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14:1 and spark plug clearance

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So - I just installed a 14:1 JE piston in my 2013 crf450.  The piston *just* hits the plug at TDC, decreasing the gap to the point where the spark is too weak to fire.  It looks like there are plugs with a low profile electrode... 

 

Does anyone have the part number?  Where can I find one??

 

Doing a little bit of research it looks like NGK R7433-8 is the correct plug?

 

 

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What about a extra thin shim or thicker crush washer on the plug itself I don't know if even possible but just a thought , could possibly get you out of a (pinch) no pun intended lol

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Is the motor all-stock except for the piston? OEM base and head gaskets used? No milling of the head? I find it hard to believe the piston could touch the plug...

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The head is ported (by RHC) but not decked. I did put an extra washer on it and that seems to have fixed it. I just want to do it 'right'.

Here's the piston at tdc. Visually it seems to be quite a bit taller than stock. Especially with the dome

ImageUploadedByThumper Talk1382744302.777478.jpg

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What gaskets did you use? Base and head?

And what's the piston part number?

Edited by Eddie8v

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Stock gaskets. I'll have to check the piston part # when I get home

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Yep, that's the correct piston. I have used that piston a few times without plug interference issues. What spark plug you running? SILMAR9A9S is the correct NGK plug.

Edited by Eddie8v

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OK.  I re-measured the clearance and the plug *is* just a hair to long.  Clearance is right at 0... It looks like a plug from a CRF250R may do the trick (NGK R0451B-8).  The 250 plug has the lower profile electrode that I need.  However, this plug is a bit hotter (8 vs. 9). 

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How did you measure the clearance?

And there's likely a colder "9" plug for the 250.

I have used a few of this same piston without issue, so I don't know how this could be happening to you? No way they would release a piston that's anywhere near hitting the plug. Maybe your piston wasn't finish-machined or something.

Edited by Eddie8v

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See the pic above for the piston finish.  

 

I used a screwdriver with a length of fuel line around it.  Keeping the fuel line hung up on the top of the hole, I inserted the screwdriver until it touched the piston.  This gives me the total allowed depth of the threads + electrode.  Not the most accurate way to measure, but it clearly shows me I don't have enough clearance.   

 

The clearance is different front to back, so I bet depending on how the electrode is oriented it may have just enough clearance.  It should be just fine with the different style electrode. 

 

I have it running with the stock plug and a washer, but I don't like to have it jerry rigged like that.  I just spent a ton of money on the motor... 

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I can appreciate that, but I am just scared something is wrong. Did you talk to Ron about this? If it were mine, I would disassemble the motor and measure the cylinder height, deck height, piston dim's and also cc the head. Maybe Ron will tell you this plug interference can happen, it just didn't happen to me.

I assume it's not a stroker crank? And is it a stock or aftermarket rod?

Edited by Eddie8v

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You can switch plugs or use index washers to provide clearance to index the electrode in the position that gives you clearance. If it were my bike I would ensure I had .030 clearance. Index washers can be purchased at Jegs, Summit....

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I did talk to Ron.  He mentioned seeing this with Kawis, and Suzukis, but not the Hondas.  He suggested I try another plug.  I'm considering throwing a different piston in there.  13:1, or 13.5:1. 

 

I'm not terribly experience with this.  At this point I'm kinda wishing I'd left it stock... 


You can switch plugs or use index washers to provide clearance to index the electrode in the position that gives you clearance. If it were my bike I would ensure I had .030 clearance. Index washers can be purchased at Jegs, Summit....

 

Thanks for the info.  I'll have a look! 

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Agreed on the index washers, but I just don't think they should be necessary... I could be wrong but I would assume a piston made for the GP would have adequate clearance with any possible plug.

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To be more specific you need .030 clearance between the plug and piston and .030 clearance between the piston and head.

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Did you confirm the proper gasket stack between the cylinder/case and head/cylinder.? 

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Gasket stack? As in are gaskets installed? Yes both head and base are there. OEM gaskets were used

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So - I just installed a 14:1 JE piston in my 2013 crf450. The piston *just* hits the plug at TDC, decreasing the gap to the point where the spark is too weak to fire. It looks like there are plugs with a low profile electrode...

Does anyone have the part number? Where can I find one??

Doing a little bit of research it looks like NGK R7433-8 is the correct plug?

It was pretty common back in the 2 stroke 125 days to use a spark plug spacer when the compression was bumped way up. I know from experience from my 125 days.

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