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Discussion Starter · #1 ·
Hello, this is my first engine rebuild. I have a few questions that maybe you guys can help me out with. It is basicaly a stock rebuild with a stepped up cam and flat tops9:1. I am using Tom Monroe's book. I will put details of build at the end of message since it is so long.

Questions.I just got the parts together and am starting short block assembly. Checked main bearing clearance with plastigage..main1-.0015 main2-.0015 main3-.0015 main4-.001 main5-.001. What is the main bearing clearance supposed to be and is plastigage that accurate? I'm a little concerned about the number 4 and 5 mains clearance being to small. I torqued mains to 100 ft lbs and made sure everything was clean. Maybe my torque wrench is too cheap..its a taiwanese.

Checked ring gap with feeler gauge and compression rings have only about .007 gap. I read in book that .016 is spec but have read elsewhere it should be higher. I bought federal mogul moly rings they are not file fit rings but I am under the impression that they can still be filed?

The engine is going in a lifted (6") 78 Bronco 4.11 gears 35" tires.
Compcams 265deh (duration in. 265 ex.275,@.050 in. 211 ex. 223,lift in. .484 ex. .510)
Cloyes true roller timing chain, flat top pistons,federal mogul moly rings,arp head and rod bolts,clevite main and rod bearings,high vol oil pump
Balanced rotating assembly,crank .010 .010
stock heads 3 angle valve job new valves,matched springs, milled to clean up
block bored .020 over with torque plates, decked to clean up
edelbrock performer egr intake, stock 2v carb for now, stock ignition, 4 core radiator and hi vol water pump
Regards, Sam

CheeseBurger Milkshake!!
3,760 Posts
IMO plastiguage is accurate and is a good idea anytime one is assembling a new lower end.

A good service manual, or a good parts man should be able to tell you the tolerances for both the rod and main bearings.

89 Posts
Here were the spec's on my main's.
#1 .0026,#2 .0026,#3 .0028,#4 .0028,#5 .0028
All my rod's were between .0019-.0022
Cylinder wall clearance was .0025
Ring gap's were .030 for the top ring and .015 for second.
The ring gap may seem wide but that what the piston manuf. recommended due to the type I'm using.
Rod side clearance were 1&5 .013,2&6 .014,3&7 .011,4&8 .011
Crank endplay was .006.
Cam was degreed in @ 106 degree's.

Your numbers look great. Tight main bearings means high oil pressure.
You checked your rods right?
The small ring gap clearence is not a problem, it will actually help.
So far sounds like you got a century motor, If you want a racer, well then those numbers are a little tight.

788 Posts
I'm not too familiar with the specs for a 400 are, but all your clearances seem a bit too tight to me. Monroe's book does not give the actual clearance specs so check in your shop manual (Chilton's or Clymer's). Look in there, and I'll bet the main clearances are somewhere in the .0025" - .0032" range.

While having .001" clearance on the mains will get you great oil pressure it doesn't leave much room for thermal expansion at all.
IMHO .001" or .0015" is just not enough.

With all the mains oiled and torqued down you should be able to rotate the crank with just your pinky. If you can't I'd visit your machinist again.

Yes your rings can be filed to get the proper end gap.
Again .007" just sounds too tight.
.010" - .020" for the comp. ring is good.

Plastigage is fine, and it's accurate enough. Out of habit though I've always double checked it's accuracy with outside and inside mocrometers just to be sure, and every time I did it showed that the plastigage is accurate.

Your specs you listed on the parts you're using sound great for a nice street motor that will last a long time if properly maintained. I would however pick up a shop manual to get the actual specs for the rod and main clearances, and go from there.

Good luck, and take your time!

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