Topic: Arrows or circles on the piston top.

[timtune]

All the manuals I have say put the arrow on the top of the piston to the front when installing them. But the two sets of pistons I have have small circles instead. Both sets have the narrower ring grooves. I thought they were early "K"s but now I'm confused. Any one able to tell me what they most likely came from?

[Jerry Rxman Griffin aka MuthaF'er]

If I am correct the 77/78 K's had narrower top rings, not the oil rings though. 1.25mm? vs 1.5mm

[PeWe]

Those pistons can be K7/K8.
If stock Honda, look for 392 marked on the side.

My K2 got a cheap version of 392 piston +0.5 from Cruzinimage, also with ring on top, EX side.
They have a little more dome than K0-K6 which increase compression and fun.

[timtune]

Both these sets are domed slightly. The first set has a complete dome from outer edge to outer edge - turns out they're from a 78F. The second have a dome that is flattened and set from each edge.

[timtune]

Can these domed pistons be swapped without concerns for valve clearance?

[PeWe]

Later F has higher dome piston. Not for the earlier CB750 with smaller chamber heads.

I heard about a K2 build where those pistons kissed the valves.
Aftermarket pistons  with higher compression no problem when valve pockets are deeper.

[Jerry Rxman Griffin aka MuthaF'er]

Both these sets are domed slightly. The first set has a complete dome from outer edge to outer edge - turns out they're from a 78F. The second have a dome that is flattened and set from each edge.

There are 2 DIFFERENT 392 pistons from 75/76F. Perhaps 3 even with the late 76 piston with thinner 2 top rings. One with a full dome and one with a partial dome that is reduced around the edge. Please show a picture of your later F piston top.

[HondaMan]

A thought:
randomly swapping pistons in/out of a given bore is not a good idea. Pistons can vary as much as 0.0004" in diameter for a given 'size' of piston, and the piston clearance in this engine must not be more than 0.0012" or it WILL burn oil. Which pistons were used to measure for the bore size? I'd suggest sticking with those.

[timtune]

As far as I know I'm slotting the pistons into the jugs they were in (can't say for sure they were in these jugs originally)

The cases were early K (73ish). But the bike was a 76F. I got these as part of a package of 2 dissassembled F's. I now think the top end was swapped for F stuff long before I got it. But honestly who knows.

[timtune]

You guys never cease to amaze. The pistons I'm using (the semi domed) are infact stamped 392 - they all have a an "A" over top 392 and opposite that 3 have a a "3" and one has a "4". I don't have a smartphone so I found this on ebay . Mine are identical.

https://www.ebay.com/itm/284080745439?epid=1323052181&hash=item4224882bdf:g:3Z8AAOSwL1tfsC10

The second set from the '78 F are stamped 410

Thanks all I'm slowing learning more of my old rides history**.

**
-Kid I hired had some 750s he took apart to fix but didn't. Traded them for some stereo equipment (google that if under 30). Guy who was supposed to pick them up didn't. Mine for $50
-built one from 2 in my basement winter of '93. Friends helped me get it up the stairs come spring (after firing it up down there to check it worked - window open of course)
-blew second gear being stupid at Ride for Sight in '98 and holed the cases when a chain let go in '99
-swapped gears and cases from a doner and flogged it for about another 3 years before it started leaking oil pretty good and it ended up under the bench
-almost 20 years later I'm rebuilding the whole bike from the bare frame up