nocigarette

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

stevesxm

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

ratman

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

BillK

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.

SB

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.

nocigarette

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

ratman

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

stevesxm

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.

Reed Jensen

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.

stevesxm

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.