Potential Group Purchase - Wiseco GS650 741cc pistons

[Old Colt]

I should be able to get thickness info later this week.

[frankenwabbit]

I'd be up for one mls headgasket. as for thickness, etc., I will defer to you guys for what's best for the application.

[Ace07]

I am not sure how to measure the deck heights and everything, so I will trust you guys on the best thickness.

[tkent02]

Same here. I'm not opening up this 550/675 until winter sometime, so to get in on this group buy I'll just go along with the group.

[Old Colt]

My Pistons arrived today, Thank you very much for doing the group purchase Andy.

Here is a used 750 piston in one cylinder,
IMG_9042.jpg

And here is a new piston
WTF only one picture per post, OK

[Old Colt]

Here is the new piston,
IMG_9040.jpg
I was going to post more pictures but this restriction is stupid.
Anyway the factory piston sits down in the bore, I did not measure the amount.
These new pistons sit proud in the bore by about 0.013"
This is with a new base gasket and the cylinder block not bolted down so the piston might well be up a few more thou. I will try to get around to getting some studs or bolts in this tomorrow.
I am working with a factory fresh case and a used cylinder block. I also have a factory new 650 cylinder to run but might start out with a used head till the new one is ported and ready.

As for head gasket thickness, my measuring at this time is not accurate enough to go by, yet.

[Ace07]

Dang that piston looks really good in there. How do you go about measuring the gasket thickness? I am still a long way from putting my Pistons in.

[Old Colt]

Dang that piston looks really good in there. How do you go about measuring the gasket thickness? I am still a long way from putting my Pistons in.

Most people chose to run a piston to head clearance around 0.030". To determine a gasket thickness one would take in this case the piston is 0.013" above the deck height, to this add the the desired clearance of 0.030 and you will come up with a gasket of 0.043" thick.

If the piston were below the deck such as the stock ones are the gasket will be thinner. Consider the piston is 0.010" below the deck. To get a desired 0.030" clearance the gasket would be 0.020"

Myself I choose to run less clearance than this. On most of the small motors I set up I run closer to 0.015" piston to head so in my case I would choose about a 0.030 gasket allowing me a more active squish area. In my case I need to get more accurate measurements to work with.

As for measuring the piston height, the proper method will be to use a depth micrometer the determine just where the piston is in relation to the cylinder deck. You also need to pay attention that the piston rocks in the bore especially when cold so multiple measurements need to be taken. Last night when i took my quick measurement I used a digital vernier which for this first setup is in the ballpark.

[Ace07]

Oh ok. Well that seems like a pretty easy way of going about it. If you increase the squish band by just .013" will it increase your performance very much?

[tkent02]

Oh ok. Well that seems like a pretty easy way of going about it. If you increase the squish band by just .013" will it increase your performance very much?

I think the thing about a tight squish band is that you can run much higher compression ratio and you can get away with more advanced timing without any detonation occuring. There's probably more to it but that's part of the reason for getting it tight.

[Old Colt]

I think the thing about a tight squish band is that you can run much higher compression ratio and you can get away with more advanced timing without any detonation occuring. There's probably more to it but that's part of the reason for getting it tight.

Correct, the way this operates is the tighter squish area forces droplets of fuel back into the center of the chamber providing a richer mixture near the plug and leaning out the outer areas that are prone to not burn fully. When this all works the engine can be run leaner therefore making more power on the fuel used as well as a cleaner burn. Simply put, a higher efficiency.

[Chuck78]

A dozen of these beauties have just arrived!!!!!!!!


(Sorry, I don't know why it turned the picture sideways, it is not that way on my phone, only after I uploaded it)

[Chuck78]

Here ya go Charlie, full size version of yours:


Question for ya - I noticed a slight bit of flash rust in your cylinder bores. I would think that you would probably want to run the hone through that again, would you not? working on some two-stroke dirt bikes with nikasil plated cylinder walls, the plating is so hard that we didn't need to bore the cylinders out any when putting new rings and piston in. We do need to hone the cylinders for proper new ring break in, which is where I read up more on proper hones to use The really long 3 Stone flex hones apparently take a lot of material off, but the ball hones are more gentle and don't take much material off at all. I assume that with any flash rusting, the fine pointed "file-tooth tips" of the honing pattern that are responsible for the bulk of the proper break in within the first 15 miles would get rusted away very quickly and not be present to get a good break in performed. I assume that you would want to hone the cylinder again then if there is any flash rusting. Correct?






FYI everyone, it is really critical to do a proper ring break in after a good hone job. As soon as the bike is started up and up to operating temperature for the first time on the new engine, you need to go out and run it up through the RPM's without lugging the engine, say 3000 RPM to 5500 RPM acceleration, then immediately close the throttle and use the engine braking to decelerate, and repeat that acceleration then engine braking deceleration cycle about 10 times under a good load but not going 100% all out full throttl pulls. Each time, increase the RPM's slightly, I think you would probably want to end up on the 10th run about 6,700 RPM, but on the fresh engine, do not take it above 7000. Breaking it in with 30 weight oil, definitely never synthetic, is a better idea also. Then for at least the first 40 miles, do not ride the bike at any steady RPM, continue this ring break in process by constantly getting on the throttle and then closing it completely. Riding on the highway is not a good idea unless you are constantly altering your RPMs and closing the throttle then accelerating. The hone pattern on the cylinder walls is sharp and pointy and act like a file to custom fit the brand new piston rings perfectly to the surface of the cylinder. If you do not do this within the first 14 miles or so of riding, the hone pattern is worn smooth and only serves as holding oil on the cylinder wall for proper lubrication to prevent glazing ( if you don't break in your rings properly, it will likely glaze over the cylinder walls immediately due to the rings not sealing properly to their full potential).

The additional cylinder pressure caused by putting more of a load on the engine actually forces the piston rings outward and pushes them into the cylinder walls more, Expediting this break in process. The break in process actually removes any surface imperfections from the piston rings as well, as even though they are brand new, they may not have a perfect service, and they really need to match the cylinder walls.

Another thing I remember now from my last engine builds, the orientation of the gap in the piston rings is critical. Make sure to read up on that and have the gaps staggered properly and in the correct orientations



I like to change my oil after 40 miles or so, and then after 100 or so miles. After maybe two hundred miles, then I give it hell and run er hard!

[Old Colt]

Thank you Chuck,
Yes my cylinders need to be touched up, these were bored 10 years ago right about the time I had a debilitating injury so this and many projects went on hold. When bored the cylinders were coated with Quickseal. Unfortunately as good as Quickseal is for building engines it has no value as a protectant during storage hence the rust. Since I could not build the motor at that time I should have oiled the parts.
I have also found a crankshaft I bought on Ebay had been washed and not oiled, it is trash now.

As for ball hones, the only time I use them is the final scuff in two strokes just to reduce chances a port may not have been chamfered. That is just me though, they can be used for a quick deglaze.

[Chuck78]

So for cylinder honing, do you use the hones with the 3 long cutting stones on spring loaded arms then? On the 2 stroke / dirtrider / kdxrider.net forums, they were a saying to use the ball hones, & that the three legged stone hones were to aggressive for most cylinders. Your opinion is a little different than that concensus but excellent advice to hear.