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Likelyhood of breaking a 64 mm crank
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bcrazy
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PostPosted: Wed Nov 18, 2020 2:28 pm    Post subject: Likelyhood of breaking a 64 mm crank Reply with quote

Did a search and found som examples of broken 64 mm cranks.


Question is how common is this?
If i were to upgrade my 63 1200 with 83 mm cylinders, 1300 heads, dual 32 PDSIT carbs and a better flowing exhaust

Should i worry about the crank breaking?
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PostPosted: Wed Nov 18, 2020 5:40 pm    Post subject: Re: Likelyhood of breaking a 64 mm crank Reply with quote

You should be fine no problem ..the only 64mm crank i would be worry about was the early 36hp/40hp 1959-60 "bastard" crankshaft those cranks were cast and would break ... but i doubt you have a cast crank since yours is a 63 ..if your worried about breakage you should take it in to a shop and have them check it by magnaflux the crank .... I would even have it counter weight but some will say no need to counterweight it ..
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Chickensoup
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PostPosted: Wed Nov 18, 2020 8:20 pm    Post subject: Re: Likelyhood of breaking a 64 mm crank Reply with quote

the issue lies when people over rev a non balanced engine. keep it under 5k, or have it balanced. or better yet just buy a 64mm c/w crank from dpr and have the rotating assembly dynamically balanced.
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esde
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PostPosted: Wed Nov 18, 2020 8:36 pm    Post subject: Re: Likelyhood of breaking a 64 mm crank Reply with quote

Somewhere here I read about having a 69mm crank "destroked" to 64, and using 1600 rods. Shocked
This way they got the thicker rod throws like the 1600, but kept it 64mm. WHY, I cannot tell you, but I read it here so it must be true. Rolling Eyes
Counterweights help minimize crankshaft flex, which helps the crank (and case) live longer.
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Chickensoup
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PostPosted: Thu Nov 19, 2020 5:30 am    Post subject: Re: Likelyhood of breaking a 64 mm crank Reply with quote

1600 rods are longer than 40hp. wont be a direct swap. plus, you would need adifferent flywheel
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bcrazy
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PostPosted: Thu Nov 19, 2020 5:51 am    Post subject: Re: Likelyhood of breaking a 64 mm crank Reply with quote

If i go down this route I will not split the case.
So another crank is out of the question

(my other option it so build on a 1600 with AB case, but do not want to discuss this here.. )

I just want to know the probability of the stock 64 mm crank breaking under ~55 hp/5000 rpm
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PostPosted: Thu Nov 19, 2020 11:25 am    Post subject: Re: Likelyhood of breaking a 64 mm crank Reply with quote

I have a 64 40hp under my workbench wit a broken crank
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esde
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PostPosted: Thu Nov 19, 2020 11:43 am    Post subject: Re: Likelyhood of breaking a 64 mm crank Reply with quote

bcrazy wrote:
If i go down this route I will not split the case.
So another crank is out of the question

(my other option it so build on a 1600 with AB case, but do not want to discuss this here.. )

I just want to know the probability of the stock 64 mm crank breaking under ~55 hp/5000 rpm

Probably pays to have your current crank magna fluxed, shot peened, and balanced, to give it the best odds of living. I've no idea what the probability of it breaking is. But, Ive seen more broken 36 and 40hp cranks than 1600, by
a large margin.
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PostPosted: Thu Nov 19, 2020 11:51 am    Post subject: Re: Likelyhood of breaking a 64 mm crank Reply with quote

If you were to offset grind a 69mm crank pin to Chevy std size what would the stroke be?

eQ
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PostPosted: Thu Nov 19, 2020 2:08 pm    Post subject: Re: Likelyhood of breaking a 64 mm crank Reply with quote

earthquake wrote:
If you were to offset grind a 69mm crank pin to Chevy std size what would the stroke be?

eQ

I know If you offset a 69 with 36hp journals gives you a 74mm Chevy journals are bigger than the 36hp journals ..but I bet it can be made to a 72,73, or maybe a 74 as well never seen an offset 69mm crank with Chevy jounals so i cant really give an awnser just a guess ..its best to go with the 36hp journals and use AApistons h beam rods back in the days guys use to use the 69crank with 36hp jounals to stroke their engines the down fall was the weak oem 36hp rods so after awhile that set up died not many do it now days but now days they make 36hp h beam rods which are stronger and which the setup would be good ...69crank offset to 36hp journals gives 74 mm with 83mm bore would make it a 1600cc and it would keep it in the vintagespeed oldschool enigine theme.....
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TinCanFab
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PostPosted: Thu Nov 19, 2020 5:09 pm    Post subject: Re: Likelyhood of breaking a 64 mm crank Reply with quote

I broke one driving normal. Here are the common reasons I've read on the samba:

-lugging/over revving
-improper machining of the bearing journal during a rebuild, causing a stress point
-someone mentioned Okrasa did tests and determined the 40hp cranks were forged at too low of a temperature?

Mine broke after using in a Bus, but it came from a Beetle. Obviously more load, did that cause the break? Take it as you will
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PostPosted: Fri Nov 20, 2020 6:42 am    Post subject: Re: Likelyhood of breaking a 64 mm crank Reply with quote

In either the Bill Fisher or Tom Wilson book it relates the use of the 83mm big bore P&C kit to crank breakage.

To paraphrase the book, the 83 big bore kit makes the little 40hp engine feel smooth and powerful, and leads the driver to keep their foot in it more, which overrevs the engine, which over time pounds out the center main allowing the crank to flex and finally break.

I can personally attest to the 83s making me want to rev out my '59 bug. I have a '61 tranny, and although I have absolutely no proof it helps, I try not to go above 15 in first, 30 in second, 45 in third and 71 or 72 in fourth.

As stated under-revving (lugging) is also detrimental.
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PostPosted: Sat Nov 21, 2020 5:17 pm    Post subject: Re: Likelyhood of breaking a 64 mm crank Reply with quote

40hp cranks used to break all the time. Get a counter weighted one from DPR and have it all balanced.

If you destroke a 1600 have have to figure out the rods (length and pin size will be wrong for a 40hp).

earthquake I think you end up with a 71mm crank when offset ground.
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Last edited by Teeroy on Sat Nov 21, 2020 6:15 pm; edited 1 time in total
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PostPosted: Sat Nov 21, 2020 5:56 pm    Post subject: Re: Likelyhood of breaking a 64 mm crank Reply with quote

Teeroy wrote:
If you destroke a 1600 have have to figure out the rods (lenght and pin size will be wrong for a 40hp).

I think you would machine the 40hp rod so it is the same thickness as the 1600 rod, same as using a 1600 crankshaft to stroke a 40hp.
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PostPosted: Sat Nov 21, 2020 6:16 pm    Post subject: Re: Likelyhood of breaking a 64 mm crank Reply with quote

True, that would be the easiest/cheapest option.
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PostPosted: Sat Nov 21, 2020 8:10 pm    Post subject: Re: Likelyhood of breaking a 64 mm crank Reply with quote

esde wrote:
Teeroy wrote:
If you destroke a 1600 have have to figure out the rods (lenght and pin size will be wrong for a 40hp).

I think you would machine the 40hp rod so it is the same thickness as the 1600 rod, same as using a 1600 crankshaft to stroke a 40hp.


yes but then youd be limited to vw journal unless you had it welded.
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nsracing wrote:
So YOU ride the bug....and the wife. Nice. Laughing

mark tucker wrote:
eye wood... lost pan of cheetos heads... cb magna spork dist

Laughing Laughing
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PostPosted: Sat Nov 21, 2020 10:43 pm    Post subject: Re: Likelyhood of breaking a 64 mm crank Reply with quote

VW rod journal is the largest/strongest anyway, only reason to go smaller Porsche or Chevy is to help for clearance on larger strokes.
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