build goals:

-Dead reliable

-Limit vibration

-A pip more power

What I got:

-1973 T140V cases

What I want to do:

-Nikasil Triples Rule T120 Big Bore Kit

-Long Rods

-10 Stud T140 Bonneville Head

Question:

I have two current choices for a crank. Choice 1 is an NOS TSS crank sans flywheel. Choice 2 is an NOS T140 crank

I can get the long rods to fit the TSS crank from Thunder Engineering, so that's no issue.

Given that, which would you choose?

I'd agree with everyone who's saying that standard T140 cranks are fine, as long as they're crack tested (Magnaflux or similar), and haven't been abused. There are quite a few ways of abusing them, from punching the sludge trap plug area to death, to mushrooming the oil feed nose with hammers (depressingly common, judging by the number I've seen for sale of Ebay that have suffered this), to running with a loose rotor nut, which hammers the rotor keyway and can even hammer the sprocket splines. I wouldn't buy a crank that had been reground, only one on std and without measurable wear. But if all these potential flaws are absent, and the crack test is passed, I wouldn't hesitate to use such a crank for a normal road bike expected to cruise at a gentle 75mph. I tend to sit at between 80 and 85 on my TR7 when on the autoroutes in France, briefly over 90 for overtaking.

The only reason for my TSS crank is that I stumbled across it incredibly cheap (£125) on Ebay, being sold by someone who didn't know what it was. I paid the 'Buy It Now' asking price and took a gamble - it looked really good in the photos, though without measurements, there was a chance it was a Harris T140 crank (which are essentially TSS cranks but with the big end journals ground to standard T140 big end diameters). I was lucky, it was a TSS. Steve Campbell told me it was the best he'd seen that wasn't brand new - hardly used, in his opinion.

I needed a set of new rods in any case so, for the £40 extra for the pair (£320 as opposed to £280 for T140 Thunder rods), it made perfect sense. I had wanted to build a really fast engine anyway, so that's where the TSS crank is going. An additional advantage of the TSS cranks is supposedly a smoother engine, due to reduced crank flexure.

My bone stock 72 T120RV has done many, many miles at 75/85. Once in Nevada I covered 90 miles in less than an hour. These old turds are way tougher than most people think.

i am personally a believer in paying more so i can worry less. my crank has seen 9000 rpm through the mile lights one time, and 8000 pretty often. i have zero worries about it cracking, bending, or breaking something else.









certainly it might be overkill for a street bike, but the day you spin the crank past the last hour of its fatigue life and it blows up, you lose the motor.

actually, with this crank, you lose the cases. this crank was blown up once already and was tough enough to come right back:

...I remember years ago reading about ARP bolts for Triumph and all the hype involved then I decided to dismantled a 79 engine that I d rebuilt to change the rods bolts that came with the new Harris connecting rods for new ARP bolts and nuts; now I just reading this about that the "last thing" is to use ARP bolts but with the latest Triumph nuts!...
-also to find a decent shop that sells those nuts and not to sell something that looks like but is not...well, could be difficult.
Anyway; I am not dismantling the engine again to follow the last hype...