Topic: 75 Super Sport Pistons

[MRieck]

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.

Don't have to cc a chamber Jerry....you can tell just by looking at it.

[BPellerine]

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.

Don't have to cc a chamber Jerry....you can tell just by looking at it.

I may be wrong but compared to a ko head the 78 392 head looks to have been decked a little to get compression.billp

[RodSOHC750]

Any carb modifications with the 836cc conversion?  Carbs have stock #105 jets.  A rebuild kit I bought includes #110 jets.

[HondaMan]

Any carb modifications with the 836cc conversion?  Carbs have stock #105 jets.  A rebuild kit I bought includes #110 jets.

No. The airflow ratios remain the same, unless you change either the cam, or the size of the inlet valve. That said, many people change to different jets just 'cuz they want to.

Most of the Keyster kits I have seen lately (if that's what yours is?) have come with #110 mainjets, but they have different taper entry vents, so they act leaner than #110 size, confusing the issue. Stick with genuine Keihin jets: hard to find right now, but they are around.

[RodSOHC750]

I have K&L carburetor kits. 

[Kawahonda]

I think K&L and Keystar are very compatible if not the same parent company, but Hondaman could confirm.

I'd trust the washers and the o-rings...maybe the gaskets...but nothing else.

[Jerry Rxman Griffin aka MuthaF'er]

Compare the air screws. Holes or no holes?! Don't think I'd worry about the mains. I'd guarantee the needles are not as smooth or the same -392 profile as the F.

[RodSOHC750]

I have been unsuccessful so far in locating a piston kit for the 836cc conversion that maintains the stock compression ratio of 9.5 and under $250.  As a fall back, I am able to locate 61.5mm pistons. 

[SOHC4 Cafe Racer Fan]

Rod,
Cruzinimage on eBay.

[Retro Rocket]

I have been unsuccessful so far in locating a piston kit for the 836cc conversion that maintains the stock compression ratio of 9.5 and under $250.  As a fall back, I am able to locate 61.5mm pistons.

Rod, you'd better be quick mate....

http://forums.sohc4.net/index.php/topic,154912.0/topicseen.html

[Kawahonda]

Yeah, what retro said.

But I wonder why the Cruizin image kit is no longer on eBay. The dude had 10+ left and the next couple days it's no longer there. I'd email the seller and ask him.

[Ilja]

Ok, from a guy with a very modified cb836 turbo, my view on the subject... spend a little bit more money and buy the 0.5mm oversize pistons if you have no interest in tuning.

The 836cc kit is nice, but requires 4mm of boring which will cost you more than a normal 0.5mm overbore.
The sleeves will be very thin after boring... to be honest, to my opinion it is really on the edge. My 836cc is running ok so far, but I'm afraid if I will push it hard, the sleeves will deform because they can't take as much heath as sleeves that are e.g. 2.5mm thick.
Also, if you screw up, you can't go bigger. So in that case you will need new cylinders or resleeving ($$).

If you don't want too complicate things, don't go up and just stay with stock.
If you like to experiment (like I do hehe), go ahead and buy the kit. ;-)

[HondaMan]

I have been unsuccessful so far in locating a piston kit for the 836cc conversion that maintains the stock compression ratio of 9.5 and under $250.  As a fall back, I am able to locate 61.5mm pistons. 

Rad: Did you write to me about the cost of modifying the DOHC750 (62mm) pistons to use in "F" engines? I just got my set back: he charged $60 for it.

[madmtnmotors]

I have been unsuccessful so far in locating a piston kit for the 836cc conversion that maintains the stock compression ratio of 9.5 and under $250.  As a fall back, I am able to locate 61.5mm pistons. 

Rad: Did you write to me about the cost of modifying the DOHC750 (62mm) pistons to use in "F" engines? I just got my set back: he charged $60 for it.

I was at least one of those that had reached out inquiring about those. That's not a bad price. If it still holds once I get my 79' motor torn down and am able to asses the condition of the pistons I will likely be reaching out to you again. Thanks Mark!

[HondaMan]

I have been unsuccessful so far in locating a piston kit for the 836cc conversion that maintains the stock compression ratio of 9.5 and under $250.  As a fall back, I am able to locate 61.5mm pistons. 

Rad: Did you write to me about the cost of modifying the DOHC750 (62mm) pistons to use in "F" engines? I just got my set back: he charged $60 for it.

I was at least one of those that had reached out inquiring about those. That's not a bad price. If it still holds once I get my 79' motor torn down and am able to asses the condition of the pistons I will likely be reaching out to you again. Thanks Mark!

Ah...I knew it was SOMEone here: I get so much e-mail now that I often can't find who it was who asked me, a month later!
Just let me know if you want to send some: it always takes him a while to do them, as he waits until it is slow in his shop, and nothing is in the lathe.

[madmtnmotors]

Ah...I knew it was SOMEone here: I get so much e-mail now that I often can't find who it was who asked me, a month later!
Just let me know if you want to send some: it always takes him a while to do them, as he waits until it is slow in his shop, and nothing is in the lathe.

These would be for future use, so turnaround time is not critical.

[RodSOHC750]

Well, I bought a set of .5mm oversize pistons for a 1975 750K for my 1975 750F.  There are apparent differences namely in the valve recesses and the F pistons having a slightly higher deck height.  Trying to figure if I should machine the new pistons valve recesses. 

[RodSOHC750]

Also, I notice the top edge of the F piston is beveled where the K piston is not.

[BPellerine]

check the the pistons for height diff by putting a wristpin in both the f and k pistons side by side,chances are the f pistons have deeper valve recess cause they are taller,you should not have to machine the k pistons.billp

[RodSOHC750]

I compared the two pistons side by side with the upper ring in both piston grooves.  As you run your finger tip along the piston top from one piston to another, you can feel the F piston is slightly higher. 

I don't want to reduce the 750 F's 9.2:1 compression ratio.  Will the K piston affect the compression ration being it is slightly shorter but has less valve recess volume. 

[Retro Rocket]

I compared the two pistons side by side with the upper ring in both piston grooves.  As you run your finger tip along the piston top from one piston to another, you can feel the F piston is slightly higher. 

I don't want to reduce the 750 F's 9.2:1 compression ratio.  Will the K piston affect the compression ration being it is slightly shorter but has less valve recess volume.

To check the piston height you have to do as Bill said, put the wrist pin in both pistons, side by side,  then measure...

[madmtnmotors]

I don't want to reduce the 750 F's 9.2:1 compression ratio.

Then you are going to have to pony up for a set of Wiseco 10.25:1 836cc pistons.

Otherwise, deal with a little loss of compression by using K model pistons.

[Stev-o]

As a proud owner of an 836cc motor, I highly recommend it. 

[Kawahonda]

There are lots of conflicting information in this thread (and in your other). Earlier part of thread says no compression loss with k Pistons. Some say gain. Now, the fact is that f pistons are not the same as k Pistons all along.

I guess the truth is that you should order some NOS oversized ones...

[BPellerine]

I cannot see it reducing comp that much to be worried about,if you are getting your head surfaced as part of the build,it will make up for a little,and new bores with fresh pistons are going to bring your numbers up to as new specs,so don't overthink it too much.billp