125 Piston Pin Fitting

[dcooke007]

Wish I could have been with you guys for Hodaka Days but just was not possible. Your stories, pictures and possible videos is much appreciated and allows me to share in your adventure. With that said I thought I would share what I was up to while you were away. Some times you get lucky and the piston pin fits perfectly with out any fitting but not always. The method that has been used to increase the piston pin bore diameter has been to use an expandable reamer. The reamer works but you can end up with the bore to large or not straight.

This is a Combat Wombat piston I am prepping for Bill in VA. I think this is a better method to fit the piston pin bore. The required tools are 9/16 through hole lap, 400 grit lapping compound and drill press. This is a link where you can purchase the lap, http://www.mscdirect.com/product/details/05060363.

To perform the lap method chuck the lap in your drill press and select a slow spindel speed. The lap diameter is adjustable and can be expanded to 15% of its listed diameter. Lap is expandable by screwing the end piece clockwise. With the drill press switched OFF place the piston pin bore over the lap and adjust the lap until it barely touches the piston pin bore. Only the middle section of the lap will expand and make contact. Apply a small amount of lapping compound to the lap and piston pin bore. When you are lapping the piston is moved up and down over the lap to size the bore. Turn the drill press on and work the piston pin bore up and down the lap for a few seconds, clean off lapping compound and check piston pin fit. It is not likely the pin will fit at this point. You need to get a feel for this method and I recommend you proceed slowly until experience is gained. From this point I expand the lap by turning the adjuster 1/4 turn, lap bore, clean and check fit. Repeat until piston pin fit is a firm thumb press fit with piston pin and bore clean and lubed.

With this particular piston and pin the fit was almost there after the last lapping session. It was so close, rather than returning to the lap I lubed the piston pin / bore and lapped the two parts together to get a perfect fit.....CLEAN OILED PARTS AT THIS POINT ONLY NO LAPPING COMPOUND. Of course you want to lap both ends of the piston pin and both sides of the piston pin bore. Aluminum vise inserts are used to prevent damage to the pin. Throughly clean and oil the parts after fitting to remove any remaining lapping coumpound.

This method may sound complicated but it is very easy, accurate and quick. For other diameter piston pins you would only need to select the proper size through hole lap.
Danny Cooke

[DGardner]

Great info Danny! thanks DG

[BrianZ]

When I install a piston I heat up the piston with a heat gun and then just push the piston pin in place. The heat expands the aluminum piston to the point where the pin is an easy fit. The same goes for removing the pin, I heat it up until I can push it out easily.

Now, is there an advantage to lapping the pin? My understanding is that when the engine reaches operating temperature the aluminum will have expanded more than the pin. Assuming that the parts were a perfect fit when they are cold, wouldn't they end up being slightly loose when the engine is hot?

Just curious as to the pros and cons of "fitting" the pin to the piston vs. using the parts as is.

Either way Danny, your work is exceptional.

Brian

[Bullfrog]

Wayeeeee toooo many Hodies have exhibited seizures due to not properly fitting the piston pin to the piston. All parts should be at the same temp. and pin should be a "thumb push fit" (NO mechanical assistance for installing the piston pin.) This has been the Hodaka recommendation since the 1960's. And I practice it on all my engines.

Ed

[dcooke007]

Hey Brian,
I am of the same opinion as Ed...piston pin should be fit to a firm thumb push fit with both parts at room temperature. I have had no issues with a properly fit piston pin but, as Ed said, I am concerned with piston siezures with excessively tight piston pin fit. Also with this method it is very easy and cheap, assuming you already have a drill press, to properly fit the piston. This is basically a honing process and removes very little material at a time and is very forgiving. I can remember the pucker factor the first time I used an adjustable reamer knowing in the back of my mind one cut to much and the piston is history. One other thing to note is that all the modern pistons, Wiseco etc., I am aware of come with piston pins with a firm thumb press fit. Better to be on the safe side in my opinion.

Danny Cooke

[dirtsquirt80]

Great info Danny hey my trans was low on oil you were right thank's Scott