The Ford Barn

macdonge · 01-21-2018, 03:18 PM

I'm in the process of rebuilding a 1939 Merc engine (Canadian keystone block) and thought I'd seek the Barn's collective advice on my rod big end dimensions. Six of the rod big ends measure at the top end of the spec (2.3597-2.3603") but two are outside, i.e one that is .003" fat (but still round) and another that is .005" fat (and out of round by ~.002 - .003").

Note that the crank pins have been ground .010" under and I have the appropriate (new) .010" bearings. From what I've read, Ford considered a total clearance (brg OD to rod ID + brg ID to crank pin OD) of .005" the limit for acceptable service. In my case, it looks as though I'll be at about .004" on the one and .006" on the other.

My question is should I get one or both of these rods resized or can I live with it given that the engine will only see light street use? I understand that racers liked to run with bigger clearances in the day, e.g. .004" - .005" total, but am I pushing it? Could I get away with this situation by using a high-flow oil pump?

Thanks in advance for any real world experience you can provide!

J Franklin · 01-21-2018, 03:48 PM

If you are going to the trouble to rebuild the engine, get the rods resized and align checked. Cheap insurance.

supereal · 01-21-2018, 04:58 PM

Have the crank pins measured. If any are scored or tapered more than .001 or out of round more than .0015, the crankshaft should be machined so the bearing clearance is .0015 to .0035. The rod bearing bores must be identical at 2.3597 to 2.3603. These are critical for floating bearings. Given that you have found differences, all rods should be examined by a machine shop before assembly.

Ol' Ron · 01-21-2018, 08:51 PM

usually increase the size of the bore by .001 over stock to begin with, I use 10/30 oil. or 15/40 in hot weather. It's important to increase the side clearance of the bearing to .010/015" and the rods to .010 for street. The stock 8BA oil pump is plenty, you need flow not pressure. High oil pressure has a tendency to flake the bearings. I use used bearings in most of my builds, have 100's of them and I can make any clearance I want. Good luck.

Dave/Green Bay · 01-21-2018, 09:45 PM

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Buy O'Rons book. Great section about your question. Dave/Green Bay

01-21-2018, 03:18 PM	#1
macdonge Senior Member Join Date: May 2015 Location: Frelighsburg, QC Posts: 166	Advice on full floater rod bearing clearances I'm in the process of rebuilding a 1939 Merc engine (Canadian keystone block) and thought I'd seek the Barn's collective advice on my rod big end dimensions. Six of the rod big ends measure at the top end of the spec (2.3597-2.3603") but two are outside, i.e one that is .003" fat (but still round) and another that is .005" fat (and out of round by ~.002 - .003"). Note that the crank pins have been ground .010" under and I have the appropriate (new) .010" bearings. From what I've read, Ford considered a total clearance (brg OD to rod ID + brg ID to crank pin OD) of .005" the limit for acceptable service. In my case, it looks as though I'll be at about .004" on the one and .006" on the other. My question is should I get one or both of these rods resized or can I live with it given that the engine will only see light street use? I understand that racers liked to run with bigger clearances in the day, e.g. .004" - .005" total, but am I pushing it? Could I get away with this situation by using a high-flow oil pump? Thanks in advance for any real world experience you can provide!

01-21-2018, 03:48 PM	#2
J Franklin Senior Member Join Date: May 2010 Location: Oregon Posts: 5,963	Re: Advice on full floater rod bearing clearances If you are going to the trouble to rebuild the engine, get the rods resized and align checked. Cheap insurance.

01-21-2018, 04:58 PM	#3
supereal Senior Member Join Date: Jul 2014 Location: Iowa Posts: 1,634	Re: Advice on full floater rod bearing clearances Have the crank pins measured. If any are scored or tapered more than .001 or out of round more than .0015, the crankshaft should be machined so the bearing clearance is .0015 to .0035. The rod bearing bores must be identical at 2.3597 to 2.3603. These are critical for floating bearings. Given that you have found differences, all rods should be examined by a machine shop before assembly.

01-21-2018, 08:51 PM	#4
Ol' Ron Senior Member Join Date: May 2010 Location: Chester Vt Posts: 8,860	Re: Advice on full floater rod bearing clearances usually increase the size of the bore by .001 over stock to begin with, I use 10/30 oil. or 15/40 in hot weather. It's important to increase the side clearance of the bearing to .010/015" and the rods to .010 for street. The stock 8BA oil pump is plenty, you need flow not pressure. High oil pressure has a tendency to flake the bearings. I use used bearings in most of my builds, have 100's of them and I can make any clearance I want. Good luck.

01-21-2018, 09:45 PM	#5
Dave/Green Bay Senior Member Join Date: May 2010 Location: Green Bay Posts: 391	Re: Advice on full floater rod bearing clearances Sponsored Links (Register now to hide all advertisements) Buy O'Rons book. Great section about your question. Dave/Green Bay