Honda 400ex top end blown. What could be the cause?

696 Views 7 Replies 2 Participants Last post by Here 2 help, 5 mo ago

GavinCarraway

Discussion Starter · 5 mo ago

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I’m new here and I don’t have a lot of experience working on engines but I’m curious to what could have caused my 400ex to do this? I read that It running lean could cause this but I’m not sure due to me not having a lot of experience. It was a brand new top end that I put on. Made sure the rings where all good before I put it in as well according to wisecos specs. I did the break in on it and then rode it and found out it was definitely lean due to it having a aftermarket pipe and all. I put a bigger jet in it after the ride. Rode it the next weekend and then this happened so not many hours on the engine (maybe at most 3). Would love some opinions on what would have caused this so it won’t happen next time. Thanks.

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Here 2 help

· #2 · 5 mo ago

It over heated - at least the piston did - I see it's a Wiseco M series piston - you say it's a new top end - what by your definition constitutes a 'new top end'?

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GavinCarraway

Discussion Starter · #3 · 5 mo ago

The new top end I put on consisted of a new cylinder, new piston and rings, and I went ahead and replaced all the valves and valve springs.

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· #4 · 5 mo ago

How did you break it in and what kind of oil did you use for break in?

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GavinCarraway

Discussion Starter · #5 · 5 mo ago

I did two cycles where I let it idle for 15 minutes and let it cool each time in between. After that I rode it really easy in low rpms for around 30 minutes. Once i did that i began to slowly start to open it up for about an hour period. After that i rode it normally. I used Amsoil metric 10w40.

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· #6 · 5 mo ago

Did you let it idle longer than a minute after riding for 30 minutes?

What was the piston to cylinder clearance set at prior to assembly?

Type of oil is not important to anything other than the clutch - if the oil does not have the JASO rating, it can cause the clutch to slip and fail.

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GavinCarraway

Discussion Starter · #7 · 5 mo ago

No I did not let it idle longer than a minute. Like I said I'm new so I never took the measurement of the piston to cylinder clearance.

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· #8 · 5 mo ago

Wiseco pistons are forged so they take longer to warm up and expand more when heated so more than standard clearance is required when at room temperature - whoever sold you the kit should have set the proper clearance, but they are not machinists and you are not either, but the proper clearance is the responsibility of the mechanic assembling the engine.

Regardless - the problem was the result of overheating and the piston got too big for the cylinder.

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