Author Topic: 75 Super Sport Pistons (Read 11764 times)

RodSOHC750 · « **on:** January 22, 2016, 09:34:24 PM »

I'm restoring a 1975 Honda 750F SuperSport. I dropped my cylinder head and cylinders off to a machine shop and they called today reporting the pistons are badly scuffed, high ring wear, and the cylinder walls are rust pitted. They recommend boring the cylinder which means oversized pistons. They offered to source the pistons but I told them I could. They recommended 0.50mm oversize pistons (61.50 mm diameter). Will oversize pistons for a 1975 Honda CB750K engine work? I don't want to go with forged pistons or higher compression pistons. I want to stay basically as stock as possible and stay within budget.

Don R · « **Reply #1 on:** January 22, 2016, 10:34:44 PM »

I believe the valve size and angle are the same. Compression could be a tad different but I'd say yes, they fit. The price of the cruisinimage 836 kit is hard to beat but I understand your point. That's 4mm of boring to get there.

Retro Rocket · « **Reply #2 on:** January 23, 2016, 01:33:08 AM »

Quote from: Don R on January 22, 2016, 10:34:44 PM

I believe the valve size and angle are the same. Compression could be a tad different but I'd say yes, they fit. The price of the cruisinimage 836 kit is hard to beat but I understand your point. That's 4mm of boring to get there.

The cruising image kit is 836cc's at very close to stock compression, for $130 bucks its a bargain...

http://www.ebay.com/itm/69-77-HONDA-CB750K-836cc-65mm-BIGBORE-PISTONS-KIT-W-HEAD-GASKET-CI-CB750KBB-/141768773488?vxp=mtr&hash=item210213cf70

Stock 0.5mm oversize pistons too..

http://www.ebay.com/itm/69-76-HONDA-CB750K-0-5mm-OVERSIZE-PISTONS-SET-4-PISTONS-INCLUDE-CI-CB750KPS-1-/131588210647?vxp=mtr&hash=item1ea344bfd7

Yoshimatic · « **Reply #3 on:** January 23, 2016, 04:34:41 AM »

This is a link to Honda piston sizes, it shows that the F pistons are the same as the K Pistons but the F1 pistons have 1mm more height above the piston pin.

https://www.4-stroke.net/library/honda-piston-size-chart

It also shows the difference from the F2 pistons.
As mentioned previously, the 836 kit is very good value for money plus it will go stronger too.

MRieck · « **Reply #4 on:** January 23, 2016, 06:19:20 AM »

I agree with the guys about that 836 kit.

RodSOHC750 · « **Reply #5 on:** January 23, 2016, 08:21:38 AM »

I ordered the $100 piston kit on EBay 61.5mm for 750k model. I asked the seller if they will work on a 750F model. He said it is not compatible and cancelled my order.

I'm not receptive to more cc's because it will effect tuning, unknown effect on drive train, and my desire to maintain bike originality and resale value.

It sounds like from the comments the K Pistons will not work in a F model.

calj737 · « **Reply #6 on:** January 23, 2016, 08:41:25 AM »

The net effect of the 836 kit on tuning is neglible. As for originality, once you bore originality is out the window anyway. So if you bore, you might as well bore for the best result and most cost benefit method.

RodSOHC750 · « **Reply #7 on:** January 23, 2016, 11:53:30 AM »

The 836 kit is a new concept to me. Does the Honda cylinder wall thickness allow for this much bore? Stock piston diameter is 61.0mm. What is the diameter of the 836 pistons?

MRieck · « **Reply #8 on:** January 23, 2016, 01:37:46 PM »

Quote from: RodSOHC750 on January 23, 2016, 08:21:38 AM

I ordered the $100 piston kit on EBay 61.5mm for 750k model. I asked the seller if they will work on a 750F model. He said it is not compatible and cancelled my order.

I'm not receptive to more cc's because it will effect tuning, unknown effect on drive train, and my desire to maintain bike originality and resale value.

It sounds like from the comments the K Pistons will not work in a F model.

They are not compatible with a 77/78 F....no problem in a 75 or 76 F. That is gospel.

Retro Rocket · « **Reply #9 on:** January 23, 2016, 02:36:54 PM »

Rod, The early F model Honda's {FO and F1} have the same head {392} as the K7 K8, its the F2 and F3 {as Mike said above} that have different head, therefore different pistons, the stock K style pistons will work in the early F models. The 836 kit in stock compression will give you some more usable power, the bike will handle it easily, there's plenty of far faster, more powerful bikes on this forum, some well over 1000cc's. The 836 kit fits easily, the sleeves have plenty of meat for this size bore... Just remember to be more specific with which model F you have, the later F engines have many different parts to the earlier FO/ F1... 836 pistons = 65mm bore

MRieck · « **Reply #10 on:** January 23, 2016, 02:47:45 PM »

Quote from: Retro Rocket on January 23, 2016, 02:36:54 PM

Rod, The early F model Honda's {FO and F1} have the same head {392} as the K7 K8, its the F2 and F3 {as Mike said above} that have different head, therefore different pistons, the stock K style pistons will work in the early F models. The 836 kit in stock compression will give you some more usable power, the bike will handle it easily, there's plenty of far faster, more powerful bikes on this forum, some well over 1000cc's. The 836 kit fits easily, the sleeves have plenty of meat for this size bore... Just remember to be more specific with which model F you have, the later F engines have many different parts to the earlier FO/ F1... 836 pistons = 65mm bore

Right. Just say you have a K motor and you'll be fine.

Retro Rocket · « **Reply #11 on:** January 23, 2016, 02:51:09 PM »

Quote from: MRieck on January 23, 2016, 02:47:45 PM

Quote from: Retro Rocket on January 23, 2016, 02:36:54 PM
Rod, The early F model Honda's {FO and F1} have the same head {392} as the K7 K8, its the F2 and F3 {as Mike said above} that have different head, therefore different pistons, the stock K style pistons will work in the early F models. The 836 kit in stock compression will give you some more usable power, the bike will handle it easily, there's plenty of far faster, more powerful bikes on this forum, some well over 1000cc's. The 836 kit fits easily, the sleeves have plenty of meat for this size bore... Just remember to be more specific with which model F you have, the later F engines have many different parts to the earlier FO/ F1... 836 pistons = 65mm bore
Right. Just say you have a K motor and you'll be fine.

That would have been too easy....

Jerry Rxman Griffin aka MuthaF'er · « **Reply #12 on:** January 23, 2016, 05:02:25 PM »

Quote from: RodSOHC750 on January 23, 2016, 08:21:38 AM

I ordered the $100 piston kit on EBay 61.5mm for 750k model. I asked the seller if they will work on a 750F model. He said it is not compatible and cancelled my order.

I'm not receptive to more cc's because it will effect tuning, unknown effect on drive train, and my desire to maintain bike originality and resale value.

It sounds like from the comments the K Pistons will not work in a F model.

Sure they will! You just don't mix 77/78 F stuff in K's without considerable work.

Kawahonda · « **Reply #13 on:** January 25, 2016, 10:06:43 AM »

Don't the cams differ from the K and the F for the 1975-1976 models?

If so, seems like the 75F with an 836 kit would be the best combo in forms of reliability and performance (and keeping it daily-driver worthy).

HondaMan · « **Reply #14 on:** January 25, 2016, 10:58:10 AM »

Quote from: RodSOHC750 on January 22, 2016, 09:34:24 PM

I'm restoring a 1975 Honda 750F SuperSport. I dropped my cylinder head and cylinders off to a machine shop and they called today reporting the pistons are badly scuffed, high ring wear, and the cylinder walls are rust pitted. They recommend boring the cylinder which means oversized pistons. They offered to source the pistons but I told them I could. They recommended 0.50mm oversize pistons (61.50 mm diameter). Will oversize pistons for a 1975 Honda CB750K engine work? I don't want to go with forged pistons or higher compression pistons. I want to stay basically as stock as possible and stay within budget.

If you use the pistons (flat-topped) from a 1969-1976 "K" engine in the F0/F1 engine, the compression will be low, below 9.0:1.To overcome this a little bit, you can mill the head about 0.5mm, which will help. The -392- combustion chamber is a little bit bigger than the -300- chamber, which causes this difference (the F0/F1 engines are not swirl-charge designs like the "K" engines were, hence the difference).

When I rebuild the post-1975 engines, I use the DOHC 750 pistons, with about 1.5mm trimmed off their shoulders, about .250"-.300" in from the outer edge, to clear the head (these have a high dome). The resulting compression ration is a little over 9.5:1. The STD size DOHC piston is the same size as the +1.00mm "K" piston, and you can still use Honda's great rings!

Jerry Rxman Griffin aka MuthaF'er · « **Reply #15 on:** January 25, 2016, 12:09:26 PM »

Quote from: Kawahonda on January 25, 2016, 10:06:43 AM

Don't the cams differ from the K and the F for the 1975-1976 models? YES

If so, seems like the 75F with an 836 kit would be the best combo in forms of reliability and performance (and keeping it daily-driver worthy).

HondaMan · « **Reply #16 on:** January 25, 2016, 06:42:30 PM »

Quote from: Jerry Rxman Griffin aka MuthaF'er on January 25, 2016, 12:09:26 PM

Quote from: Kawahonda on January 25, 2016, 10:06:43 AM
Don't the cams differ from the K and the F for the 1975-1976 models? YES

If so, seems like the 75F with an 836 kit would be the best combo in forms of reliability and performance (and keeping it daily-driver worthy).

Yep! The cams for the post-1976 bikes push the power up in the RPM band, making a few more HP for the tradeoff. But, the 836 kit would bring the midrange torque up quite a bit, so overall it makes a nice difference. Don't forget: use HD studs, like the ones from APE, when doing the big-bore kit!

MRieck · « **Reply #17 on:** January 26, 2016, 04:36:23 AM »

My 75F (and other 75F heads I have purchased on ebay) have the early style chamber. Basically the chambers have more material in it them will increase compression compared to a 76F or later K heads.

RodSOHC750 · « **Reply #18 on:** January 26, 2016, 06:00:10 PM »

Does the cylinder head have any identifying marks to identify which particular chamber volume it has? There is a "X" and "3" cast in the external surface between #2 and #3 intake ports.

The pistons have a 392 on them. I assume this is the stock standard piston 61.0mm diameter P/N 13101-392-000. A .5mm oversize piston should be p/n 13103-392-000.

I am getting strong pressure to go for 386 big bore pistons. After researching and viewing other comments, I'm leaning towards staying with the 61.5 over bore pistons. I don't have enough confidence yet the big bore would not result in compression ratio change, inadequate valve clearance, affect tuning and smooth idle, and require stronger studs.

scottly · « **Reply #19 on:** January 26, 2016, 06:27:23 PM »

Quote from: MRieck on January 26, 2016, 04:36:23 AM

My 75F (and other 75F heads I have purchased on ebay) have the early style chamber. Basically the chambers have more material in it them will increase compression compared to a 76F or later K heads.

What's the volume of the 75F chambers? My K7 head was between 22 and 23cc, IIRC.

Retro Rocket · « **Reply #20 on:** January 26, 2016, 06:51:44 PM »

Quote from: RodSOHC750 on January 26, 2016, 06:00:10 PM

Does the cylinder head have any identifying marks to identify which particular chamber volume it has? There is a "X" and "3" cast in the external surface between #2 and #3 intake ports.

The pistons have a 392 on them. I assume this is the stock standard piston 61.0mm diameter P/N 13101-392-000. A .5mm oversize piston should be p/n 13103-392-000.

I am getting strong pressure to go for 386 big bore pistons. After researching and viewing other comments, I'm leaning towards staying with the 61.5 over bore pistons. I don't have enough confidence yet the big bore would not result in compression ratio change, inadequate valve clearance, affect tuning and smooth idle, and require stronger studs.

With the cruising image kit there would be virtually no negligible change in compression, maybe slightly higher which won't hurt or change anything, the valves will have NO clearance problems unless you fit a high lift cam, seeings though you aren't , that problem doesn't exist, your carbs effect "smooth idle", there is NO effect on the tuning with bigger pistons that you wouldn't experience with a stock bore 750, maybe slightly larger jets to help with the improved mid range power, and I would put stronger studs on any 750 engine I'd have built, its has more to do with keeping the oil in and is something I recommend to every rebuilt 750 engine, high performance or not, one of the biggest problems with these bikes is the inability to put enough tension on the head with stock studs... I'm not sure why you think a stock compression piston kit will "effect tuning", There's no "unknown effect on drive train", thousands of us have had far more HP than you'll get from this 836 kit, going through these engines, they are tough as nails, I'm not trying to talk you out of the 61.5 kit, just letting you know that most of your fears are misconceptions. You can keep everything else stock with the 836 piston kit and the only change will be, you'll have a bit more usable midrange power, and it won't effect originality any more than a 1st or 2nd oversize bore, which is zilch, nothing, nada.....

Kawahonda · « **Reply #21 on:** January 26, 2016, 07:03:12 PM »

Retro: <if> he has the larger cc head (392), then won't the cruising image kit actually reduce the compression?

I thought from what some others are inferring that iit would be stock compression with that kit only if he has the earlier head (300).

Mriek was saying that his 75F came with the smaller cc head. Sounds like Rod needs to verify the casting # on the head.

Thanks for the information thus far! I'm interested in this discussion because this may influence my future build!

Jerry Rxman Griffin aka MuthaF'er · « **Reply #22 on:** January 26, 2016, 07:21:59 PM »

If anything the -392 head has fewer cc's and will bump your compression slightly not decrease it. The -392 engine is 9.2:1 and the head is part of that reason. Now, my disclaimer, I have not measured combustion chambers but have owned a 75 750F for 41 years.

RodSOHC750 · « **Reply #23 on:** January 26, 2016, 08:33:31 PM »

Great comments. The 836 piston kit uses 65mm pistons. Is there enough sleeve thickness to accommodate that much boring? How do I tell the cylinder head combustion chamber size? Is there a marking on the head?

Retro Rocket · « **Reply #24 on:** January 26, 2016, 08:41:23 PM »

Quote from: RodSOHC750 on January 26, 2016, 08:33:31 PM

Great comments. The 836 piston kit uses 65mm pistons. Is there enough sleeve thickness to accommodate that much boring? How do I tell the cylinder head combustion chamber size? Is there a marking on the head?

Plenty of sleeve thickness, this is a common kit for the 750's, although most choose the Weisco kit with its higher {10.25 to 1} compression. Plenty of guys here have fitted the cruising image kits as well. If you have an FO/F1 head it will be marked "392" on the casting on the back of the head somewhere near number 3 intake... Mike Reick that commented earlier is a head expert, he does brilliant porting work and is the man to ask about any 750 head...

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Author Topic: 75 Super Sport Pistons (Read 11764 times)