CruzinImage GT750 Piston Question

[Suzukidave]

The list has found in the past where the tolerance of the cast blocks and where the bore was varied a fair bit , with the early models they shimmed the big end and miss placement of the bore would side load the ends of the rod which as we know spins 360 deg very fast at times . Where shimming the small end that only rocks back n forth and allow the big end side room for tolerances in the bore was a much better plan . The only problem i have seen on the small rod end that was too loose was it allowed the wrist pin bearing to ride out the side of the rod , this was when late model pistons with the wider gap at the pin location for the thrust washers was used without the thrust washers on a early engine .

[Vintageman]

Suzukidave,

Makes sense.

1mm looser on top may are may not be enough for bearing to move and be real issue. But, would imaging the step in washer is in and out of bearing as engine runs. EOM it stays in place. Maybe flip the washer for cruzinimage versus adding shim. Maybe do nothing trivial. Not sure? Suppose pop and engine open after a few k miles and take a look see.

I may ask John Noonan USA -Wossner what they did on their piston for GT750 if I go that way. But those are more pricey than even OEM. Not sure needed for GT750, but maybe would have been a better piston for the looser Cyl to piston clearance of a T500 and high RPM running.

As far as clip question? If Cruzer is ~ same diameter and width groove OK to use Suz clip else use one came with it my 2 cents. If wrist pin loose in piston it moves a lot but, if tight it seams like they don't walk or at least rattle about too much is my experience so less long term worry.

Good stuff.

[Coyote]

Vintageman, The Wiseco pistons I had were for a GT550. They were the 'old' style with dykes rings. All NIB. the paperwork in each box clearly stated 'overbore' .002.
I understand your thinking of .003 - .0035. In reality, the pistons only real job is to carry the rings. Rings create the compression, not the piston. The piston can raise or lower compression by increasing or decreasing the volume of the combustion area. Shaving the head is a common method to increase compression as it decreases the combustion volume.
But, we're talking about ancient 2 stroke technology here. I would say .002 - ..003 clearance would be fine for any of these old engines. Bigger woulg still work, but then you get into the area of piston slap. Not keen.

[Vintageman]

0.002" for a 61mm forged piston is amazing, That is just about what Suz calls out and those are cast piston.

Forged usually expand more than cast significantly when you compare clearance percentage difference, but stronger in general. Cast usually expand much less but in general not as strong.

[Suzukidave]

the paperwork in each box clearly stated 'overbore' .002.

http://www.2strokeworld.com/forum/index ... ic=12283.0" onclick="window.open(this.href);return false;

[Vintageman]

Sorry Suzukidave I was in the middle of typing my post and you posted yours ->


0.002" for a 61mm forged piston is extremely good! That is just about what Suz calls out and those are cast.
In general forged expand more than cast and quite a bit if you compare differences in clearance percentage. And in general forged are a stronger piston.

I hate piston slap for various reason and more so in air cooled engines. I like to hold to spec centered. I hate seizing even more.

HVCycle for example had pistons made for Yam R5. Those remakes are cast, and guys were using Yam cast spec and they were seizing. HVC later stated to use 2.0mm not the smaller number called out by yam. That is fine, just need to know. I used Wossner on R5 and they wanted 0.0028" Yam was 0.0018" I think wisecos were ranged at 0.003"-0.0035" (dyke ring top). That is for 65 mm piston


Mike S

[chance johns]

i have scored OEM pistons. well at least one. the compression on all three cylinder are all around 125psi

i was considering picking up these after markets as i have the top end off to address a faulty base gaskets, but if my compression is good and all are roughly the same, maybe i dont need to replace?....whats caused the score is the big question.

[Vintageman]

Joost


What did you do about this->

"but on the cruzinimage pistons this is almost 1mm too wide. I was thinking about installing extra shims to solve this, but haven't really went and bought some.

fyi ->
http://www.thebearingcompany.co.uk/prod ... oduct=7282" onclick="window.open(this.href);return false;
http://www.thebearingcompany.co.uk/prod ... oduct=7281" onclick="window.open(this.href);return false;
http://www.thebearingcompany.co.uk/prod ... oduct=7280" onclick="window.open(this.href);return false;

My OEMs 1975 gt750 are 1.024" which is 26.00mm spacing for rod and OEM washers.

Has anyone ever checked this dimension on Cruzin image piston?


Thanks

Mike S

[Vintageman]

No one ever measured the crusin image piston ????

I had someone in Canada checked aftermarket piston and came up with 26.5mm. He said his supplier was out of Japan. I believe the ones that were from Australia (on eBay a while ago) were also coming out of Japan and Crusin image states "made in japan". Are they the same supplier? Seems plausible… I wonder who this may be...

When I measured the two washers attached to wrist pin I come up with about 0.012" shorter than 26.0mm. So I think EOM piston allowed for a little expansion and ease of assembly (others have said this too).

So if these new Japan after market pistons wider by 1/2 mm we will have ~3/4 mm play side to side versus the ~1/4mm EOM had put in place.

The steps in washer I believe are simply for ease of assembly only.

Wrist pin bearing is 18mm x 23mm x 24mm, needle length 1mm shorter than cage (if I understand). I think this little extra side to side OK and will check rod eye width to see how bearing still rides (just did an edit for messed this up last time )

I am going to get new washers (inexpensive) and new wrist bearings of course. It does seem that my cage is a little shorter (worn) shorter than 24mm and imagine washer is a little worn too.

[Vintageman]

Bought Crusin set.
I hope if not same only 0.5mm more
I'll post are real short reply (impossible) when i get them as too what i measured there for size
Anal-yses paralysis

[Vintageman]

Joost was correct 1mm wider not 0.5mm.

[Suzsmokeyallan]

One of the issues with top end bearings is they get narrower with mileage, considering the bearing is a steel race and the piston is aluminum you would think the face of the piston where the ends of the top end bearing spin onto would cut a noticeable groove. You hardly ever see that much wear on the piston compared to a paring down of the top end bearings cage ends.
Be very careful with used top end bearings, with enough mileage the worn ends start to crack inwards to the needle slots, after enough of those micro cracks happen, a bearing cage explosion will be imminent.

[Vintageman]

That's one reason you see engines (for example Rotax engines specified for Ultra lights) use cage less bearings .

Besides the 1mm side to side slop, I did not like how loose the pin fits in piston (Its piston not pin for pin tighter in OEM piston), and the casting for the wrist pin is a little out of align with machined hole to accept the pin, thinner towards bottom versus OEM which is well centered in casting.

for my peace of mind I bought OEMs pistons. I have a lot of money in this GT750 to worry about a little more. I am keeping a bike build journal on this one and think this decision would be appreciated if ever it gets a new owner.

[dyrberg123]

Just a quick drop off from me here. I had a set of Cruzings pistons installed in a RD400, and on the RD forum the Guys with experience with those recomended me to bore ,0017" witch i did. The shop told me it was pretty tight, but the result was really great. Running very silent, and no problems. I used the supplied snap rings