Topic: 75 Super Sport Pistons

[Retro Rocket]

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...

F0/F1 are the same as K7/K8 {392 head}, F2/F3 {410 head} are very different and won't work. While the F0/F1 pistons are slightly different to the earlier K pistons, as Bill has said above, the difference is minimal and they will work...

[HondaMan]

Maybe to help defray some of the confusion:

The OEM compression of the K0-K6 engines is 9.0:1.
The OEM compression of the F0/F1 engines is 9.2:1 (despite some of the ads Honda put out, showing it higher).
The OEM compression of the K7/K8 engines is the same as the F0/F1.

Installing the 836 CruisinImage pistons in the F0/F1/K7/K8 engines makes the compression just over 9.0:1, IF the quench band is cut into the head to match the bores. If not, the compression is nearer 9.25:1, but breathing is restricted a little bit.

Installing the K0/K6 pistons into the F0/F1/K7/K8 engines brings the compression down to about 8.8:1. On most of these heads there is room to mill off the 1mm extra height applied to the combustion chambers, bringing them back down to the size of the K0-K6 chambers. This brings the compression back up to about 9:1 again. Some riders worry about "valve clearance" in this conversion, but my clay-on-top-of-the-pistons tests have showed this clearance to be more than 2mm in all cases.

If you take some DOHC 750 pistons (1979-1982) and trim their shoulders down 1.5mm for about 6mm in from the outer edges, the compression comes to about 9.6:1 in the end, and the bore size is +1.0mm over stock bores (760cc).

[RodSOHC750]

I happen to have a used piston I replaced from my 1979 750LE DOHC engine.  Note the high ridge along the center of the piston top. Will this interfere with the head?

Picture below are of the new 1975K piston and the 1979LE piston.

[BPellerine]

they would give higher comp in early f or k,and maybe clearance issues.billp

[Jerry Rxman Griffin aka MuthaF'er]

The -392 F/F1 pistons ARE different than the -392 K7/K8 pistons. The F has a 'full' dome and will give a slight compression bump over the K7/K8. I have posted comparative pictures somewhere.

[madmtnmotors]

Should be no clearance issues with the flat tops. Poised to install some "dolled up" cruzinimage 65mm into my K8:



  

[SOHC4 Cafe Racer Fan]

Should be no clearance issues with the flat tops. Poised to install some "dolled up" cruzinimage 65mm into my K8:



  

Looks like DFL coating on the skirts!

[HondaMan]

I happen to have a used piston I replaced from my 1979 750LE DOHC engine.  Note the high ridge along the center of the piston top. Will this interfere with the head?

Picture below are of the new 1975K piston and the 1979LE piston.

You must cut down the whole crown from around the edge to about 6mm inward, about 1.5mm deep, to let them fit the K7/8 or F0/1 heads. I did one of those last Spring.

[RodSOHC750]

Just to make sure I understand correctly, here is a sketch of the piston modification.  In correct machinist terms, this would be a 1.5mm x 6mm chamfer all-around.

[HondaMan]

Just to make sure I understand correctly, here is a sketch of the piston modification.  In correct machinist terms, this would be a 1.5mm x 6mm chamfer all-around.

Here's the ones I did last Spring. The -392- piston is on the left in each shot, the modified DOHC on the right.

[RodSOHC750]

Thanks for the photos of the DOHC piston modifications.  I don't see any problem boring 1.0 mm oversize and netting a slight increase in compression ratio (9.6 vs 9.2).  I would need to also order rings for a 1979 DOHC 750 standard bore. 

[RodSOHC750]

Just an update on my progress.  I bought eight used 1979 Honda CB750 DOHC pistons off of Ebay.  I'm taking them to a shop this week to have the top edges beveled 1.5 x 6 mm.  Four pistons are for me and the other four (if they turn out to be good condition) will go to my son for a future rebuild.