Honda ATV Forum banner
1 - 11 of 11 Posts

· Registered
Joined
·
76 Posts
Discussion Starter · #1 · (Edited)
EDIT - Looked it up, the 300ex pistons should work but they are lower compression rating. . Namura piston sets + gaskets for top end were only around $100. If they work well, I may just get that.

Any reason to run the 12:1 versus a stock 300ex compression rating piston? Any detriment against the X running well?
______________________________________________

I have a 250x. Found some parts including a cylinder that has some good scoring and needs a bore and new piston.

All the ones I have found on eBay are 12:1 compression, some don't have C.R. listed but Wiseco number is, and that number comes up as 12:1.

Is this too much for pump gas, stock valves/clutch + springs, and non-hard faced rockers?

Only lower C.R. pistons are std size which I can't use.

Also, any larger piston should be fine with any head gasket, as it's the OD of the cylinder sleeve, correct? Or might I need a larger head gasket with, say, a .40 over bore?

Thanks
 

· Premium Member
Joined
·
18,211 Posts
wiseco makes a nice topend kit that includes everything you need to rebuild the top I use a 12:1 at .060 bore with no issues you should be fine with your stock springs and rockers.
 

· Registered
Joined
·
76 Posts
Discussion Starter · #3 ·
I saw that, it was maybe $155 or something of that sort. Gaskets, rings, piston, circlips, valve seals...

Sure I will be alright, as you have 1stage 1 cam, High performance coil, jet kit, 400EX carb, K&N filter, FMF power Core 4 full exhaust listed in your sig. I was hoping for even a 10.25:1 or 11:1 max.

I know it's not recommended on my 350X to go more than 10.25 to one without hardfaced rockers, port and polish, jet/exhaust/filter, valve springs, maybe cam.

Didn't find a full .pdf manual (I did see some pictures from your album, thanks!) to see stock compression ratio, but 12:1 just seems real high??? I suppose I can run AV Gas from an airport or something.
 

· Premium Member
Joined
·
18,211 Posts
are you building a 250X engine or 350X engine?
I have a full PDF of the 350X trike actual honda service manual if you need it. pm me your email. and I'll send it over
 

· Registered
Joined
·
76 Posts
Discussion Starter · #5 · (Edited)
PM sent.

Have a 350x trike that's getting fully rebuilt. This will be a separate 250x quad. Later on down the road I may build a nicer 350X engine for my quad, so I'd rather not go all out on rockers and such for the 250 engine. (But I do want it assembled, needs a piston)

Thanks
 

· Premium Member
Joined
·
18,211 Posts
They made the 300EX compression lower because it dosne't hav a decompression system, 11:1 is about the max for the 300 so you don't have premature starter failure. the 250X has the system so if you can kisk it you can put it in. LOL
 

· Premium Member
Joined
·
18,211 Posts
Just buy the 250X piston kit. Why would you want a 300EX piston? 12:1 is fine in a 250 even with no other internal mods I gaurantee it. the stock bore for a 300EX is 74mm the stock bore for a 250X is 74mm, the only thing that possibly could be different would be the deck height since the 300 has 5mm more stroke.
 

· Premium Member
Joined
·
18,211 Posts
you can run pump fuel with the 12:1 or you can run the 100 LL AV fuel I run both depends on my wallet for that day..lol. If you're not planning on boring it and just want to freshen up the engine go to Honda and buy a stock piston.

Wiseco kit 200 bucks everything you need Less the oil.
 

· Registered
Joined
·
16 Posts
I have a 300ex and I know this is off topic but I was riding one day and all of a sudden all my top end power was down the drain I can't go more then half throttle with out it loosing power I took the top end apart and the piston is dirty but no wholes or anything the spark plug was pretty "darkened" but that was really it I don't know what I want to do I still want more power out of it when I am done what would you guys do high compression or bite it out
 
1 - 11 of 11 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top