hi comp piston worth it?

[Mad*Ex*Rider]

im going to need to put a slip in my cylinder pretty soon and am wondering if it would be worth it to install a high compresion wiseco piiston at the same time? and most importantly how will this affect the reliabitlity of the 400ex?

[plkmonster2]

Get a JE or Ross. They are lighter. If you go with a 416 or below, there will be no loss in reliability. Just a high comp piston doesn't add much. Get a cam while you are at it, and it will be much better.

[jak1389]

yea get a je piston. it wont make you loose any reliabitlity.

[TC426EX]

a hi-comp. piston, filter and a cheap hotcam w/a decent exhaust will really wake your bike up for pretty cheap

[Mad*Ex*Rider]

i alredy have it jetted with a hot cam, decent exhaust and a filter, do i have to bore out the motor to get more compression though?

[wilkin250r]

You don't have to bore out your cylinder for higher compression, you can get high-comp pistons in stock bore, 406, 416, 426 whatever.

However, it may not be a bad idea to bore to the next size up, just to give you a clean slate to work with.

[Mad*Ex*Rider]

but when i do this i wont have to screw around with the crankshaft or gearing right?

[bwamos]

Nope. Only "major" difference between a high comp piston and a low comp piston is that the high compression on has the pin a little lower on the piston so the top face is higher when it's fully compressed.

You have to make no changes other than putting in the new piston, and honing the sleeve if it's not a new sleeve or bored. You need a fresh hone so the rings will seat.

If you do the piston get a ring compressor. (very cheap, kina looks like a piece of sheet metal rolled up with a clamp on it.) Will make getting the piston inside the sleeve much easier. ;)

[big diggin7]

wheres a good place to get the je pistin ive been hearing about i want to puit the 416 kit in, you just haveto bore the sleeve out to drop the pistin in?

[jak1389]

call up c&d racing.

[big diggin7]

how much is this upgrade from c&d?

[jak1389]

its $120 for the whole je 10.8:1 416 piston set up.

[wilkin250r]

Originally posted by bwamos
Nope. Only "major" difference between a high comp piston and a low comp piston is that the high compression on has the pin a little lower on the piston so the top face is higher when it's fully compressed.

Close, but not quite. For most aftermarket pistons, the pin is at the same spot as stock, and the edges of the piston are exactly the same height. The difference is in the shape and volume of the dome, it sits a little higher and takes up more volume of the combustion chamber.

The very edges of the piston dome are usually very close to the head to begin with, even with the stock setup (the exception is the 400EX). You generally don't have much room to move them upwards.

[F-16Guy]

Originally posted by wilkin250r
Close, but not quite. For most aftermarket pistons, the pin is at the same spot as stock, and the edges of the piston are exactly the same height. The difference is in the shape and volume of the dome, it sits a little higher and takes up more volume of the combustion chamber.

The very edges of the piston dome are usually very close to the head to begin with, even with the stock setup (the exception is the 400EX). You generally don't have much room to move them upwards.
Which brings up a good point. The gap between the edge of the piston and the head on a 400ex is a little too big to create the most efficient combustion, so make sure you put it together with the thinnest base gasket you can get (or none, if you're comfortable with using just a sealer like Yamabond). You should also use a thinner head gasket. I'm a big fan of OEM Honda head gaskets, so I recommend the XR400 gasket, which is a little over half as thick as the 400EX gasket (if I remember right).

[wilkin250r]

Yeah, the 400EX is an odd design, I have no idea why Honda would make it like that. Even with thinner gaskets, it's very difficult to bring the deck height to where it needs to be.

It's not terribly bad, certainly not dangerous nor detrimental to reliability, it's just not the most efficient, and makes upgrades difficult.

If I were ever doing engine upgrades on a 400EX (my girlfriend's is stock, and will probably stay that way) I would probably talk with JE or Ross about a custom piston to correct the deck height issue without resorting to removing the base gasket. They usually have a minimum order of 4 pistons when doing custom designs, but I'm sure I could sell the other 3 to cover their own costs so that I'm only out the price of one piston.

I wonder what negative problems are associated with moving the pin down 1mm...

[03pissedoff426]

check out ATV unlimited i got my 426 13:1 JE piston from them for around $85

[Mad*Ex*Rider]

what is the stock compression?

[2004exrider]

not tryin to hyjack the thread, but whats wrong with wiseco? ive been thinkin about doin a 11:1 compression stock bore piston. ive probly got a little over 40 hrs on it. do you think it will be out of specs or can i just hone it and put it in?

Jimmy

[jak1389]

Originally posted by 2004exrider
not tryin to hyjack the thread, but whats wrong with wiseco? ive been thinkin about doin a 11:1 compression stock bore piston. ive probly got a little over 40 hrs on it. do you think it will be out of specs or can i just hone it and put it in?

Jimmy

well weisco ratios are not accurate since they got those compression ratios from the xr400 motor. so the 11:1 would not be an 11:1. to get closer to the ratios, you could use a xr head gasket. you would need to rehone the cylinder to put in the new piston.

[bwamos]

Originally posted by wilkin250r
Close, but not quite. For most aftermarket pistons, the pin is at the same spot as stock, and the edges of the piston are exactly the same height. The difference is in the shape and volume of the dome, it sits a little higher and takes up more volume of the combustion chamber.

The very edges of the piston dome are usually very close to the head to begin with, even with the stock setup (the exception is the 400EX). You generally don't have much room to move them upwards.

Well that's the way it works on the 300ex at least.. Going from a stock piston to an 11:1 weisco. The pin is noticably lower than the stocker. I'd take a photo.. but one of them is in the quad.. lol. ;)

But, a 300ex wasn't ever built for performance.. it was lawyerized.. so it probably didn't have a good design to begin with.

You are very correct in regard to 99% of the pistons out there. I was over simplifying it.


As far as the dynamic vs. static.. the static is really the only number they can measure with accuracy. Dynamic changes with the RPM of the engine. Cams are tuned to a certian RPM and at that RPM the compression is at it's peak.. but at other RPMs it is lower. This is due to the fluid properties of the air. Suction, velocity, bla bla bla... It's a lot like a tuned pipe on a 2-stroke. Different pipes are going to give you different dynamic compressions at different RPM's.

Static compression is easily measured, and standardized.

[400sEXridr]

stock is 9.1:1

[mcshaw16]

Originally posted by bwamos
If you do the piston get a ring compressor. (very cheap, kina looks like a piece of sheet metal rolled up with a clamp on it.) Will make getting the piston inside the sleeve much easier. ;)

Just a quick question. This is my first time rebuilding a top end, I bought that type of ring compressor and I'm having some trouble with it. It seems that the only way it would work is if you had the piston separate and could slide it down through the top of the cylinder? Am I wrong?

Because I'm at a point where I have the piston already on the rod, so I don't see how it would work. Is there a different type of ring compressor that I need?

Thanks

[wilkin250r]

Originally posted by mcshaw16
Just a quick question. This is my first time rebuilding a top end, I bought that type of ring compressor and I'm having some trouble with it. It seems that the only way it would work is if you had the piston separate and could slide it down through the top of the cylinder? Am I wrong?

Because I'm at a point where I have the piston already on the rod, so I don't see how it would work. Is there a different type of ring compressor that I need?

Thanks

Me thinks you're not using it correctly. It HAS to separate somewhere to slip around the rod. You can't put the piston pin in after it is already installed in the cylinder, it just doesn't work that way.

Any ring compressor in the world has to come apart somewhere so you can pull it off around the rod.

[F-16Guy]

Put the piston in the ring compressor and slide it in the cylinder from the bottom, but only enough so that the rings are in. You need to be able to slide the wrist pin in. Put the base gasket in place. Now take the cylinder with the piston sticking out of the bottom, put it over the rod and line it up, and then slide the wrist pin through. Now make sure you have a rag around the base of the rod and over the opening in the case (unless you like to fish) and install the circlips. You could also start with one already installed. Once that's done, move the rag and slide the cylinder into place, and you're done (except for the rest of it, ofcourse).

[lostcreekz450]

yes way worth it. my bike is a 450 and puts out the same horse power that the alba racing 470 stroker Z puts out 470 low comp.
450 high comp, 52ish horse power.