From: Dave Baker
Newsgroups: rec.crafts.metalworking
Subject: Re: ream clearance largish hole?
Date: 15 Jul 1999 02:31:07 GMT

>From: mulligan@advinc.com
>
>In article <19990714133104.17422.00000112@ng-ck1.aol.com>,
>  pumaracing@aol.com (PumaRacing) wrote:
>
>> You have to preserve the perpendicularity of the little end hole to
>> the main rod axis to tiny limits. If there is misalignment the piston
>> will sit in the bore at an angle and you will end up with serious
>> engine problems.
>
>Interesting concepts.  What kind of tolerances are you discussing,
>and how does one define the rod's long axis?  When doing this,
>do you have to ensure that the big end's bore is parallel to the
>pin bushing?  Do you make up some kind of fixture to allow the
>rod to be mounted with the big end axis vertical, and then bore
>the small end?
>
>If the machine had fairly loose tolerances, would it be acceptable
>to use a piloted reamer?

Tolerances of ideally less than 1 thou per inch. Clamping the big end round a
vertical fixture is a good way to ensure alignment of the little end providing
the clamping system puts no distortions into the rod - it relies on very
careful shimming up of the little end before boring. However all rods should be
originally machined relative to the side of the big end journal and clamping
down on this is normally sufficient to ensure accuracy at the little end

A reamer is not an ideal way to finish any fully floating little end bush due
to two things. Piston pins are not always made to a size that enables a
standard reamer to generate the correct fit. Secondly even if they were so made
it is not normally possible to guarantee a reamed hole to sufficient limits.
Ideally a floating pin would want the bush machined to perhaps 2 to 4 tenths
clearance. Best way is to hone until the pin just slips in.

Press fit pins directly into steel rods usually require between 7 and 15 tenths
interference fit. Reaming is normally sufficient for the wider tolerances on
this type of assembly.

Regardless of machining system it is good practice to check alignment of big
and little ends after such work - especially on a race engine. I do this on the
lathe as follows.

Turn a piece of bar to the same size as the I/D of the big end journal (without
the bearings in). Clamp the rod round this with a couple of thicknesses of
paper under the cap end to ensure the rod is clamped tight against the beam
side of the big end journal. The rod is now held exactly perpendicular to the
axis of the lathe. Put the lathe in lowest gear or lock the chuck if possible
with the rod vertical.

Fit a piston pin into the little end (polish down an old pin until it just fits
if the rod is a press fit type).

Now run a dial guage along the pin using the saddle and a magnetic base. More
than 3 or 4 thou error across the length of an average piston pin and the rod
needs a tweak. For a race engine work on as close to zero error as you can.

Alternatively don't be a cheapskate like me and go out and buy a proper rod
checking machine. I'm a cynic as well as a cheap skate so I only trust my own
method even though it takes a while longer. You never know just how inaccurate
these damn newfangled machines are !! especially the ones made in the USA.

(ducks and covers till the flak stops flying)




Dave Baker at Puma Race Engines (London - England)  - specialist cylinder head
work, flow development and engine blueprinting. Web page at
http://members.aol.com/pumaracing/index.htm

From: Dave Baker
Newsgroups: rec.crafts.metalworking
Subject: Re: ream clearance largish hole?
Date: 15 Jul 1999 02:57:49 GMT

I suppose I should finish this off with a quick note on actually straightening
a bent rod. You can use a flypress but I don't like doing this. It is easy to
turn 3 thou of bend one way into 20 thou of bend the other way if you aren't
careful. I tweak the rod while it is still on the lathe fixture.

Get a largish lump of ally or similar to hold against the little end to act as
a shock absorber. Give the appropriate side of the middle of the beam a good
tap with a brass hammer or another lump of ally. Whichever was the high side of
the pin is the side of the beam you want to tap. It is essential when doing
this to creep up on the rod so it remains unaware it is about to be hit.
Whistle innocently, look the other way, lull the rod into a false sense of
security - then whap !!

Let the bugger know you are about to thump it and it'll fight you - give it a
good sharp tap while its attrention is diverted and you can correct a fair
amount of bend without using too much force. Why I should think that inert
lumps of metal are sentient is possibly a mystery to you - it certainly is to
me - perhaps it is just me going steadily more and more mad. The more I machine
bits of metal though the more I become convinced the little bastards have a
mind of their own. Anyway I think the valium is wearing off so I'd better get
to bed :)


Dave Baker at Puma Race Engines (London - England)  - specialist cylinder head
work, flow development and engine blueprinting. Web page at
http://members.aol.com/pumaracing/index.htm