Number of crank bearings

[Dave Sherman]

I was pondering the other day. My MG has 3 crankshaft main bearings. Seems to me this was one of the weaknesses in the Triumph 1500 and early MGB engines, since this makes two crank throws that oppose each other, with nothing supporting between them. I know the 1500 is notorious for worn out thrust bearings and crank bearings. Later MGBs went to a 5 main bearing engine. Is it safe to say that most 4 cylinder engines today would have 5 mains?

 

[Donald]

Some even worry about how many bolts hold on the main bearing cap. On GM small block V8 for cars its a 2 bolt main. On truck and marine its a 4 bolt main.

 

[chrisri]

As far as i know all ohc motors have five main bearings.Old pushrod engines had only three,but they rev less.

 

[OVERKILL]

Originally Posted By: chrisri
As far as i know all ohc motors have five main bearings.Old pushrod engines had only three,but they rev less.


The old Ford 300 i6 had 7 mains.

 

[92saturnsl2]

VW's VR6 engine also has seven mains, and independent rod journals for each cylinder.

I would imagine most inline 6's would have seven.

 

[Chris142]

My jeep has 7

 

[chrisri]

Yes but OP asked about four pot motors.

 

[Dave Sherman]

Actually, it was focused on 4 bangers, but is it more common to have one throw between bearings? If inline 6 engines typically have 7, then that would seem that is the case. For that matter, do flat fours and sixes (Subaru, for example) have 5 and 7?

 

[ls1mike]

Originally Posted By: Donald
Some even worry about how many bolts hold on the main bearing cap. On GM small block V8 for cars its a 2 bolt main. On truck and marine its a 4 bolt main.

If you live in the 60's to the mid 90's

...

LT1 Y body cars and some F-bodies had a 4 bolt main.

All LS based motors are a 6 bolt main.

 

[expat]

Originally Posted By: Dave Sherman
I was pondering the other day. My MG has 3 crankshaft main bearings. Seems to me this was one of the weaknesses in the Triumph 1500 and early MGB engines, since this makes two crank throws that oppose each other, with nothing supporting between them. I know the 1500 is notorious for worn out thrust bearings and crank bearings. Later MGBs went to a 5 main bearing engine. Is it safe to say that most 4 cylinder engines today would have 5 mains?


Some inline 4's (like the early Austin Seven) only had a Two bearing Crank
http://173.203.80.233/media/listings/image2/2011/01/Phoenix_A7_Crank.jpg

The Triumph Crank is interesting, in that the engine was developed from an original size of 803cc.
Great effort (and little money) was spent enlarging the capacity (in the 4 cyl versions) to ultimately just under 1500cc
This involved increasing the Bores to the point where the Bore centers had to be moved into two pairs, resulting in the 1300cc version having very little land between cyls 1&2 and 3&4

But in order go to an even larger capacity (1500cc) the crank then had to be stroked.
and when doing so they decided to make the old 3 bearing crank 'Beefier' and (30lbs) heavier, and the Bearings Bigger.
Weather this was a Good Idea is debatable!

The Thrust was washer issue is another matter entirely, and has nothing to do with the # of crank bearings or size. There are 'Fixes' for this issue. (PM me if you are interested)

Interestingly, on engines like the MGB and Austin Seven where the # of main bearings have increased. Many Racers prefer to use the Older versions, with Less # of Main bearings claiming that they "Rev better"

 

[Astro14]

My Packard has nine mains. Inline 8 engine...and very, very smooth when it runs, much smoother than a V-8...

 

[OVERKILL]

Originally Posted By: 92saturnsl2
VW's VR6 engine also has seven mains, and independent rod journals for each cylinder.

I would imagine most inline 6's would have seven.


Many do. Some don't.

Slant-6:

 

[Donald]

Originally Posted By: ls1mike
Originally Posted By: Donald
Some even worry about how many bolts hold on the main bearing cap. On GM small block V8 for cars its a 2 bolt main. On truck and marine its a 4 bolt main.

If you live in the 60's to the mid 90's

...

LT1 Y body cars and some F-bodies had a 4 bolt main.

All LS based motors are a 6 bolt main.


I suspect there are a whole lot of GM small block V8s in boats still running around the water.

 

[supton]

Originally Posted By: OVERKILL
Originally Posted By: 92saturnsl2
VW's VR6 engine also has seven mains, and independent rod journals for each cylinder.

I would imagine most inline 6's would have seven.


Many do. Some don't.

Slant-6:

Anyone read this month's Hot Rod, in the back they have a pic of a Slant Six con rod. Folded in half. Supposedly the motor was still motoring on.

Not quite related crank bearing count, but related to stoutness and slant sixes.

 

[SteveSRT8]

Originally Posted By: Dave Sherman
I was pondering the other day. My MG has 3 crankshaft main bearings. Seems to me this was one of the weaknesses in the Triumph 1500 and early MGB engines, since this makes two crank throws that oppose each other, with nothing supporting between them. I know the 1500 is notorious for worn out thrust bearings and crank bearings. Later MGBs went to a 5 main bearing engine. Is it safe to say that most 4 cylinder engines today would have 5 mains?


IDK, but as an owner of two different Triumph TR-4's I can tell you they had a Massey Ferguson engine design that was quite stout when machined and set up properly. Just can't be revved to the moon...

 

[Hokiefyd]

Originally Posted By: Dave Sherman
Is it safe to say that most 4 cylinder engines today would have 5 mains?


Yes, I think it's safe to say that most 4-cylinder engines have five main bearings...even going back to many decades ago (like the Pontiac Iron Duke 2.5L, etc).

 

[440Magnum]

There are a whole lotta factors that come into play that are more important than pure main bearing count. Case in point, most inline sixes have 7 mains, so any inline six you find with fewer than 7 mains is probably lower powered or short-lived by comparison.

Hmmm... yet the Slant-6 has 4 main bearings. Fewer mains meant a shorter, stronger crank in that case, and it was deemed worth the trade. Especially since it also helped the whole engine be shorter (along with a waterpump tucked beside the slanted block).

The old Chrysler flathead 8-cylinders came in 5- and 9- main bearing flavors in the 1930s- the Imperial engine being the 9-main. However, only the 5-main engine survived out of that decade and into the next before getting replaced by a V8 in 1952.

 

[rationull]

Originally Posted By: Dave Sherman
For that matter, do flat fours and sixes (Subaru, for example) have 5 and 7?


Yes. A flat 4 crankshaft is effectively the same shape as an inline 4 crankshaft (flat plane, one crankpin per piston, but probably shorter than an "equivalent" I4 crankshaft). A Subaru 4 would have 5 bearings and a Subaru 6 would have 7 I believe.

Looking at photos of old VW flat 4 crankshafts, looks like maybe they only had 3 bearings though!

 

[440Magnum]

Originally Posted By: rationull
Originally Posted By: Dave Sherman
For that matter, do flat fours and sixes (Subaru, for example) have 5 and 7?


Yes. A flat 4 crankshaft is effectively the same shape as an inline 4 crankshaft (flat plane, one crankpin per piston, but probably shorter than an "equivalent" I4 crankshaft). A Subaru 4 would have 5 bearings and a Subaru 6 would have 7 I believe.

Looking at photos of old VW flat 4 crankshafts, looks like maybe they only had 3 bearings though!


For flat-fours, there is a big gain to be had in the engine balance if the opposing pairs of cylinders can be put as close as possible to being in the same veritcal plane. It can't be done, of course, because the crank pins have to be offset... BUT if you don't put a main bearing between those pins, the offset can be minimized. So its a trade between more crank support and better inherent balance. There are examples of both. 3-main boxer 4:

5-main boxer 4:

 

[jrustles]

Originally Posted By: 440Magnum

5-main boxer 4:

Interesting. Subaru upsized the mains, and downsized the rod journals with the change to FB