Crankshaft materials

[zefarelly]

Im gathering bits to build an engine (ford kent block, precrossflow) and am considering crankshafts. . . . .

my little brain has so far absorbed this much

standard cast iron crank, max ravs 6500, 7-7500 lightened, balanced and tuftrided.

Cast iron, from billet, better quality, bit stronger, safe to 7-7500

nodular cast iron ???? no idea (fomula ford apparently?)

forged steel (eg farndon) rev through the roof, mega strong, but prohibitively expensive at the moment

any ideas, corrections or advice greatfully received.

Ive been offered a billet crank which sounds like a deal @ £100 . . . . . . .

[KC]

Nodular iron is made from sintered iron powder and then is drop forged and pressed into shape. It makes for a very strong and light weight crank, but expensive.

Whatever you buy, if new, demand to see a metal certificate for the material used. Any good crank maker will have them. They get the cert from their supplier. It is the makers guarantee that the material is of good quality. If you get a used one if might be worth it to you and your engine to get the used one magnafluxed for cracks.

[zefarelly]

ok, thanks KC . . . .someones got a nodular one, as new with bearings for £350 . . .says its done 1 hour, Id like to know why its for sale/only done an hour , but if its good it could be the christmas present Im after, what sort of revs will they take? 8000?

[avsfan733]

i'd go for the billet because of the singular metal matrix qualities that the billet naturally posses being expensive inexpensive is a moot point because the way u are looking at this with each increase in price u not only get the increase in strength which allows u more revs/power, but the increase in strength also gives u better wear meaning more longevity and probably saving u money in the long run

[KC]

The billet crank is probably the best way to go for longevity. What do you expect from your engine? Is it going to be a screaming high rev engine or more a stump pulling torque monster?

[zefarelly]

thanks people . . . . .I think I'll go for the billet, given that this will be my first engine, and the constraints of my budget Im not planning on building a rev monster, I was thinking along the lines of a kent 244 cam or similar, keep it around 7k RPM max and hope its strong enough and reliable, thus maximising the time I get on the track so I can learn a bit of racecraft whilst not having to sell my soul to rebuild the thing every 1/2 hour of running.

the current plan is billet crank, 125E rods, twin 40's etc, that should hopefully give me about 120 BHP with a drivable rev range . . . .and reliability

[boyracer]

A good standard cast iron crank will put up with a lot of abuse, especially if you nitrided it and had it properly balanced.
The biggest problem at around 7000rpm would be the harmonics. In other words slight out of balance which then cause resonance in the crank.
Know of a 4AGE (Toyota 1600cc) that produced around 240bhp (flywheel) powering a westfield (similar to a caversham, a light car anyway) that did a crank every season of racing.
Lasted longer after we took LOTS! of weight out of the flywheel, but the crank was stock standard with no prep at all. By thw ay redline was 7300rpm.

[zefarelly]

thanks boy racer, I think thorough balancing of rods and pistons is vital, as well as crank, and a steel flywheel lightened too is probably a very good idea . . . .hopefully that'll survive intermitent 7500 peaks . . .the next question is camshafts!

[coln72]

From memory sintered iron and nodular iron are two different things. Nodular iron (or ductile iron as it is also known) is regular cast iron with magnesium added to convert the carbon present from flakes into nodules. This improves the strength and fatigue life of the iron.

I beleive in using stock components where possible. the bottom end in my Corolla is untouched and sees regular excursions above 7000rpm (sometimes above 8000rpm) and has done for the last four years with no sign of any problems.

[Lee Janotta]

So much of it's about properly breaking in and maintaining the engine. I remember reading about the Penske race team braking in their Trans-Am engines by letting them sit idling in the raceshop's parking lot with a garden hose running water through the cooling system.

[coln72]

True, very true. My old mans Corolla was never run in properly by its previous owner and now after a couple of events its (blew the dipstick out of the motor). To the point that people are offering him pistons etc. to fix it.

[zefarelly]

I'd hazard a guess that a Corolla crank is manufactured to slightly higher tolerances than a 40 year old Daghenham iron stick . . . .by the time you've lightened and balanced your probably nearing something like a bog standard jap equivalent made in the 80's or after !

[coln72]

Shame that the original motor I was using was manufactured in 1973. Only changed it because I had an offer too good to refuse . I agree though, the japs seemed to have their act together a little bit more than the Poms or the Yanks back then.

[Red Dog]

zefarelly, the ford engine is a stonker, don't be fooled. BTW I am not a Ford fan. The Kent is very short stroke and with a steel bottom end and decent cam will hit 9000 rpm and not break! Yes the Nips build their production stuff to better tolerances but a racing engine does not run tight tolerances and a good machine shop will do the grind or rebore spot on. There are some excellent tricks to help the engine bed in quickly. The most important being to have the thing machined and built properly!

[Dave Brand]

Originally posted by Red Dog
but a racing engine does not run tight tolerances and a good machine shop will do the grind or rebore spot on.

You seem to be a little bit confused between tolerances & clearances. Doing a grind or rebore 'spot on' implies working to very tight tolerances!

[Red Dog]

Absolutely right Dave Brand - that's what I meant! It was one minute to three in the morning though....!

[Dave Brand]

Originally posted by Red Dog
Absolutely right Dave Brand - that's what I meant! It was one minute to three in the morning though....!

Try getting a little bit of sleep now & again!

[zefarelly]

I have no experience building 4 stroke engines, Ive only ever built two strokes, I imagine bore clearances arent so critical, but cranks and rod bearing ones must be . . . .kent engines are a doddle to build though . . . .assuming the machinings done properly what can possibly go wrong!!!

[boyracer]

Exactly... most of the art in engine building though, is checking that everything has been machined spot on. Everyone f**ks-up now and then, even machinists.