rebuild tolerances for 200 six

[edgewood bronco]

In checking the tolerances for bearings in this engine, they're coming out at what I understand are at or near the absolute maximum. The main bearings all plastigage to 0.002 - about a half thousandth more than what I have as ideal. The rods are the same, with two creeping in to the 0.0024 zone.
My crank shaft end play is 0.011 - I have 0.012 as the wear limit.
So, while technically within tolerances, I'm putting together a worn engine, am I not? do bearings come in half thousandth sizes?
What about the end play? Can I get a bearing with a thicker thrust bearing surface?
What are my options to get this better within spec. I only want to rebuild this once!
THanks!!

 

[MustangSix]

If I were building a modern engine such as a Zetec or Duratec and using a 5w-20 synth oil, I'd shoot for a tighter tolerance, but on an old school 200 running heavier oil such as 10W-30 or 10W-40 I'd be ok with your bearing clearances in the .002 to 0025 range. You'll get plenty of mileage out of it.

I think the crank end play is OK as well. You probably didn't measure it but the end play was probably twice that when you took it apart. Again, I'd be more inclined to try to get tighter if I were building an engine that had a really high pressure clutch or sustained high rpm requirements (race engine).

 

[edgewood bronco]

Thanks Jack.
The plan is to run synthetic, but 10W 30. Would that make a difference?
THanks.
Andy

 

[MustangSix]

No difference as long as the viscosity is the same.

 

[rbohm]

i have to agree with jack. you tolerances are within limits, though the end play is close, but i see nothing wrong with your numbers though. good choice on the oil as well.

 

[wsa111]

Are the tolerances with old bearings or with new bearings installed on the unturned crankshaft??
Your measurements are within specs, but at the max. Your choice while you have it apart.

 

[edgewood bronco]

This is the newly ground and polished crank and 0.010 oversize bearings. Which is why I ask if there's a possibility of getting 0.015 oversize bearings That'd be perfect.
I cleaned off the bearings and journals of the two connecting rods that were a bit more than 0.002 and re- checked them twice and I got 0.002 for both of them, both times.
The machine shop's work is spot-on, I just wish I'd been a bit more careful about specifying what tolerances I wanted.
Now on to zero-decking the block...

 

[Econoline]

If the crank was ground and polished you should expect the tolerance to be good. My crank was ground 10 under and all of the mains and rods were spot on @ .0015". I'd talk to the machinist about what you've found. What brand bearings are you using?

 

[MustangSix]

I hesitate to mention it because I've never tried it on a 200 (or any other engine), but WAY back in the day, I was helping rebuild a 32 Ford Model B four banger at the machine shop where I worked as a kid. The main bearings were a little too loose so the old man gently took the inserts out, sanded the mating edges on a flat surface ever so slightly, then used thin aluminum foil as a shim between the insert and the cap. He put foil under both sides of the bearing and ended up reducing clearance by about .001". He told me that he had done it with paper shims as well.

Now I've used spacer inserts to put 350 cranks into a 400 block - no problem. And as far as I know and remember, the Model B engine never had and issue either.

But that was a low speed, low output engine. It topped out at about 3500 rpm, so you can do some research on your own. That may be an option, but unless you are blueprinting, what you have is within spec and I would button it up and not worry about it.

 

[edgewood bronco]

I'll talk to him tomorrow (unscheduled closure today) and see if we can do anything. Otherwise you guys have convinced me that I'll be in good shape and I'll go with it as is.
Thanks!
Andy

 

[CNC-Dude]

Your housing bores in the block can be at the high side of their tolerance and cause the bearing clearances to be toward the high side. If your block wasn't checked, then that can be the reason, not the crankgrinder. As Jack mentioned, you can sand the main caps lightly on a thick piece of glass or precision parallel with some 120 grit sandpaper and some varasol(parts cleaner) to close the housing bores up slightly. All professional engine builder do this, so don't think it is shadetree by any means. But you have to be careful to not make the cap crooked, and sand the cap away from you 2 or 3 strokes in only one direction(not back and forth), and flip the cap in your hand 180° and do it again 2 or 3 strokes using an even pressure on both. Rinse off and recheck your clearance.

 

[edgewood bronco]

Thanks for the tip CNC. some what ifs to ask you:
I assume I want both ends of the cap on the sandpaper at the same time when I do that.
Would it be a good idea to install a main and its bearings without the crank, torque it to specs and use a feeler gauge to see if there's any space between the end of the bearing ends (not sure if I have a fine enough feeler though)? My thought is that I don't want to shorten the caps if the bearings are just going to stick farther out from the ends of the caps and cause some sort of distortion, or am I overthinking this?
I also assume that if I do one, I have to do them all, whether it makes a difference or not.
Thanks!
Andy

 

[rbohm]

Thanks for the tip CNC. some what ifs to ask you:
I assume I want both ends of the cap on the sandpaper at the same time when I do that.
Would it be a good idea to install a main and its bearings without the crank, torque it to specs and use a feeler gauge to see if there's any space between the end of the bearing ends (not sure if I have a fine enough feeler though)? My thought is that I don't want to shorten the caps if the bearings are just going to stick farther out from the ends of the caps and cause some sort of distortion, or am I overthinking this?
I also assume that if I do one, I have to do them all, whether it makes a difference or not.
Thanks!
Andy

yes you want both ends of the main cap on the sandpaper at the same time, you need them to be sanded teh same amount.

as for the bearings, i can tell you that is there is any space between the bearing halves when installed, you will have a spun bearing. the bearing inserts were designed to have a slight crush to them to keep them from spinning.

as for bolting the inserts into the caps, and torquing them in place, engine builders do that all the time to check the bearing clearances.

in the end i think you are over thinking this whole thing. .002 clearance is just fine, so i wouldnt bother trying to tighten up the clearances, unless you plan on racing this engine in a serious class, and i dont think you will do this.

 

[CNC-Dude]

You are only taking a few tenths of one thousandth off of the bearing clearances. As rbohm mentioned, the bearings are designed to have a certain degree of crush when they are installed in the caps and block, so just use your plastigage to verify your results. Also, yes on both ends of the cap on the paper at the same time. Also, to give you an idea of how much one tenth of one thousandth is: A human hair is approx. .005" thick. Cut it length ways into 5 equal diameters. You will end up with 5 strands being .001" each in diameter. Now just take one of those .001" diameter strands and slice it length ways into 10 equal diameters. Just one of those is .0001", or one tenth of one thousandth.

 

[MustangSix]

I think maybe I wasn't clear, but the Old Man actually sanded the inserts, not the caps. He was making room for the shims.

I wonder if that engine is still running? That was almost 45 years ago.....

 

[edgewood bronco]

...unless you plan on racing this engine in a serious class, and i dont think you will do this.

Yeah, I'm not planning on breaking any land speed records in this, just want it rock solid so that I can forget about it. I'll put it together as is and remember that this thread exists if I ever get into another one.
Thanks folks!