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<  Performance issues  ~  other OEM options for CB750 big bore pistons - Kawasaki GPZ?

fang
Posted: Wed Jan 14, 2009 9:07 am Reply with quote
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Joined: 19 Oct 2005 Posts: 23 Location: Austin, Texas, USA
I have been looking for a replacement 67mm Henry Abe piston for my 900cc kit. My general lack of success inspired me to broaden my search a bit.

So I have been looking for two things: (1) an alternative source for a 3-piece oil ring for these old H-A 67mm pistons. And (2) the possibility of a completely different 67mm piston that could drop into the CB750. If such a thing exists, it might also be a source for better quality piston rings for the H-A pistons -- or better yet, it might lead to a much better quality big bore piston package than the old H-A pistons.

The focus of my search is to find a relatively common either stock-size or factory over-sized setup that is about 67mm in diameter from a motor with 2-valves per cylinder and 15mm wrist pins (just like our SOHC CB750s).

The other day I was helping my brother work on a 1984 Kawasaki GPZ750 (non-turbo). It is a DOHC motor, and I thought it had 16 valves. But I was wrong; these have an 8 valve engine. Today while searching for big bore CB750 pistons I noticed that the 1980-1985 Kawasaki GPZ750s also have a stock bore of 67mm as well as 15mm wrist pins. (This is for the naturally aspirated motor, NOT the turbo version, which I believe has larger wrist pins.)

I need to actually get my hands on some of these pistons to measure and compare other variables like skirt length, crown volume/height, etc, but they might actually be an option for us! I also saw that there is a somewhat common Wiseco 69mm (+3mm/810cc) big-bore kit out there for the 1980-1985 GPZ bikes. If it all fits, the GPZ750's 810cc/69mm kit would be about 942cc in a SOHC CB750. The Wiseco part number for the kit is 'K810.'

I am excited about the possibility, and I am interested in anyone else's thoughts and experience.

Peace and grease,
-fang
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Satanic Mechanic
Posted: Tue Jan 20, 2009 9:17 pm Reply with quote
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Joined: 04 Sep 2005 Posts: 309 Location: Munich, Germany
Now that sounds interesting! I tried a few piston manufacturers sites hoping I would find some more data, esp. compression height but no luck so far. Nonetheless, this may be an exciting opportunity!

Cheers,
Axl
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t1m32sh1n3
Posted: Wed Jan 21, 2009 11:02 pm Reply with quote
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Joined: 01 May 2008 Posts: 18 Location: central ohio
the gpz has a stroke of 54.14mm, and the cb750 has a stroke of 63mm. anyway with the diff. in stroke between the 2........ i'm also almost positive the gpz piston is shorter as well, not sure how much.
i know cb900 rods are 1.5mm longer.......if you needed to make up lost throw!
a buddy of mine has a shop ill probably swing by there sat. and try and find a gpz piston.
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RC30
Posted: Fri Jan 23, 2009 5:45 pm Reply with quote
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Joined: 22 May 2006 Posts: 6 Location: Hessen, Germany
i`m use kawa gpz750 2-valve pistons in my cb350 k4 engine - so its no problem to install these pistons, with bigger liners, in cb750 - some people take cb350 series piston (64mm bore) to build 810cm.
the gpz750 pistons have a very high piston dome, so you must check the valve pockets and drill out the combustion chamber.
I send axel some pictures of this pistons.

Yours,
michael
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t1m32sh1n3
Posted: Sat Jan 24, 2009 12:22 am Reply with quote
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rc30 do you have any specs on the piston??? i called the guy i know and he doesnt have a gpz piston. but..... im still looking so as soon as i find one.................
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RC30
Posted: Sat Jan 24, 2009 10:04 am Reply with quote
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the standart bore is 66mm, the 4. oversize is 67mm, piston pin is 15mm, piston clearence 0,04 - 0,05 mm, the pistons are von ART, the company also build the pistons for honda.
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Satanic Mechanic
Posted: Mon Jan 26, 2009 7:58 pm Reply with quote
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Joined: 04 Sep 2005 Posts: 309 Location: Munich, Germany
Michael (RC30) sent me the pictures he made of CB350 pistons and GPZ 750 pistons. Here they are:

CB350 and CB750:


CB350 and CB750:


CB350 and CB750:


GPZ750 and CB750:


GPZ750 and CB750:


GPZ750 and CB750:


GPZ750 and CB750:


CB350 and GPZ750:


CB350 and GPZ750:


Thank you, Michael!


Last edited by Satanic Mechanic on Wed Feb 04, 2009 10:27 pm; edited 2 times in total
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fang
Posted: Wed Jan 28, 2009 3:40 pm Reply with quote
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Joined: 19 Oct 2005 Posts: 23 Location: Austin, Texas, USA
Quick question:
In these pictures, which one is the GPZ750, and which one is the standard CB350 piston? Is the one marked CB350 with the taller dome the GPZ? Sorry about the confusion.

-fang
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fang
Posted: Sun Feb 01, 2009 9:04 pm Reply with quote
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Joined: 19 Oct 2005 Posts: 23 Location: Austin, Texas, USA
I bought a set of used Kawasaki GPZ 750 pistons of eBay today. LINK. Worked out a deal with the seller and he only is mailing me the pistons without the cylinder block to save shipping. I also am going out of town tomorrow for about three weeks. When I get back I will take a good look at them and post some comparison pictures of them with my Henry Abe pistons. That might be as late as early March.

peace and grease.
-fang
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Satanic Mechanic
Posted: Wed Feb 04, 2009 10:25 pm Reply with quote
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I got an email from Michael who explained what is what on the piston pictures. I have edited my previous post with the data from Michael.

Compared to the stock pistons the GPZ pistons have quite a dome. They might even be an option for F2 heads. Hmmmm....

Cheers
Axl
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fang
Posted: Sat Feb 07, 2009 4:28 pm Reply with quote
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Joined: 19 Oct 2005 Posts: 23 Location: Austin, Texas, USA
Thanks. That helps a lot.
My GPZ pistons will arrive some time soon. I will post some additional pictures with my thoughts. I also am now looking for a "spare" F2 cylinder head to see how that fits... I maybe found one online, but the seller does not really know what it came off of, and I can't remember which bikes came black. Was it only the 77-78 F2 models? I am trying to get the seller to verify that it is not just dirty.... Here are two pics. What do you think -- does this look like a decent, big valve F2 head?





Thanks!
-fang
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Satanic Mechanic
Posted: Sat Feb 07, 2009 7:33 pm Reply with quote
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It is a '77/'78 head at any rate. If it was black from the factory, then it's a F2 (or F3) big valve head. (Only '78 heads have eight knock pins).
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fang
Posted: Thu Feb 19, 2009 6:14 pm Reply with quote
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Joined: 19 Oct 2005 Posts: 23 Location: Austin, Texas, USA
I got my GPZ pistons, but I broke my camera and have been too busy to mess with them.
Their valve pockets look good, but the piston dome is a few mm taller than my Henry Abe pistons. They might be perfect for the F2/F3 motors. But I don't have any of those parts anymore for a comparison

I don't have time to mess with this aspect of the project anymore. I've decided to just stick with what I know -- there is no reason to change plans now when I am almost finished with the motor. I really just need to put some Henry Abe pistons back in there. So if anyone wants my four GPS pistons they can have them for free -- just pay for shipping -- USPS flat rate is about $9.00 anywhere in the USA.

Just PM me to work out the details. Otherwise I will shelf these for a couple years and get back to the idea when I build my next big CB750 motor.

peace and grease
-fang
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Terry in Australia
Posted: Wed Mar 25, 2009 8:17 pm Reply with quote
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Joined: 25 Mar 2009 Posts: 2 Location: Melbourne, Australia Mate!
Hey what a great thread, I have a NOS 65mm "Action Fours" big bore kit for a KZ650 Kawasaki (718cc) that I bought on EBay years ago to use in a CB750 engine, but like Fang, discovered that the domes were a tad high for my "K" bikes, but didn't think about my F2?

I've currently got a K6 top end on the F2 but I'd much rather add to the original F2's performance with an 836cc kit that I've had sitting in a box for the last 5 or 6 years, ha ha! Thanks Fang, you've got me thinking! Oooher, but then again, I've got my own boring bar, so maybe I'll go buy a set of GPZ750 pistons! Anyone need a NOS 836cc kit for their F2? Cheers, Terry.
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Voxonda
Posted: Wed Mar 25, 2009 8:22 pm Reply with quote
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Joined: 01 Nov 2006 Posts: 42 Location: The Netherlands
Anyone need a NOS 836cc kit for their F2?

I know I would like to!

Great meeting you here, Terry.
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