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Discussion Starter #1
Has anyone ever heard of the AZ Quads 745cc, 106.5mm Athena Big Bore kit being sold on Ebay and their website? 745 Big Bore Kit

I wasn't aware that Athena had a big bore cylinder kit for the 700.

Are these any good? I know that Athena makes other quality big bore kits. I'm looking to pair the 12.5:1 version with my Cuervo +1 ported head and 5050 cam w/ +3 TB. I know I'll have to run a bit of race gas in it but i already do this for my motorcross bikes.
 

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Well, it used to be that 105.5mm was the biggest you could put in stock cases without machining. Now comes along a 106.5mm with no case machining. Where did the extra 1mm come from? It's a thinner skirt on the cylinder and I'm not sure that's a good thing.
 

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Discussion Starter #3
Thanks. I asked them if its actually an Athena cylinder and they said yes it was made custom for them. I'll inquire some more about the skirt thickness.
 

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Cylinder bore may be pressed in, it can be thinner allowing extra room in the case. Athena has made these for motorcycles.
 

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I've had some Honda 250R big bores in factory cylinders with very thin skirts. I think P12 or maybe P14 bores. I would put a THIN layer of non hardening Permatex , YamahaBond or similar between the skirts and cases to somewhat stabilize it. It is a bitch to get apart though, using brake clean help dissolve it.
 

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Discussion Starter #6
So i received the 106.5mm 745cc kit. It's a custom Athena cylinder and CP 12.5:1 piston. The skirt is right around 1.75mm thick. I have a concern with the top ring gap though. I'm measuring 0.026". CP general requirements are 0.019" to 0.023" depending on how radical the setup is. It looks like others on here generally shoot for 0.020" on the 734 kit. .My second ring is ok at 0.028" but Im concerned about the top ring gap. I've emailed CP for their feedback but they will probably tell me to check with my engine builder which will probably tell me its fine. Thoughts?
 

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I would call CP customer service, tell them it's out of spec. Chances are they sent 105.5 rings for a 106 piston. I think I would want the second ring tighter towards minimum specs.
 

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Discussion Starter #9
Thanks guys. I'm going back and forth with CP about it now. The second ring is supposed to be a looser fit according to their setup instructions. They give a range of 0.017" to 0.034" for a bore this size. I'll run the 0.028" clearance on the 2nd ring by them too though.
 

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Thanks guys. I'm going back and forth with CP about it now. The second ring is supposed to be a looser fit according to their setup instructions. They give a range of 0.017" to 0.034" for a bore this size. I'll run the 0.028" clearance on the 2nd ring by them too though.
On most 4-stroke engines the 2nd compression ring usually has a wider gap, and gap range, than the top ring.
Yes, it is in spec, but I would agree on liking it closer to the low side of the spec. But, it'll probably be hard to get that one replaced since it's technically good.
For curiousity; have you measured the bore? I wonder if it's on the big side of spec as well?
 

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Discussion Starter #12
I need to get a bore gauge to accurately measure the cylinder bore. Heres a pic of the top ring. I can actually fit a .028" feeler in there.
20200130_181152.jpg
 

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I had to file mine to fit on my CP piston. It will be fine, is on the high end. Take to a machine shop to measure if you don't have a dial bore gage and micrometer.
 

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I need to get a bore gauge to accurately measure the cylinder bore. Heres a pic of the top ring. I can actually fit a .028" feeler in there. View attachment 138212
I had to file mine to fit on my CP piston. It will be fine, is on the high end. Take to a machine shop to measure if you don't have a dial bore gage and micrometer.
Measure the bore in 3 or 4 different places. See if it's straight.
Might as well measure the piston too.
 

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Discussion Starter #15
CP got back with me today and said that since the rings are so thin, they need extra gap for expansion and 0.028" could be okay. They said to check with the supplier of the kit who hasn't got back with me. I plan to verify the bore size this weekend though.
 

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CP got back with me today and said that since the rings are so thin, they need extra gap for expansion and 0.028" could be okay. They said to check with the supplier of the kit who hasn't got back with me. I plan to verify the bore size this weekend though.
That’s entirely possible, the ring material can be an issue as well. Recently had a 110.5mm cylinder and diamond piston wiped out do to too tight of ring gap. Was in diamonds spec, but was still too tight.
 

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CP got back with me today and said that since the rings are so thin, they need extra gap for expansion and 0.028" could be okay. They said to check with the supplier of the kit who hasn't got back with me. I plan to verify the bore size this weekend though.
If the rings are thinner, then there is less material to expand.
Their statement sounds backwards.
That they "could" be ok, and to check with the supplier.
Wth? They should send a ring that's in spec for the top, and good business would send another 2nd compression ring to try and get the gap smaller.
Small details that wreck a good and expensive build.
Bad business; as in, how much does a set of rings really cost them?
....not much.
 

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If the rings are thinner, then there is less material to expand.
Their statement sounds backwards.
That they "could" be ok, and to check with the supplier.
Wth? They should send a ring that's in spec for the top, and good business would send another 2nd compression ring to try and get the gap smaller.
Small details that wreck a good and expensive build.
Bad business; as in, how much does a set of rings really cost them?
....not much.
Thinner material gets hot faster, which means it could expand more.
They build pistons to suppliers specs. Whoever is putting the kit together is who should be responsible to ensure the parts work together.
Cylinder could be too big or not round. Supplier could’ve spec’d wrong size piston for correct wall clearance. Supplier could’ve sent wrong rings with piston. Too many variables to jump on CP for doing anything wrong.
 

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Thinner material gets hot faster, which means it could expand more.
They build pistons to suppliers specs. Whoever is putting the kit together is who should be responsible to ensure the parts work together.
Cylinder could be too big or not round. Supplier could’ve spec’d wrong size piston for correct wall clearance. Supplier could’ve sent wrong rings with piston. Too many variables to jump on CP for doing anything wrong.
I always thought CP supplied the rings with the piston when ordered. So it would be CP to make it right. UNLESS, the seller already gapped the rings and took to much off.
 
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