TR4/4A Prepping Crankshaft

[TRMark]

I was going to do some rear seal work to cure a nasty leak. Then I looked at the camshaft.

So apart comes the engine. While I have it apart............
I was Road America last September and talked to the TR4 racers about keeping the engine together if I installed a more aggressive cam and wanted to use higher RPM. I was kinda surprised to find out the guys I talked to were using a very "carefully prepared" stock crankshaft along with Carrillo rods. Thought I would hear Carrillo and "Moldex" or some other custom billet crank. The Carrillo rods are doable but I am not sure what is meant by "carefully prepared." Any suggestion as to what that means and/or where I can send the crankshaft.

 

[CJD]

Prepped crank envolves:

Carefully inspect the crank you intend to use using visual, die penatrent, and x-ray methods for any surface or internal cracks and/or voids.

Once you have a decent crank, die grind all surface flash and imperfections to be smooth.

Now, have it ground .020" or more under, paying absolute care to the radius where the grinding meets the sides of the crank pins. The idea is to grind the largest possible radius into the sides of the crank pins.

Now send the crank out for balancing. You must know which rods and pistons you are using and have them sent too. Of note...use the lightest rods and pistons you can find, as RPM is what kills a crank, and the forces from the mass of the rods and pistons increase exponentially with RPM. The lighter your rotating/reciprocating masses, the less stress on the crank.

Now preparation diverges. We need to improve the fatigue resistance by one of 3 methods. The classic is to tape the freshly ground pins and have the crank shot peened...especially the new radiuses at the pins. This is not some guy in a speed shop, but a metal prep shop that is able to shot peen to aircraft mil-specs. Option 2 is to have the crank chemically nitrogen treated, or tufrided to harden the surface. Option 3 is ion nitriding. Of the 3 options, I still lean toward the old school shot peening.

The above is basic prep. You can actually go farther, by adding lightening holes through the crank pins...but that becomes a bit of an art learned through talking to guys who have done it before.

If you prep a stock crank well, the time and money will be a larger investment than a new billet crank! You could safely increase RPM by about 25% over stock red line for road racing, and up to 50% higher for drag racing.

 

[TRMark]

Thanks John, that helps when I go to the local machine shops, although I am doubtful they will be of much help, mostly domestic hot rod stuff around here. Do you have any suggestions of a specialty shop I could send the crank that understands the necessary techniques? I have heard also of chamfering the oil holes at their outlet in the rod and main journals to improve oiling, is that something to consider.

 

[CJD]

Billings. Wow, I guess I'm spoiled down here in Fort Worth. We have Bell Helicopter, Lockheed, and General Dynamics. When I used to regularly build engines I googled metal treatment and preparation and got 3 local mil spec shops. That may be very hard in Billings.

I used to do the clean up grinding myself, but used the treatment shops to x-ray and shot peen. If I wasn't in a hurry, they would slip my little cranks into batches they were doing for a big client, and barely charge me anything for it. Of course any machine shop that has a balancer can do the balancing for you. If there is a local race track in town, at least one machine shop will be familiar with adding decent radiuses to the journals.

If you don't have any luck finding local shops, maybe someone knows of a shop that supplies finished prepped cranks?? They're a dime a dozen for Chevy, surely there is one or two for our Triumphs?!?

Oh...forgot about your other question...
Yes, chamfering is important for the oil passages. I would frequently even go a step farther by grinding a "V" shaped notch in the journal as a feed to the main journal oil holes. When combined with main bearings that have the upper half notched, the idea was to have a continuous supply of oil to the crank rod journals.

 

[TRMark]

Thanks again John. I copied and pasted your comments so I can file them in the build file. The internet and this forum are a huge help to those who live in more "remote" parts of the country. It's not like Billings is at the end of the Earth, but we can see Wyoming, which of course is.