cr85 Top End Rebuild Question

[Nick's #1 Fan]

Hopefully this isn't a dumb question since I am new to the CR85's.

I am doing a top end rebuild and I noticed that when I removed the piston there were 2 small (pin) holes on the exhaust side of the piston. The holes are directly on top of one another and they are in line with the post in between the two exhaust ports.

What could have caused this or could they have been added for performance reasons? (I said I was new so be nice :) )

It is a 2003 cr85 with a ported and polished cylinder. My new stock piston doesn't have these holes so I want to make sure that I am not missing something before I reassemble it.

Thanks for the help!

 

[Isobareng]

Why I am not familiar with the cr85 it is not uncommon for hole to be drilled in a piston to line up with port bridges. These holes are there for lubrication to flow from under the piston to points that due to their lower surface area, mass and exposure to heated combustion will see greater heat then the rest of the cylinder.

Hope this helps

D

 

[Nick's #1 Fan]

That makes sense....I am not comfortable drilling those holes myself, am I going to see issues if I don't put the holes in the new piston?

By not adding the holes will I need to rebuild the top end more frequently?

 

[Patman]

Who is the piston kit from?

 

[Nick's #1 Fan]

I am just using a stock Honda piston.

If I should go with something else please let me know...Like I said I am new to these motors.

 

[Patman]

If you bought the correct part then don't worry about it. No piston should need extra holes but it's possible the vendor of the one you are replacing incorporated them.

 

[Jasle]

On several occasions I have bought aftermarket pistons that came with a note indicating that i needed to drill these holes if there was an exhaust bridge.


Exhaust bridges get more heat and expand more than the rest of the cylinder. Personally i'd drill the holes. they won't hurt as long as you line them up right and hit them with some sand paper to knock the burr off when your done.

 

[Patman]

Hmmmm, learn something new every day.

 

[whenfoxforks-ruled]

I sure thought by now all pistons came with the holes, or directions on how to do it. Well, all pistons that run on an exhaust bridge.

 

[griffbones]

Every one of my Wiseco pro lite pistons have come with directions saying to drill the holes, but the holes have always been pre-drilled, generic directions I guess?

 

[Nick's #1 Fan]

You have all big a big help.....

Is there somwhere that I can locate some directions on how to do it?

Do you think there is any problem with doing it to a stock Honda piston? Perhaps it's more for aftermarket pistons?

Thanks again for all the help...

 

[griffbones]

I think Wiseco may suggest this on their forged pistons because they will expand with heat more than a stock cast piston. But I'm quite sure there are other members on here that could give you a definitive answer on this question.

 

[Clipity]

Yes its for extra lubrication for the exhaust bridge. I believe Wiseco is the only company that you need the holes. I know on the CR500 its a must or you run the risk of it seizing,you all so need to relieve the the exhaust bridge. I made a jig out of some scrap wood to hold the piston in place so I could drill the holes with my drill press.

 

[BigRedAF]

Put the piston on the rod, leave the rings off then install the cylinder. Get a sharpy then reach inside the exhaust port and trace each side of the bridge on the piston. Use a very small drill, ~.030 or less and drill the two holes centered in between the lines. Use a drill twice as large and use your fingers to drill a very small chamfer to remove any flashing. Some guys don't drill they instead use 600 grit sand paper and relieve the bridge. I'm thinking that your bike has a plated cylinder so you need to drill not sand the bridge. If memory serves me correctly Honda relieves the bridge and the stock piston doesn't have any holes...

The holes allow cooler gas/air to cool the bridge. The exhaust gas is very hot and causes the bridge to swell and scuf the piston causing a sezure if it's not drilled or relieved, I hope that helps...

 

[Nick's #1 Fan]

That helps a ton......

I have one other question, I am running a ported and polished cr85 cylinder with a stock head and I am wondering if I can switch back to regular gas? He is going to race it in this form for awhile until he get confortable with the feel of the motor. I had been using 110 but since I am back to stock compression will it make much difference to go back to pump gas.

Thanks again for the help!

 

[BigRedAF]

If you have stock compression you may actually make more power with pump gas...

The most power is made when running the least octane that your engine requires. :whoa:

 

[whenfoxforks-ruled]

Depending on who "ported and polished" the cylinder it may be a good idea to replace it. The powerband has a good midrange snap, not much off bottom. Ill advised work on the cylinder can make that worse, hit farther up in the rpm's. Short of Eric, I have no idea even who to trust with a porting tool.

 

[Patman]

Short of Eric, I have no idea even who to trust with a porting tool.

NOBODY!

 

[whenfoxforks-ruled]

The "I have done this 1000 times, or we flowed it, they have done other racers" are still a dime a dozen. What works in other forms of racing does not hold true to dirt. No, I did not go to college, I drive by one a lot! Wait a minute, it is like this: