How does one machine engine mains?

[JimSvarv]

Hi
I was wondering how the mains on a longer engines could be made if one doesn't use a lathe or boring head.
On a 2 main engines circular interpolation on each side could be used.
But how is something like a five main block made?

If one cut the mains with a ball nose then it would go out of round when the caps are torqued down, no good.
One could I guess make the mains one by one and then mount them to the engine?
But I would prefer to make it in one piece.

Any advice?

 

[gbritnell]

You have two choices. The first one takes accurate machinery and tooling to accomplish. This would be line boring. Whether on a lathe with the part mounted to the cross slide (if you have a lathe with a flat surface for the the compound) or by standing it up on a mill against an accurate angle plate and knowing that you mill will be parallel to the axis of the crankshaft holes over a long distance.
The second option, which I have done on several engines, is to machine the bearing areas in the block as a rectangular cutout. Meaning run an end mill the length of the block to whatever width and depth that would be appropriate for a bearing insert. The insert would be bronze and rectangular to match the cutout dimensions. You can even make it wider so that the bearing mounting bolts go through both the cap and the insert thereby securing both. It's not prototypical but it sure is easier to accomplish.

 

[JimSvarv]

Thanks! So kinda like a Offenhauser with bearings pre bolted beforehand.
I have a cnc router I'm putting together and been thinking about a lathe.
But if I follow the second option of your advice and use build up crank/camshaft then I should be able to start something without a lathe for now.

A second question, if ball bearings are used for the mains then pressure lubrication can't be used for the rod journals right?
Seen a few builds like that, feels like the small end wouldn't get lubrication with just splash.

Just want a engine that lasts if I put in the effort to make it and I saw some stories here about model plane engines that had to be rebuilt ever few runs...

 

[gbritnell]

Usually some designer/builders put ball bearings only on the outboard shafts and plain bearings inside. That way you can use a splash oil system and everything will get lubricated.

 

[Nerd1000]

Thanks! So kinda like a Offenhauser with bearings pre bolted beforehand.
I have a cnc router I'm putting together and been thinking about a lathe.
But if I follow the second option of your advice and use build up crank/camshaft then I should be able to start something without a lathe for now.

A second question, if ball bearings are used for the mains then pressure lubrication can't be used for the rod journals right?
Seen a few builds like that, feels like the small end wouldn't get lubrication with just splash.

Just want a engine that lasts if I put in the effort to make it and I saw some stories here about model plane engines that had to be rebuilt ever few runs...

There are ways to get pressure lube through a ball bearing crankshaft, for example oil can be fed in from one end.

Splash oiling is usually adequate for models, it's good enough for Briggs and Stratton mower engines which see endless abuse!

 

[SmithDoor]

Hi
I was wondering how the mains on a longer engines could be made if one doesn't use a lathe or boring head.
On a 2 main engines circular interpolation on each side could be used.
But how is something like a five main block made?

If one cut the mains with a ball nose then it would go out of round when the caps are torqued down, no good.
One could I guess make the mains one by one and then mount them to the engine?
But I would prefer to make it in one piece.

Any advice?

I have done just hand full of pillow bearing and a boring bar shart fits the pillow blocks and small than the bore.

It is very simple

Dave

 

[JimSvarv]

I have done just hand full of pillow bearing and a boring bar shart fits the pillow blocks and small than the bore.

It is very simple

Dave

Hi Dave.
Could you post a picture or a link to your work?

 

[Steamchick]

Splash lubrication, where the con-rod dips into the oil bath, actually creates a mist of oil which washes all the surfaces inside the bearings and provides adequate lubrication. However, modern car engines with plain bearings need a large flow of pumped oil through the crank bearings to cool the bearings. Only very high performance engines need pumped oil to cool small end bearings.
So go with whatever the Designer decided was OK.
High performance Motorcycle engines have built-up cranks with rolling element bearings, but still need low-pressure high-flow oil feed to keep them cool.
K2