4-1/8” crank

T

theriddler56

Senior Member
4-1/8” crank

Has anyone ever tried offset grinding a Merc 4” crank using 21a rods and rod bearings from a 98-2000 and up 4.6L ford engine. It appears to be doable as the 4.6 rod housing bore is only .018” larger than the 21a rods. Using 4.6 rod bearings that are 1.000 mm undersized (.040”). It would appear one would have to grind the merc crank to 2.046”. Is there enough meat on the merc rod journal to accomplish this?
 
Re: 4-1/8” crank

What is the outside OD of the mentioned bearing and the width.? Given that the 21A rods are designed for full-floater bearings (which are very thick), is hard for me to imagine that you'd have enough bearing thickness to fit the 21A big-end bore and have an ID of 2.046.

Now, I'm just talking out loud here . . . so bare with me . . .
 
Re: 4-1/8” crank

Back in the day the 296 was king and this was made from a Merc crank 21A rods an 3 3.8 pistons. I mad a few of these, the last one in 08 for Ron Horrell and he just retiredd it for a blown 294, I told him I'd freshen it up, but he said, it doesn't burn a drop of oil and still rund like a champ, with many miles on it left.
 
Re: 4-1/8” crank

I apparently wasnÂ’t clear enough in my post.
21a housing bore is 2.195-2.200”, 4.6L housing bore is 2.2388-2.2392” difference of .018”. That would mean hone 21a rods out .019”.
4.6L crank journal is 2.086-2.0867” bearings are available in 1.000mm undersize (.040”).
That would mean turning the merc crank journal down to 2.046”. The merc crank rod journal is 2.139” this makes a difference of .093” remember you are offset grinding, which means most material is removed from the inside of the crank throw. My question was is there enough meat to do it. 4.6L bearing width close to same size as a stock merc bearing.
 
Re: 4-1/8” crank

WHY? STD 2” full floating bearings are not expensive & work perfect. So why go to the trouble of reinventing the wheel. I have built many 4 1/8 stroke offset ground using the 21A or 91A rods.
 
Re: 4-1/8” crank

Floating bearings are $300+, 4.6L bearings are $30+. Tangs on bearings are only for locating in rod they have nothing to do with holding bearing in place. That is accomplished by’bearing crush’. Most 21a or 91a rods should be resized before using them. Crank has to be reground to accomplish 4-1/8” anyway, so reason for doing is simply less expensive.
 
Re: 4-1/8” crank

You are saying the rod bearings for a 35-39 221 engine with either 21a or 91a rods are found for less than $100?
 
Re: 4-1/8” crank

Standard size yes that is correct. $100 or less. When I have my 4 inch cranks offset ground it’s always to stand at.
 
Re: 4-1/8” crank

theriddler56 said:
Floating bearings are $300+, 4.6L bearings are $30+. Tangs on bearings are only for locating in rod they have nothing to do with holding bearing in place. That is accomplished by’bearing crush’. Most 21a or 91a rods should be resized before using them. Crank has to be reground to accomplish 4-1/8” anyway, so reason for doing is simply less expensive.
Click to expand...

I hear you, but the floater bearings have two rods per crank journal. I’m not saying the modern insert bearing converted to floater won’t work, but now you may have the sides of the two rod bearings trying to float while also rubbing against one another. Not saying it will be an issue, just something you'll need to be mindful of when setting it up.
 
Last edited:
Re: 4-1/8” crank

I have several hundred sets of "USED" Floating bearings. Always saved the good ones when dissembing annengine for rebuild. Most are from the 29a rods. When assembling the bearing kit for the stock car it was cheaper to select a used baring than have the crank reground to size, Also we were having trouble with bearing flaking. Early in my career as an engine builder for the stockcar, I realized I didn't know much about them, and the learning process was expensive. But we never lost an engine. Over the years we discovered that some of the cad /silver bearings dressed in cosmoline were prone to flaking when running hi oil pressure. I had the pressure jacked up to 70/80 lbs hOT. This also broke one crank from catenation erosion of the filit radius of the crank. I even have a picture of it. I think allot of guys back then were just learning about the flathead and it took some of the old timers to convince us that 30/40 lbs was all we needed. For the next 5 or 6 years I just used a stock pump. After a few years we went to a full flow filter. I still use the "Used" bearings to get my clearances..
Gramps
 
Re: 4-1/8” crank

Tim Ayers said:
I hear you, but the floater bearings have two rods per crank journal. I’m not saying the modern insert bearing converted to floater won’t work, but now you may have the sides of the two rod bearings trying to float while also rubbing against one another. Not saying it will be an issue, just something you'll need to be mindful of when setting it up.
Click to expand...




Hi Tim,


I believe what Riddler is saying is he intends to fit the 4.6 bearing inserts as non-floaters....he's removing the locating tangs and using the bearing crush of the resized-to-4.6 spec 21A rod housing bore ID to locate and hold the bearing shells. That'll work fine, just have to be careful to center the shells in the rods on assembly.



Riddler-

I like info on options like this, and appreciate the research and sharing of info. Never know when parts supplies may dry up and we have to reinvent that old wheel. Thanks for sharing!


Good info in this thread...thanks to all for contributing. :)


Terry
 
Re: 4-1/8” crank

I get it too...actually, some Canadian engines used lock in shells in 21A rods. Basic part number of those shell bearings was C41A 6211. These are like unicorn scat to find these days. Point is, if one sized the big end of the rods to suit these modern bearings, and notched to suit, what the op is suggesting would be quite doable.
 
Re: 4-1/8” crank

cadillac512 said:
Hi Tim,


I believe what Riddler is saying is he intends to fit the 4.6 bearing inserts as non-floaters....he's removing the locating tangs and using the bearing crush of the resized-to-4.6 spec 21A rod housing bore ID to locate and hold the bearing shells. That'll work fine, just have to be careful to center the shells in the rods on assembly.



Riddler-

I like info on options like this, and appreciate the research and sharing of info. Never know when parts supplies may dry up and we have to reinvent that old wheel. Thanks for sharing!


Good info in this thread...thanks to all for contributing. :)


Terry
Click to expand...

Ah. I get it now. Thanks, Terry.
 
Re: 4-1/8” crank

Riddler, it sounds like you may be proposing a new idea, one that has not been tried or thought of before. As always, there are many ways to skin a cat and if you have the facilities to try such an idea then go for it and keep us updated on how it goes. What may be easy for one person, due to abundance of parts stashed over years, may be not easy for someone starting from scratch.
Mart.
 
Re: 4-1/8” crank

theriddler56 said:
Tangs on bearings are only for locating in rod they have nothing to do with holding bearing in place.
Click to expand...

That is true but it would be a good idea to machine some tang slots in the rods so the bearings will be positively located in the right place.
There is usualy a large radius in crank rod journals that have been reground for racing so it is imperitive that the bearing does not rub on the radius.
 
Re: 4-1/8” crank

Pete said:
That is true but it would be a good idea to machine some tang slots in the rods so the bearings will be positively located in the right place.
There is usualy a large radius in crank rod journals that have been reground for racing so it is imperitive that the bearing does not rub on the radius.
Click to expand...

Wait, the tang on both halves of a two piece locked-in type of bearing does not do anything in helping it to stay in place? Once seated on the crank journal, it would need to be pushed inward in order to release itself from the tang slot, no?

When you spin a locked-in type of bearing in a motor, that is a mechanical failure. The halves sit on top of one another when the rod cap is connected to the main portion of the rod body.

How can't the tang not help to keep the bearing shells from spinning?
 
You must log in or register to reply here.
Back
Top