Date: Fri, 14 Aug 1998 07:46:01 -0600
From: Bill Larson
Dear List members,
I need some advice on the safest way to open up the small end I.D. of my crank rod "in situ." The fitted pins on my new Venolia pistons are between .0005- and .001-inch larger than stock size. So I need to open up the small end a tad, and I don't want to split the cases and pull the rods out just to do that! I've read in the INOA Tech Digest that one should absolutely NOT use emery (or crocus) cloth on any internal aluminium engine parts, as the abrasive material embeds into the aluminium. But when I asked our resident machinist here at work what he recommends, I was surprised to hear him suggest emery cloth wrapped around a three stone wheel cylinder hone, and attached to a drill. When I mentioned the bit about embedded abrasive material in aluminium, he said just spray it first with some WD-40 and don't worry about it. What say all you talented machinists out there? What's the safest way to open up the rod end bore while still attached to the crank? Thanks in advance!
Bill Larson '74 Norton Roadster w/ new 10:1 Venolia pistons
Date: Fri, 14 Aug 1998 10:01:51 -0500
From: Colin Sharpe
Bill Larson Wrote:
>Dear List members, I need some advice on the safest way to open up
the small >end I.D. of my crank rod "in situ."
[Snip]
I don't know what the INOA Tech Digest is, but they are right about not using abrasive. Your resident machinist seems to have a problem extracting his cranium from his...er...rectum.
The way to do this job is with an expandable or adjustable reamer. If you can fabricate a bracket to hold the rod steady to the barrel mounting studs, that will be of immeasurable help, since reaming a rod which is slopping around is almost certainly going to result in a funny shaped hole. Stuff the crankcase mouths with paper towels, to prevent chips of aluminium from falling in, and be sure to use cutting oil (NOT engine oil) to lubricate the reamer. Take tiny cuts, since this will reduce the tendency of the reamer to chatter, and you can always take another tiny cut, but it's much harder to replace metal once it has been removed. Turn the reamer in a clockwise direction only, do not "Unscrew" it as you take it out, as this is considered a way to dull the reamer.
This method is well documented in many older books on motorcycle maintenance, and in Harley Davidson service manuals. In the good old days, and on Harleys today, small end bearings were/are bronze bushings which can be pressed out when worn and replaced with new ones. Because the bushing will be squeezed when fitting, and will probably not end up exactly the right size for the piston pin. The bushings were therefore supplied undersize, so they could be reamed to give a perfect fit.
Hope this is of help, and sorry that the tool is much more expensive than a sheet of emery cloth! And shame on Venolia for making their pins a non-stock size!
English.
-
Date: Fri, 14 Aug 1998 16:47:33 +0100 (BST)
From: Philip Pick
On Fri 14 Aug, William Larson wrote:
> Thanks for the reply. This is exactly the same advice I just
got from Pete
> Serrino. I'm going to look for one of these reamers at a local
tool supply
> house. I'll use my old pistons as a test before cutting into
the rods.
> I'm thinking about making the rod end I.D.'s slightly undersized,
and then
> making up the difference by heating up the rod end with a hair dryer
(or?).
> That way I shouldn't have to worry about a loose fit, right?
If you are going to work in situ by reaming please let me suggest the following dodges.
Buy the pilot for the adjustable ream you are going to use. Clamp the rods together with two pieces of wood and a clamp so that they are in line, i.e. with one piece of wood to the front of the rods, and one behind. A hole in the centre of these wood batons would allow a nut/bolt to be used as a clamp.
Use the taper sleeve supplied with the pilot to achieve in line reaming. The extra support will help solve a lot of chatter and vibration surface marking.
Make a number of passes without adjusting the size of the reamer. A small amount of material will be removed with every pass.
A slightly slack small end causes MUCH less damage than a tight one......
Good Luck
Date: Fri, 14 Aug 1998 11:36:23 -0500
From: Colin Sharpe
Bill Larson wrote:
> Colin,
>
> Thanks for the reply. This is exactly the same advice I just
got from Pete
> Serrino. I'm going to look for one of these reamers at a local
tool supply
> house. I'll use my old pistons as a test before cutting into
the rods.
> I'm thinking about making the rod end I.D.'s slightly undersized,
and then
> making up the difference by heating up the rod end with a hair dryer
(or?).
> That way I shouldn't have to worry about a loose fit, right?
>
> Bill Larson
Sorry, but no! The pins are an interference fit in the piston, that's why you have to heat the piston before inserting or removing the pin. Since the pistons have to rock on the rods as the angle changes due to the rotation of the crank, the pin should be a running fit, and not an interference fit in the rod. Doing what you say with a hair dryer will result in an interference fit, which will prevent the piston from rocking. What you need is a running fit, with just about enough clearance for the oil film. No slop. That's why the small end of the rod has that neat little hole with a countersink in the top of it, to let the oil in.
English.
Date: Fri, 14 Aug 1998 10:52:43 -0600
From: Bill Larson
But my pistons have a close running fit with the pins! No heat required. So why would I need a running fit with the rod end too? There is an oil hole in the pin boss of the pistons. I am sure a company that's been in the racing business as long as Venolia knows what they're doing with pin fits.
Bill Larson
Date: Fri, 14 Aug 1998 15:59:30 -0500
From: Colin Sharpe
Bill Larson wrote:
> But my pistons have a close running fit with the pins! No heat
required.
> So why would I need a running fit with the rod end too? There is
an oil
> hole in the pin boss of the pistons. I am sure a company that's
been in
> the racing business as long as Venolia knows what they're doing with
pin
> fits.
Ah, so the answer might be that you need to do nothing! Maybe Venoliain their wisdom have designed the oversized pin to be an interference fit in the rod. Maybe you don't need to ream the rod, just heat it and fit the pin? .0005"-.001" should be in the range for this type of fit.
I think the answer at this stage of the game would be to call Venolia pistons and ask them : "What is the suggested clearance between piston pin and connecting rod small end when running your pistons?"
They should know the answer, since they obviously have tested their product before marketing it. Do whatever they tell you to. I have never used Venolia products, and (Stupidly) assumed their pistons to be very much like everyone else's.
Let me know what they say!
English.
Date: Fri, 14 Aug 1998 18:22:17 EDT
From: Colin Steer
As well as warming up the small end, a good wheeze is to put the gudgeon pin(aka piston pin ) in the deep freeze for an hour or so before fitting. This should shrink it to fit easily within a half to one thou fit in a warm orifice.
Surprised no-one else suggested this first.
Colin Steer
Date: Fri, 14 Aug 1998 22:07:31 -0700
From: Vernon Fueston
Been following this thread with much interest.
My advice is put in Hepolites with stock pins,,,,,,, all problems solved and we know they work.
Failing that, tear down the engine (its ready for mains and a new cam anyway) and fit the pins with a Sunnen hone.
Reaming sounds good until you try it. Its hard to hold the rod steady, turn the reamer smoothly while not putting undo pressure sideways or up and down, keep all the chips out of the lower end, and not develop a chatter that ruins the finish. With the bronze bushings, when you ruined one you stuck in another. With a Norton you change rods! Phillip Pick's hints are very good, but do not underestimate the degree of skill involved. Also there is considerable cost in acquiring a quality reamer, anything less precludes a successful outcome.
I would advise against heating the rod to install the wrist pins. Your pins may be free in those forged pistons now, but they often grab when hot as the piston try's to revert to it's previous shape. Cast pistons do not do this, but forged pistons are really extruded, and I have seen them distort through the pin bosses when hot. If the pin is tight in the rod, and becomes tight in the piston, you will either seize the engine or (more likely) snap the small end of the rod. This can ruin your whole week.
Vernon Fueston
"It's hard to make things fool-proof
when fools are so ingenious."