'81 GS650G: Aftermarket oversize pistons hit head

[Berti]

Hello everyone,

After a long abstinence here I'm looking for some knowledge and advise.
I bought a cheap GS650G, lots of issues, came very close to quit, but actually made it my pandemic project and learning experience (along with toooo many $$$ spent, but hey, gained a lot of experience!)

Here I was looking for oversize pistons...
Here I was about to quit...

Sooo a pandemic and gazillion years later here I am. Absolut total rebuild of the engine: balanced and polished crankshaft, all!! new bearings and gaskets, complete rebuild of the head, machined cylinders for oversize pistons and... the infamous cruzinimage 740ccm bigbore kit.

Last weekend I finally wanted to put it all together and that's where i ran into issues where I'm looking for advices.

Turns out the shape of the cruzinimage pistons are significantly different than the original ones. And thats where they hit the head. Here are some pictures to give you some idea. I probably need to clear around 5mm until the piston won't hit the head anymore....
Collage.jpg

Soooooo, what should/can I do???

[zed1015]

That's certainly not right .
Depending on crown thickness you may be able to machine enough off to get the correct clearance and compression ratio or space the block but at a guess i'd say they sent you the wrong pistons.

[Buffalo Bill]

IOM: Those are so out sized, they could never be machined to fit.

[Nessism]

Are you sure the pistons are hitting the head? Could the valves be mistimed causing them to hit the pistons? If it's the pistons you might want to put some clay on the piston top so you can figure out where it's hitting.

[Berti]

Thank you so far for your feedback.

That's certainly not right .
Depending on crown thickness you may be able to machine enough off to get the correct clearance and compression ratio or space the block but at a guess i'd say they sent you the wrong pistons.

I'm not sure if I want to alter the pistons. Taking stuff off sounds kind of wrong.
Unfortunately I bought these pistons a while ago. I will contact cruzinimage and see what they say.

IOM: Those are so out sized, they could never be machined to fit.

Yes, I agree.

Are you sure the pistons are hitting the head? Could the valves be mistimed causing them to hit the pistons? If it's the pistons you might want to put some clay on the piston top so you can figure out where it's hitting.

I'm 110% sure the pistons are hitting the head. I have not yet put in the cams therefor all the valves are fully in.
Also no clay needed, the head leaves some tiny scratches on the piston. The problem is the big dome shaped "crown" of the pistons.

https://www.thegsresources.com/_foru...38#post2603938 This post shows the difference where it becomes obvious.

[hillsy]

We had a GS drag race guy who worked in our shop and he would often cut a spacer base gasket out of aluminium road sign material to lower compression for turbo applications.

Could be worth a try if you don't want to relieve the head or machine the pistons straight up.

[Chuck78]

Well, I bought these pistons and this is the result...

https://www.thegsresources.com/_foru...23#post2651323

Awww man that SUCKS! They basically sold you a hemi head's domed piston, nearly identical to the stock 77-79 GS750 piston...
I would DEFINITELY be contacting them to complain, at least so they pull them and don't screw over anyone else... Unless perhaps they actually did mix up and send you GS750 65mm stock hemi pistons vs big bore GS650 65mm closed chamber head pistons...
MAJOR BUMMER...
But... an 80+ 8v GS550 head may potentially work on that piston with minor dome machining around the perimeter. There's no way you could machine that big hemi dome down to the 650 dome shape/size, you'd cut through to the back side...

[Chuck78]

So this isn't just a "stock photo" of a similar item... The dome design differences here is very plain to see, & CLEARLY will not work with the head designed to be used with the 650 piston...


vs

[Chuck78]

Berti, do you have a good accurate set of calipers to measure the piston skirts on those?
You might be able to drop in a set of these Wiseco 740 pistons into those bores. I "might" possibly be conviced to sell one of my 2 spare sets. I was holding onto one for a friend who didn't have the $ to buy a set at the time, & has since had 2 kids and hasn't gotten around to putting the 650 top end on his 550... he's too busy with his kids, wife, job, PE175 Suzuki, & GS450E...

[Berti]

No, it's not a stock photo and the actually image. I sent cruzinimage an email explaining the situation. Let's see how they react. I'm not really happy with them.

Now that my cylinder is already bored oversize I don't have too many options here.

I don't actually want to go the road with extensive altering head/piston or even heads from different bikes. Just frustrated at the situation.

If you let me know what exact measurements you need, I have ways to precisely measure!

[wymple]

What kind of compression ratio is that end up with anyway? In the old days we would double up on head gaskets, but those were simpler engines & desperate measures.

[fastbysuzuki]

I have no experience in fitting that kit but that looks to be something that cannot be resolved by fitting a thicker head gasket, the only other way would be to have the piston dome's machined to the same profile as the Oem ones.
Personally I would be contacting the supplier to check if they have sent the wrong ones as if extra machining or thicker head gasket (unlikely) is required that should surely be explained in the kit write up on the website.

[Berti]

Thank you all for your help!

I contacted the Manufacturer and so far no response...

To everybody who ever comes across this thread - i highly advise to not buy these specific pistons. May work for other models/manufacturers. The 740ccm big bore kit does not fit the original Suzuku GS650G head! I guess I paid for this lesson

¯\_(ツ)_/¯

[650Skull]

From the cruzinimage site description for the GS650 - 740 BBK. Cruzinimage state the pistons are a different shape

81-83 SUZUKI GS650 740cc BIGBORE PISTONS KIT 65mm PISTON

-DESCRIPTION-◆10-GS650BB◆
*81-83 Suzuki GS650 Bigbore pistons kit with Head Gasket
Aftermarket
Piston size: 65mm( 3.00mm Oversize )
Head gasket: Bore size is 68mm
*Set includes: pistons, pins, clips,rings and head gasket
Note: Cylinder boring required
These pistons are aftermarket.different design from of the Suzuki genuine pistons

[650Skull]

A bit of left field thinking............the GS650 barrels and top end is used on the 550 as a big bore kit............

Pics from cruzinimage show the standard size and up to 1mm oversized GS650 pistons as original shape No crown to speak of