Thread: 350 crank main cap problem

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alright.. while installing my new crank into my 350. i have a slight problem. the crank freely spins with the rear main cap in place, and also with the center cap in place.. and with the front cap in place... but when i put on either of the remaining 2 caps, the crank will not spin at all. do i have an end float problem? (i measured it to be fine). all my bearings seem to be good as well as my main caps clean. should i tear back down and check under the block bearings for debris? any info would be apprieciated. im 18 attempting to build my first small block with the help of some family and friends.

You need to plastigauge each bearing to see what your actual clearances are. Mistakes happen right out of the box with bearings, etc. It is the only way to be sure.


Don

alright ill go back and check that. what clearances am i looking for?

You should have a Chev manual that is specific for your engine that has the torque specs and the clearances for that motor. For a given clearance, for instance the main bearings, there is a high limit and a low limt in the spec and your measurement should be in that range.

There are tolerances for the piston to bore clearance, the piston ring end gaps, the rod bearing clearance, the main bearing clearance, the rod side clearance, the crankshaft end play, etc. Every clearance needs to be checked to verify that it is within the tolerance range.....parts wear and engine machine shops make mistakes.

Again, get the torque specs and clearances.

Mike in Tucson

By the way, if you are not familiar with Plastigage, it is a neat product that has been around for a zillion years. Plastigage is a squishable plastic material that is made to a very precise diameter. When you put it between two parts and squeeze them together, the plastigage flattens out to the thickness of the gap between the two parts. Thus, since the diameter was known, the cross sectional area is known. From that, the width of the squished part is a function of the thickness of the squished part....the area stays the same as before the squish....from a pictorial width chart on the package, you match the width of the squished section of plastigage to the chart and it tells you the thickness of your clearance gap. It's easy and requires no expensive tools.

It works for rod bearing clearances and main bearing clearances. The plasticgage comes in several thickness ranges and is color coded so you know which one you are using.

mike in tucson

You could have a bent crank, or the block could need align boring. If the Plastigage doesn't find the problem, you'll need a dial indicator and stand to check for wobble in the two main journals. Hope its the bearings!

let's see now---first engine
18
friends and family

Do you have the correct caps in the correct location? and turned the correct way?

My rule of thumb
Check all clearances yourself with the machinist present before you pay him.
Im also curious as to how you checked your end play?
This should be around 4 thou and you need dial guage for it once all mains are in place?

Also have the parts been balanced Crank, Rods, Pistons, Flywheel/Flexplate and Balancer?
You do not want to install unbalanced!!!!
If you have dial guage place crank on block no mains and see if it stays on zero all way round?

If all else fails---put the caps on in the correct locations

Jerry is correct make sure the caps are in the correct order as was disassembled.if your not sure of the order the caps are usually marked but it very hard to see.a friend of mine rebuilt a engine and found this out the hard way,starter wouldnt turn engine over.he also learned to roll motor over after each bearing and componet was installed.also seen a brand new set of bearings out of the box that were wrong size so use the platigauge as suggested ( cheap insurance)