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nac-nacer

Drilling holes in piston?

61 posts in this topic

I just got a new wiseco piston kit and i'm getting ready to do a top-end. The last time i did a top-end on the bike i didn't drill the holes for the exhuast bridge but on Dirtrider.net i just read about a guy who didnt and it destroyed his plating and did horrible damage to the piston.I've got the cylinder off and it looks perfect plating wise, should i still drill the holes.

Thanks

___________________

91 cr250

04 crf100

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If you have a bridged exhaust port, and that bridge has not been relieved, you should drill the holes. They will allow un-burnt pre-mix from the crankcase to cool the exhaust port bridge and prevent it from expanding and possibly pinching the piston. If the holes are not needed, being there will not do any harm.

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call me un-educated but i'e never heard of drilling holes in a piston prior to installation, is this for wiseco pistons only? i've done several 250 topends and always use oem parts and have never done this

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Just did the same for my son's 2002 CR250. Starts right up, but he says that it now acts like it's very rich. I haven't ridden it since the rebuild, so I can't vouch for his diagnosis. It was jetted just about perfectly before the rebuild, but it also had about .006" of piston clearance and the ring endgap was over .035". Perhaps increasing the compression caused it to run richer.

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call me un-educated but i'e never heard of drilling holes in a piston prior to installation, is this for wiseco pistons only? i've done several 250 topends and always use oem parts and have never done this

This is only to cool and lubricate the bridge. Not all 250's have this.

It is good practice on any mfg's piston.

Back in the day of iron bores even with the bridge relieved no holes meant the bridge expanded with heat and pushed into the piston notching the ring and shortening the whole package life.

The reason they are not predrilled is some oe cylinders have the bridge out of spec to port or starboard and the predrilled holes miss the bridge and spew the charge out the port.

Hmmm Vertex a Wiseco step sister now.

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What does it mean to "relieve" an exhaust bridge?

This is the jug out of my CR500. See how the bridge is relieved?

DSC01051.jpg

Basically, you take about a couple of thousandths off the bridge.

You only drill Wiseco 's! Wiseco's are forged and expand funky versus cast pistons. Cast pistons do not need to be drilled.

cr500wiseco.jpg

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This is the jug out of my CR500. See how the bridge is relieved?

DSC01051.jpg

Basically, you take about a couple of thousandths off the bridge.

You only drill Wiseco 's! Wiseco's are forged and expand funky versus cast pistons. Cast pistons do not need to be drilled.

cr500wiseco.jpg

Very nice illustartion!

The Bridge is expanding into the piston not the reverse

Again This is good practice with any piston cast or forged.

The Thermal expansion issues of Forging's went away many years ago with new alloys. Heat treat and forging design. The Old tales are hard to die. Forgings can now be set up as tight or tighter piston to cylinder clearance than casting,s.

My own 03 KX 250 is using the single ring Seizco at .0018 the OE casting is set up at .0023-.003

RC,s RM250 in 05 used a similiar single ring forged Seizco to win both sx and mx championships. I did not notice Goose warming it up any different than anything else

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So for my kx 125, would i need to do this? I have used it without the holes, no damage has seemed to occur yet.

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So for my kx 125, would i need to do this? I have used it without the holes, no damage has seemed to occur yet.

Thats what he thought too!

scoredpiston2.jpg

Actually he has a base gasket leak also. But you can see the scoring on the intake skirt due to lack of lubrication holes. Easy way to tell...pull your pipe off, shine a light up in there and have a look-see.

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I looked at it when i did the top-end this winter. There was a little blow by on the exhaust side (maybe 1/4 in.) with about 30 hours on it.

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I looked at it when i did the top-end this winter. There was a little blow by on the exhaust side (maybe 1/4 in.) with about 30 hours on it.

Keep an eye on your compression. You did a top end.....re-ring?

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Thats what he thought too!

scoredpiston2.jpg

Actually he has a base gasket leak also. But you can see the scoring on the intake skirt due to lack of lubrication holes. Easy way to tell...pull your pipe off, shine a light up in there and have a look-see.

Actually we are looking at the exhaust side of the piston pictured.

The Exhaust side is the Hot side that includes that exh.bridge on some models.

Was this a test to so who was paying attention?

The failure shown is all the work of the extreme lean condition of the base gasket leak. Exh. bridge damage will be more limited to where the bridge contacts the piston vertically. Tight ring end gaps can/will also have more tension on that area causing accellerated wear. And those cooling/lube holes only cool/lube the bridge not slobering it all over the exh.side of the piston.

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