Crankshaft Cutting
 

Is There A Special Tolerance Fot Cutting A Marine Crankshaft After A Bearing Has Damaged It

Re: Crankshaft Cutting
 

the short answer is no. rod and main clearences are specific to about 20 dif variables all of which are service dependent... i.e what size the motor is , what the compression ratio is, what temps you expect to run, whether you are runnin hard bearings or soft... some will tell you that "marine motors " want "bigger clearences" because they are "marine" but i have yet to hear a convincing technical reason to justify this... if you have dmaged your motor and it is going back together in the same configuration it came apart AND you know why the bearing failed after a long and happy service life, then i would be tempted to measure the clearences you HAD and stay with them. if that is not an option then your best source of information will be the bearing manufacturer ( clevite are best in my opinon) ... describe to one of their engineers your configuration and operating parameters and consider that ans authoratative.

also... remember... just because the machine shop SAYS the crank is that size, don't believe them. use a dial bore gage on the rods at proper torque and crush and mike the crank yourself and read the numbers for real. assuming someone else has done this is asking to have your ass handed to you later on

Re: Crankshaft Cutting
 

have the block align honed on the main brg saddles, dont run a crank cut more then .010/.010 if you plan on making big power, 2.5 to 3 thousanths clearance on the mains 2.7 is what i run
2.5 on the rods. i would resize the rod big ends at the same time. getting the correct bearing crush is very important for the bottom end to live. good luck, rm

Re: Crankshaft Cutting
 

nossig,
Like Steve said, there is no difference in regrinding a marine engine compared to any other performance engine. I personally think that the surface finish, taper and roundness is more important than anything else. As long as the clearances end up within the manufacturers tolerance range, you should be fine.

rat,
Not grinding a crank more than .010" is an old wives tale. There is absolutely no reason not to grind a crankshaft as much as is needed to repair it as long as bearings are available. The Nascar teams are using Honda size bearings in thier engines, they are probably 1/4" smaller than a normal small block. Guys offset grind big block cranks to small block journal size all the time with absolutely no problems.

Re: Crankshaft Cutting
 

If this is a steel crank you need to find what kind of treatment it has from the manufacturer since some only run a few thou's past the surface..

Most OE GM steel cranks are nitrided. A 10thou cut on these will probably go past the treatment will need to be retreated.

I'm bringing this up because no one in my area can do this - it needs to be packaged up and sent out.

With machining costs, shipping, nitriding, and the shipping back here, it is typically cheaper for us to get a new crank.

If it is a cast crank, you do not have all these issues.

Re: Crankshaft Cutting
 

Thanks Guys, Rm Bad Ass Boat Have Fun With Her..........cant Wait To See It Painted Old School

Re: Crankshaft Cutting
 

polishing a crank will usually remove the nitrating, its only a few thousnths thick. i personally wont run 1000hp on a crank cut .020 or .030 tho i know others have done it. dont want to buy another block and heads if it lets go :eek:

Re: Crankshaft Cutting
 

just as a point of curiosity , what technical reason would use to explain why you wouldn't run a 20 under crank... also... nitriding does not build a surface... or change its dimension in any significant fashion... it just makes the surface itself a bit tougher... you can grind right thru the nitriding and get a perfectly good and usable finish on the steel crank that as it is.

the only significant reason not to run a heavily cut crank is that in extremely high compression applications, the babbit thickness becomes such that it it is suceptable to extruding... but the good bearing mfgs now just use more copper underlayment and keep the babbit the same... so no dif.

Re: Crankshaft Cutting
 

Some builders prefer larger clearances because they run thick cold oil. I set my engines up with .003 for the mains and .0025 on the rods. The critical thing is to make sure the bearing clearances ARE THE SAME for all the rods and mains. If you have one bearing that has slightly larger clearance, the oil will act like water and "take the path of least resistance". This means the rod or main with the larger clearance gets oodles of oil and the remainder suffer.
The big difference between marine and automotive engine clearances are between the piston and cylinder wall. Since marine engines run under a heavier load that auto engines the pistons tend to run hotter ( and larger due to heat expansion ) while at the same time the cylinders tend to stay dimensionally stable because of the cooler running temps of the blocks. So marine engines tend to be a little bit bigger on the clearance between the piston skirts and cylinder walls. If your oil is 10-40 multi viscosity like I used to run then standard automotive bearing clearances on the bearings are fine. I prefer to install the bearings in both the rods and mains after the parts are reconditioned and then measure the size of the bearings with a very accurate bore gauge and sizing gauge. Then have the crank ground to the exact size you need for each bearing. Sometimes I've found different width bearings have different sizes by as much as .001. On Chevs you can purchase overside shells to compensate for the difference. I just ground the crank and rod journals to match. Normally all the rod bearings are the same. I've never seen an appreciable difference between bearings in a set. But on the mains I've seen considerable differences.

Re: Crankshaft Cutting
 

im not sure i agree with a lot of this. assuming i understand what you are saying...

you talk about big clearences for thick cold oil but thats crazy... the oil doesn't stay thick or cold for very long and if it DOES then the rest of the motor is taking a real beating. oil... ANY oil needs to be 200 degrees to work as designed... to stay clean and to flow properly for the application and viscosity it was designed for. and once it DOES get hot for any reason, your film strength is gone because of the big clearence and the failures occur...

and you are suggesting that you selectively grind EACH journal differently on the crank to match EACH individual component assembly ? why on earth would you do that ? have the crank ground properly to the correct size within very good tolerence. have the block align honed CORRECTLY so that IT is the right size and STANDARD have the rods done the same way with proper bolts at the correct preload and then, using your dial bore gage as you say, simply set your clearences by using the 0 or + 1 or + .5 bearings or whatever you need to buy for proper bearings... and you are done and the rotating assy is perfect AND all the hard parts are a standard size ... i can not imagine doing it the way you suggest.

as for the piston to wall clearence... this is a discussion we have had here before... i believe ( exluding blower motors etc) that the vast majority of these motors are over cooled... running a carb motor w/out a thermostat at 140 is just wrong and TERRIBLE for the motor. and the piston to wall as well as other clearences are the reason and one significant reason why the service life of marine stuff seems so low compared to other industries... it has NOTHING to do with the mythical " marine" loads... trucks see the same loads over 10 times the hours that any marine motor EVER does... my f 350 towed a 20, 000 lb trailer up and down the mountains with a 460 gas for 150,000 miles... you think your 502 at WOT for 30 mins a month exceeds that ?

no. there is no magic to any of this. any big block correctly machined to nominally correct tolerences and clearences suited for the application and components specified will work JUST fine without any magic or voodoo. more often than not it is EXACTLY the magic and voodoo... the solving of " problems" that you don't really have, that ends up being the root cause of the next failure.

Re: Crankshaft Cutting
 

I totally agree with the oil issue. Oil should be 200 to 215 for the best lubrication and flow. The temp boils off any moisture. The engine should not be overcooled. When I posted I was talking about generalities. Some clowns prefer to run 50 weight oil at 70 degrees. I sure don't. The only reason to grind a journal differently is if the bearings are of a different size. MOST ARE NOT. But I prefer to check the size before grinding. In automotive applications (Aston Martin), I've seen differences between main bearings, and since a set of main bearings from England for a six cylinder engine are 700.00 dollars, I damn well make sure the crankshaft is going to run in them before I start telling the grinder what size to make the journals. The only reason for more clearance between the piston and cylinder is to allow for more expansion if the engine is SUPERCHARGED. If you are running a normally aspirated engine it probably won't make any difference.

Re: Crankshaft Cutting
 

I Realize That Everybody Has There Point And Good Ones At That, But When I Go Throgh My 540 This Winter And I Send The Crank And Block Out What Should I Tell The Machine Shop About Where I Want My Tolerances At...............

Re: Crankshaft Cutting
 

as i said earlier.. there is no one size fits all answer. depends on the complete configuration and expected service usage... my advice was and remains, define the cr, define the hp, define the oil and side clearences, define the temps, define the expected torque and then call the bearing mfg you intend to use. your 540 and someone elses that works well with his clearences may be completely wrong for you.

Re: Crankshaft Cutting
 

thanks steve as always appreciate the help.................i will call clevitt today unless you know of a better bearing company

Re: Crankshaft Cutting
 

nope... they make the best bearings there are.... find an engineer there that is familiar w/ the winston cup motors....

Re: Crankshaft Cutting
 

stevesxm - somewhere above you mentioned grinding past the 'nitriding treatment' and using the resultant surface is okay.

I'm no metalurgist or expert on it so I have always ASSUMED that these should be re-nitrided for proper dependability/ function.

I'd like to know / hear your reasoning behind this because I am curious and want to know if I should be doing more research into this for myself.

Thanks.

Re: Crankshaft Cutting
 

Shoot for .0025 for the rods. .003 for the mains. Some builders like .0035 for the rear main due to crank flex but I set my mains on a 540 at .003 straight across. That is a safe clearance for your engine if you are running a good multi-viscosity oil such as 20-50 or 10-40. These are pretty standard clearances for a big block Chev engine.

Re: Crankshaft Cutting
 

I'm running .0028 on rods and .003 mains in my 540 but its blown. If it was N/A I would go another .005 tighter on both and run a little thinner oil. I run 20/50 or 50 wt depending on water temp/time of season to maintain some viscosity when fuel starts to dilute the oil and hold oil pressure when ran hard.I also use all -12 hose,full flow fittings,2 qt filter and the melling factory blueprinted pump plus decent size cooler. Motor runs 70 psi oil pressure under full boost hot and 40 at idle after a real hard run. There are ALOT of stock 502's running .002 or less on mains and close to .0015 or less on rods for hundreds of hours w/15/40 and std volume oil pump. The first machine shop screwed my mains up royally in my 540,its important the proper crush and perfect main alignment is maintained when your machine work is done,if you have sloppy machine work than you need extra bearing clearence to bandaid it. My stock 502 when checked on the stand after beating on it for 50 hours in stock form and approx 150 hrs w/blower still had mains within .0001 of each other so keep in mind blocks are often screwed up by line boring/honing when they didn't even need it in the first place. As far as machining off the nitriding,I'm not sure on a stock crank but I specifically bought a crank thats perma-toughed .080 deep for future rebuilds and I already been thru the motor once and crank and bearings still looked like new,Smitty

Re: Crankshaft Cutting
 

nitriding is a process where the surface is is subjected to a diffused nitrogen process and, as such merely changes the SURFACE properties of the material... no material is added and no dimensions are materially changed altho it is a hi temp process... the point is that it is not a heat treatment in the traditional sense.. where there is material change in dimension and the heat treatment itself extends into the substrate to a measureable depth... with nitriding you start with a perfectly good crank made of perfectly good material that you could run just the way you have it... and make the bearing surfaces a little bit more resistant to scuffing and casual abuse... but if you grind thru it to the next undersize... you still have that same perfectly good crank you started with... it was a very popular system when crank materials wern't very good and you could toughen up the journals and reduce simple wear.

and smittys analysis of the clearences is spot on

Re: Crankshaft Cutting
 

You might not be the most popular guy around but you TELL IT LIKE IT IS!! Keep it up :D ,Smitty