Thanks for the info - sounds like you have done the research! That being
so, I have a few questions. BTW, the original poster was the guy breaking
BMC cranks - I'm currently running Ford based engines. I ran A series a
few years ago, and like you I ran essentially stock cranks, well prepped.
The question: I'm running a stroker version of a TwinCam bottom end. I
think one of the problems is that a critical vibration harmonic is
occurring right in the center of my power band, while in motors with
shorter strokes the harmonic happens at a higher RPM than the motor turns
to. From what I've read in books, the FFord has the same harmonic problem,
occurring at about 6800 - 7000 rpm, and they say that FFords that are run
there a lot tend to fail the cranks faster than FFords that don't.
Here's the stroke data: I'm running 82.2 mm stroke, giving 1800 cc (the
engine was designed for the 2 litre sportscar class, in case anybody shouts
"cheater"!). The Twin Cam, apparently safe to well beyond 8,000 rpm, has
about a 72 mm stroke. The FFord, which I've heard has the harmonic around
7,000 rpm, has a 76 mm stroke (I think). Any thoughts on where the
critical vibration is with my stroke of 82.2 mm?
Thanks to anybody who can answer this!
Brian
At 11:08 AM 07/10/2000 -0400, you wrote:
>Group,
>
>A few years ago, after breaking two MGA cranks, I did extensive research
>and analysis of crankshafts. I'm sure everyone here has reported their
>personal experience accurately, and that experience is as valuable as any
>"theory".
>But there are some immutable laws of physics that override some of the
>speculative thoughts expressed in some previous messages. To clear up a
>few myths:
>
>In a 3-main 4-cylinder engine, BY FAR the most destructive stresses on the
>crankshaft are caused by the reciprocating masses acting at high RPM. In
>a nutshell, it is NOT horsepower that is breaking these cranks, it is high
>RPM. The inertial forces are proportional to the engine speed squared --
>e.g. 8000 RPM creates over twice the amount of stress as 6000 RPM (8**2 =
>64; 6**2 = 36). Lots of other considerations came up in the analysis, too
>much to detail here.
>
>An obvious implication -- lighten the reciprocating masses. Running stock
>pistons & rods isn't a real good idea at high RPM. Lighter forged
>aluminum pistons, and aftermarket racing rods will reduce the stress on
>the crank considerably.
>
>Another myth: nitriding EN 16. It will have relatively little benefit.
>Conventional nitriding only works when the steel has a fair amount of
>certain alloying elements (notably chromium or molybdenum). EN 16 doesn't
>have either, so it won't form the beneficial nitrides you're looking for.
>That's why Runciman noted that "it hardens less than EN40. EN 40 B
>nitrides very well (3% Cr, 0.6% Mo). The correct method for heat treating
>EN 16, or any other non-alloy or low-alloy steel, is Tufftriding -- but
>very few heat treaters still perform Tufftriding. There are a couple
>other methods that may work, but I don't have enough data on those (I
>Tufftride my cranks).
>
>Another myth: that EN 40 is somehow "more rigid" than En 16. All steels
>have essentially the same modulus of elasticity. In simple terms, they
>are all "equally rigid". Certainly some are STRONGER than others, and
>heat treating affects strength -- but the modulus of elasticity remains
>essentially the same at 30x10**6 psi for all steels.
>
>One more: balancing. Yes, it's a good thing, and there is no reason not
>to balance ... but it does relatively little to reduce the stresses on the
>crank. It DOES tend to reduce the force that the engine transmits to the
>frame, and hence it "feels smoother". But the inertial forces due to the
>reciprocating masses in the engine are still there, and just as large as ever.
>
>When I was studying this a few years ago, the "billet crank" manufacturers
>in the U.S. were Moldex, Scat, COLA, and LA Enterprises. I think some of
>those may be out of business now. In England, the billet crank source was
>Phoenix. I have heard good things about Moldex and I own one Moldex
>crank, not installed yet, but appears to be well made. The typical billet
>crank is made of 4340 steel (in the U.S.), nitrided, and shot peened. In
>the U.K they typically use EN40B.
>
>One of the most critical aspects which determine life of a crank,
>particularly a poorly-supported one like a 3-main 4-cyl, is the fillet
>radius of the journals (particularly the main journals). A common mistake
>occurs when a machine shop regrinds a crank, and reduces this fillet
>radius. It should be kept as large as possible, without disturbing oil
>flow out of the bearing. On the MGA, 5/32 radius was a good number. A
>fillet of 1/16" will reduce life by something like 80%.
>
>Another avenue for improving fatigue strength is shot peening of the
>fillets (journal surfaces are masked). This works even better if the
>crank is properly heat-treated first (e.g. Tuffrided or nitrided if
>appropriate alloy).
>
>Hope this helps you, Brian, and clears up some of the mis-information out
>there. For what it's worth, I have chosen to use NOS factory cranks in my
>MGA engines (these cranks are EN16), had them Tufftrided and shot peened,
>and theone currently in my car has done about 50 race weekends with no
>cracks (yet). I try to limit myself to 6500 RPM but go to 7000 when
>necessary. I use aftermarket rods & pistons. Not sure on the
>availability of NOS cranks for the A-series, I got mine from England.
>
>Regards,
>Mark Palmer
>BSME, MSME
>
>
>
>
>>From: Jim Runciman <Jim.Runciman@donhad.com.au>
>>Reply-To: Jim Runciman <Jim.Runciman@donhad.com.au>
>>To: "'Brian Evans'" <brian@uunet.ca>, Taiju Kobayashi
>><portago@xc4.so-net.ne.jp>, vintage-race-digest@autox.team.net
>>Subject: RE: Spridget 1275 crankshaft
>>Date: Sat, 8 Jul 2000 20:01:48 +0800
>>
>>I note the exchange on this subject regarding spridget cranks and think that
>>you may be considering unnecessary expenditure. I run an A series 1275
>>engine in a Cooper S and I stopped using the EN40B cranks about 10 years
>>ago. The EN16 cranks have proven far more durable.
>>We wedge the crank and nitride it (doesn't come up to the same hardnes as
>>the EN40 B unit), balance the rods ( I have a set of Carrillos in one
>>engine) and make sure the flywheel runout is no more than 0.001". Obviously
>>everything is balanced.
>>I use one of two US sourced cams and we have an engine that produces +135HP
>>on 100 octane fuel and will rev to 8500 if required. In ten years I have had
>>no blow ups and the only incident with a crank was a crack started to
>>propogate from the corner of the keyway in the end of a crank but it was
>>picked up before any damage ensued.
>>These engines are running 30 tto 50 hours between strips and they are super
>>reliable.
>>problem with th e EN40B cranks is that they are too rigid for the BMC
>>engine. The material is fantastic in almost any 5 bearing engine.
>>Hope you find this of interest.
>>
>>Jim Runciman
>>Jim.Runciman@donhad.com.au
>>Phone:08 9279 7611
>>Fax:08 9279 7173
>>
>>
>>-----Original Message-----
>>From: Brian Evans [mailto:brian@uunet.ca]
>>Sent: Saturday, 8 July 2000 12:12
>>To: Taiju Kobayashi; vintage-race-digest@autox.team.net
>>Subject: Re: Spridget 1275 crankshaft
>>
>>
>>While I had good luck with stock BMC crankshafts when I was running them
>>from the outright failure point of view, I often had to show up with three
>>or four cranks to find one that wasn't cracked after magnafluxing (what is
>>it about magnafluxing that cracks so many crankshafts? pisses me off...)
>>
>>I do know from talking to guys who ran works BMC cars in Canada in the good
>>old days than Cooper S EN-40B nitrided cranks were lifed at three or four
>>races, which suggests that simple fatigue is the real answer here. The
>>stock cranks are old, they have a lot of miles on them, and they weren't
>>designed to last seasons of racing at high HP levels, which I believe is
>>more important than RPM's alone.
>>
>>I too am probably going to be shopping for a billet crank soon. Who do I
>>call, who has the good prices and delivery, and who has the quality?
>>
>>Thanks, Brian
>>
>>At 09:55 PM 07/06/2000 -0400, Taiju Kobayashi wrote:
>> >Dear fellow listers:
>> >
>> >We have been racing a Spridget w/1275 motor quite successfully, however the
>> >crankshaft was chopped twice in 2 years.
>> >The crankshaft is nitrided and balanced, and we kept it revved under
>> >7500rpm. And the engine is professionally built and maintained by
>> >ex-Formula 3000 engineer.
>> >
>> >Now that the well-prepared stock crank proved themselves unreliable by
>> >nature, we are in search of a steel billet or some other better crankshaft.
>> >Does any of you have recommendation or real experience with that sort? Let
>> >me know how it works.
>> >We also keen to know what were wrong with our stock crankshafts. Any tips
>> >to avoid troubles?
>> >
>> >Cheers,
>> >
>> >Taiju Kobayashi
>> >Tokyo,Japan
>> >'69 Tecno Formula Ford(coming soon)
>> >'63 AH Sprite MkIII(our friends', myself as 2nd driver for endurance race)
>>
>>Brian Evans
>>Director, Strategic Accounts
>>UUNET, An MCI WorldCom Company
>
>________________________________________________________________________
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Brian Evans
Director, Strategic Accounts
UUNET, An MCI WorldCom Company
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