Can anyone point me to a definitive resource on main bearing and rod bearing tolerances? I'm finding only guys talking about what worked for them.
My FORD ENGINE OVERHAUL MANUAL tells me for the main bearing oil clearance:
"Generally, the clearance should be about 0.001 to 0.002 (for exact specifications, see the Haynes Automotive Repair Manual...)
Well, I have that manual too and it does not list it anywhere that I have found yet. Only the endfloat tolerance is mentioned.
If any of you have a better resource handy, please let me know what you need to know. 351c, 4v, H code, no drag racing(well maybe once).
I have seen mentioned online that 0.002 to 0.003 is the way to go.
Others had said .0025 to 0.0035.
We have the Plastigage and have not yet used it to see where I stand.
I'm incline to say that anything between 0.0015 and 0.0035 is fine, but I defer to those of you with more than my zero engine build experience.
(My father-in-law is helping me and he has countless Chevy builds, but this is his first Ford so we are erring on the side of caution.
Thanks for anything you can offer,
Zac
I agree...excercise caution. You may consider calling a machine shop in your area that has experience building Ford motors. A good machine shop should provide this information to you and or provide a reference source you could go to to get this information. To be sure you get the right answer, you may call 2 shops for corroboration.
Anywhere in the .0015 to .003 is acceptable with .0015to .002 being optimal. Also described as one and a half thousandths to three thousandths, being acceptable, and one and a half thousandths to two thousandths, being the best. Machining and tolerances have changed as far as late model engines go. But when rebuilding an older engine, you need to stick with the older way of doing it. As they are no where near as precisely engineered, as todays engines. They need the extra clearances due to the twisting and flexing, that normally occurs in an engine under normal operation. Newer engine are stiffened, to eliminate this torsional flex, thus they can run tighter tolerances. Hope this helps JTS 71 Mach1
(09-06-2012, 11:23 AM)JTS71 Mach1 Wrote: [ -> ]Anywhere in the .0015 to .003 is acceptable with .0015to .002 being optimal. Also described as one and a half thousandths to three thousandths, being acceptable, and one and a half thousandths to two thousandths, being the best. Machining and tolerances have changed as far as late model engines go. But when rebuilding an older engine, you need to stick with the older way of doing it. As they are no where near as precisely engineered, as todays engines. They need the extra clearances due to the twisting and flexing, that normally occurs in an engine under normal operation. Newer engine are stiffened, to eliminate this torsional flex, thus they can run tighter tolerances. Hope this helps JTS 71 Mach1
Definitely helps validate what I have learned so far on the subject.
Thanks!
Glad I could help. JTS 71 Mach1
It's all subjective as to what one can get away with in a street motor. A lot if you know what your doing. Balancing is very important for main bearing logevity, remember pieces of a wearing main bearing can get sent right down to the rod bearing through the oil hole. You need a good bore gauge and mics reading in .0001". Forget about plastigage, it's useless to a professional engine builder. The cranks oil holes should be chamfered. A crank should have no runout. The rod and main bores need to be within .0002" preferably closer to 0. Go through all your rod and main bores with the bore gauge and note their size, so you can adjust your clearances easier for the desired bore. Always use the high reading on the cranks journals when setting the bore gauge. The cranks journals need to be within .0003 at the most or I'd be looking for another standard crank. Turning a crank is always your worst option because you loose the ability to buy oversized bearings once it's been turned. I have seen some .011 over bearings but I don't know about availability, worth looking into if your stuck with a .010 under crank and are having problems. You can mix bearings from different mfgrs to try to get different clearances. If you have a set of ball mics that read in .0001 you can go through all your bearings and try to find different bearing thicknesses and use them accordingly to adjust the clearances. In standard you can buy over size bearings in .001 & .002 and if the clearance isn't right you can half shell them in the bores, one bearing half standard and one bearing half over size in the bore, whatever it takes to get you in the right range. If you are running on the ragged edge of loose you should use a high volume oil pump with a good 20-50 weight oil. IMO for a 351C street motor. Rods, no less than .0018 no more than .0027. Mains, no less than .0022 no more than .0032. For main bearings I prefer 3/4 groove or full groove because the crank is not cross drilled. 1/2 groove bearings do have more structure but giving away full 360* oil IMO is a bad thing. Stay away from race bearings, they are much too hard for a street motor. Restrictor kits are not necessary in a street engine and if I ever use them I always drill them out bigger. Make sure you seat the thrust bearing the right way. Pry the crank in either direction when torquing the thrust bearing. Then get a piece of brass and a small sledge. Put the brass inbetween the sledge and crank and strike the snout and the flange firm but not hard. Then check the play with a dial gauge with a magnetic base. You should always check the rear main seals crush. It should be around .020. I always put the seal in by it's self and tighten the cap down and check it for roundness after checking the crush, you don't want to get caught with a rear main leak.
(09-19-2012, 02:56 PM)furray Wrote: [ -> ]It's all subjective as to what one can get away with in a street motor. A lot if you know what your doing. Balancing is very important for main bearing logevity, remember pieces of a wearing main bearing can get sent right down to the rod bearing through the oil hole. You need a good bore gauge and mics reading in .0001". Forget about plastigage, it's useless to a professional engine builder. The cranks oil holes should be chamfered. A crank should have no runout. The rod and main bores need to be within .0002" preferably closer to 0. Go through all your rod and main bores with the bore gauge and note their size, so you can adjust your clearances easier for the desired bore. Always use the high reading on the cranks journals when setting the bore gauge. The cranks journals need to be within .0003 at the most or I'd be looking for another standard crank. Turning a crank is always your worst option because you loose the ability to buy oversized bearings once it's been turned. I have seen some .011 over bearings but I don't know about availability, worth looking into if your stuck with a .010 under crank and are having problems. You can mix bearings from different mfgrs to try to get different clearances. If you have a set of ball mics that read in .0001 you can go through all your bearings and try to find different bearing thicknesses and use them accordingly to adjust the clearances. In standard you can buy over size bearings in .001 & .002 and if the clearance isn't right you can half shell them in the bores, one bearing half standard and one bearing half over size in the bore, whatever it takes to get you in the right range. If you are running on the ragged edge of loose you should use a high volume oil pump with a good 20-50 weight oil. IMO for a 351C street motor. Rods, no less than .0018 no more than .0027. Mains, no less than .0022 no more than .0032. For main bearings I prefer 3/4 groove or full groove because the crank is not cross drilled. 1/2 groove bearings do have more structure but giving away full 360* oil IMO is a bad thing. Stay away from race bearings, they are much too hard for a street motor. Restrictor kits are not necessary in a street engine and if I ever use them I always drill them out bigger. Make sure you seat the thrust bearing the right way. Pry the crank in either direction when torquing the thrust bearing. Then get a piece of brass and a small sledge. Put the brass inbetween the sledge and crank and strike the snout and the flange firm but not hard. Then check the play with a dial gauge with a magnetic base. You should always check the rear main seals crush. It should be around .020. I always put the seal in by it's self and tighten the cap down and check it for roundness after checking the crush, you don't want to get caught with a rear main leak.
Thank you for your input. Super awesome knowledge.
