426 Big Bore Kit - Honda TRX Forum
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post #1 of 15 Old 10-10-2018, 01:15 PM Thread Starter
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426 Big Bore Kit

Bought this kit (2 pics below) and wondering if anyone here has installed the similar kit. Instructions say to install, torque, reassemble to point that engine can run. Run up to operating temp then shut down and let cool (heat cycle). Then they suggest tearing it apart to re torque the bolts. I've asked then the question to confirm if they require removal of the head to access the cylinder bolts. Seems crazy to me.

If anyone has installed this kit, I'd like some feedback. I still have time to return it...
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File Type: jpg 426 Big Bore Kit 2.JPG (121.0 KB, 5 views)
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post #2 of 15 Old 10-10-2018, 08:34 PM
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It's considered a cost of racing!

It is not necessary to remove the head and re-torque the cylinder bolts, but it is imperative to re-torque the head to prevent the head gasket from failing. Also, it is best if the head cover gasket is replaced after removal to re-torque the head.

Follow break in instructions to the letter; use the proper break in oil, change the oil when specified and always change the oil filter each time the oil is changed. Although heating and cooling cycles are important, it is also important to not let the engine idle more than 10 minutes when cold or more than 3 minutes when hot. You can't break in an engine by just running it, you have to put it under a load - it's best to "pasture ride" it - let it pull the machine, let the machine push it - you can give it full throttle, just don't over rev it (short shift it) and at least 10 heating and cooling cycles. If you break it in properly, it will last an acceptable period of time before needing to be rebuilt.
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post #3 of 15 Old 10-10-2018, 09:59 PM Thread Starter
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Your feedback is appreciated.
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post #4 of 15 Old 10-14-2018, 04:05 AM Thread Starter
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Was removing the old piston and noticed the machine had a hot rods crank. I'm not sure the 426 kit I ordered will work. The pistons are both wiseco and the new one is taller from the top of the wrist pin by .67mm. I am reaching out to Hot rods to figure out what crank this is and Wiseco to figure out if the old piston is a stroker piston.

Have a look at the pics. Any feedback is appreciated.
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post #5 of 15 Old 10-14-2018, 02:33 PM
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I checked with Wiseco first - the old piston is an 89mm 435cc (440 kit) 11:1 compression

The new piston is 88mm 426cc 10:1 CR - if you had gotten piston 4628M08800 it would be a an 11:1 CR and probably be the same crown height as the 4667 piston

Wiseco does not provide piston measurements other than diameter.

The stock stroke is 70mm - if you just put the old piston (without rings) on the rod and slip the old cylinder over the piston, you can use a dial indicator (or possibly the slide caliper) to measure the stroke. A stroker crank increases the stroke to 74mm.
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post #6 of 15 Old 10-14-2018, 02:54 PM Thread Starter
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Thanks for the feedback. I guess I figured a 10:1 piston would allow more space in the combustion chamber at TDC. Meaning that the measurement I took should be less not more. This is why I figured a stroker crank. I like the idea of measuring the stroke. Will do that right away. Always learning when i get into these types of rebuilds.
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post #7 of 15 Old 10-14-2018, 03:23 PM Thread Starter
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So I out the piston on and cylinder on place and checked the stroke a few times. 70.24mm so i assume stock stroke. This being the case, the 426 piston should be fine. I'm still confused why a lower compression piston would measure higher than the higher compression piston.

426 (10:1) piston is .67mm taller than the 440 (11:1) piston. Does CR take into account the bore size as well? I'll google this as well.

Maybe someone can make some sense of this for me?
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post #8 of 15 Old 10-14-2018, 05:06 PM
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Yes, increasing the bore without making any other changes will increase the compression ratio.

"Say we have a 500cc single-cylinder engine with a bore of 100mm and a stroke of 63.66mm. Let's also say its cylinder has a combustion chamber volume of, for the sake of easy calculation, 50cc. With the piston at BDC, the total volume of the cylinder is 550cc. With the piston at TDC, the volume is reduced to 50cc. Its compression ratio, therefore, is 550 divided by 50, or 11:1. But if we give the engine a 2mm overbore and make no other modifications, its displacement is now 520cc. Total cylinder volume is up to 570cc, reduced once again to 50cc at TDC. Dividing 570 by 50 gives 11.4:1."
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post #9 of 15 Old 10-14-2018, 05:12 PM Thread Starter
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Thanks, I did a little research s well. Learn something new every day.
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post #10 of 15 Old 10-14-2018, 05:21 PM Thread Starter
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Onto the next concern.

After looking at the damage on this motor, it appears the guy used the wrong bolts for the upper timing gear. I can tell because it is clear the threads and heads were cut shorter with a zip wheel, and the heads are still taller then the OEM allen bolts. The longer bolts, ground against the inner valve cover, allowing aluminum filings to enter the oil system. I removed the oil screen and it was plugged with these filings so my guess is the motor was starved on oil. The piston and cylinder clear got extremely hot because it appears the piston seized then loosened when the motor cooled? (it turned over before I disassembled). I've since purchased the 426 top end kit.

My question is, what about he bottom end? The crank appears to be tight as well as the connecting rod. Should I be looking for any other signs to indicate the bottom end requires work?
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