Vintage Dirt Bike Q & A

Putting new piston in MR, mainly because I'm running a 73-74 piston and want the peace of mind of having ring ends in the right spot, and because the stock '76 wrist pin I have in the Wiseco 337 is a tighter fit than it should be. I'm putting in a Wiseco 338. Bore is .40, or 1mm, over. New piston, wrist pin and bearings.

The piston I'm removing has been in there one year and is in pretty darn good shape. Carbon on top, but sides look strong. I had not heard of drilling extra lube holes when I put that one in, so I didn't do it.

Local race guru says he never does it, but he also changes pistons after about every three races. I don't race at all, just ride trails and dirt rodes.

Any thoughts on whether I need to drill where the exhaust bridge is?? Judging from the condition of the piston I removed, I'm a little worried I may cause more harm than good.

Thanks.

If you want a professional opinion, I would call Wiseco. For the '79 CR250R they recommend extra lubrication holes for the exhaust port bridge and provide instructions on where they should go. I have run without - an oversight - and the piston lasted exactly 1/2 lap of the Elsinore GP. If you do have a bridge, I cannot see what harm these would do, but that is just my opinion. (Your mileage may vary.)

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

Good question! The stock piston does not have holes in it, so why do it?

I'm in the process of rebuilding an MR motor, have a new Wiseco piston with the drilling instructions.

I don't have a problem drilling, I'm just curoius as to what others have done.

Anyone?

One additional twist. I have been told that Wiseco pistons run tighter tolerances than OEM. That may be one reason for the holes where the stock pistons don't have them...

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

The stock pistons did not have the holes because the bridge was relieved from the factory. The bridge gets hot and swells up from the exhaust heat and closes up the clearance on the piston. Usually when people bore engines they do not relieve the bridge, so they seize, same with a replated cylinder. The holes supply some lubrication to the bridge area, and more importantly they cool the bridge and keep it from swelling. With all that said, I do not drill my pistons, but I always make sure the bridge is relieved. As for Wiseco pistons running less clearance than stock, that is not true, Wiseco pistons are forged, and the factory pistons are cast, a cast piston does not swell as much as a forged piston when hot, so a cast piston does not require as much clearance. The old Wiseco pistons were very bad about seizing, the new stuff is top notch, still I have not been able to run an engine as tight on the clearance as what they say and not scuff the piston, at least not on the high horsepower air cooled stuff.

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Sam Alexander

1974 Honda CR250M
1976 Honda CR250
1974 Maico MC440
1974 Maico MC250
1977 Maico AW400

...definitely a Joda moment.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

what exactly do you mean when you say "relieved" ??

I'll take a guess. The exhaust port bridge is shaved down slightly so when it is cold it is further away from the piston than necessary, but as it warms up to operating temperature it grows slightly towards the piston ending up with the correct tolerance. Sam's point is that if you do not relieve the bridge you have to stop it growing and getting to close/tight with the piston. This is done by cooling it with pre-mix from under the piston crown that bleeds out through the lubrication holes.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

I have always drilled the holes in my new Weisco pistons for the extra lubricant. Sam & Mike what good info & tech stuff. Nice to know!

But what the #$% is a Joda moment??

Relieved is after a couple of Coronas

Joda, funny green chap with amazing insight and knowledge. Popped up in the Star Wars movies.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

Do You mean Yoda, in spanish or Hebrew it would be Joda, I guess
Some great info on the piston drilling options and information, I would assume that "relieving" the bridge would mean removing .002-.004 of material from the bridge to allow for expansion(?)

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Garry Mainer
#33m (retired)
Honda Cr250r 1980
Honda Cr250r 1980
Yamaha TY250C 1976
Arctic Cat 650 H1 2007

..sorry I had a European moment.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

The 1979 Honda has a Chrome bore. I never grind the exhaust bridge on any Chrome bore just drill th piston it can't hurt anything to do so.

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Proformance Cycle
c/o Lee Curry
58 Wyoming Ave.
Wyoming, PA. 18644
570-693-5811
mailto:proformance58@cs.com

This has come up before, but remember that you need different rings for a stock chrome bore v one that has been sleeved....or it will seize.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

The chrome bore on the 79 was done by Honda at the factory, if you look at it real close you will see it is relieved, on all of the Honda engines it is about .004" right in the middle of the bridge, and feathers out at the top and bottom. If you replate the cylinder a good shop will relieve the bridge, even on Nikasil. My suggestion is that if you replate the chrome bores, go Nikasil, less headaches and they seal better. I have bored the 79 oversize, we relieve the bridge before it goes to plating, then if they don't relieve it, we do it when it comes back. We are extreme here on race engines, that charge being blown out the exhaust port is unacceptable in my opinion, but like I say, I am extreme and like to get every last bit of power out of the engines. Got a 75 CR250 in here right now getting some very trick work done, it will be interesting, it should be faster than my 74 engines, which is kinda scary to think about!

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Sam Alexander

1974 Honda CR250M
1976 Honda CR250
1974 Maico MC440
1974 Maico MC250
1977 Maico AW400