Topic: Cb750 piston dome measurements

[Pbeattie89]

Does anyone know the dome measurement for the different stock pistons (300, 392, and 410)?

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[PeWe]

No, but 392 pistons is a good upgrade for CB750 K0-K6. A little bit more compression that compensates for today's thick head gasket.
Cruzinimage.net sell +0.50mm and +1.0mm kits.

My CB750 K2 has +0.50mm 392 pistons (for F1,K7/8).
It takes almost no oil between oil changes. The last change was after 5500km. The Spectro HD 20W-50 mineral oil  still looked and felt good

[bryanj]

As long as YOU knoe whats fitted and document it you can fit any, its only bad if you try to sell it as a thing it isnt

[Pbeattie89]

I currently have 392 pistons in my build and a 62 mm by 0.030" mls head gasket. I was playing with a compression ratio calculator, but I didn't know what to put in for the dome volume. I was just wondering what my compression ratio would roughly be?

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[bryanj]

At a guess similar to F1

[PeWe]

I think that 410 pistons (F2/3) will get valve contact in a K0-K6 engine with stock cam.
Probably with F1 K7/8  head too.
My K6 has a ported 392 head.

This must have been up before on this forum.

When using MLS head gasket, rigid HD cylinder studs are needed to tighten the head with 21-22 ft lb so the gasket will crush and seal.
Remember how much you tightened it.
If the head must go off you can reuse the MLS head gasket. Tighten it a  little bit harder next time.
I did so on my K6. First time close to 29Nm, next time 0.5 Nm more.
Gaskets are not  cheap.

You can get a little bit more compression with an RCS base gasket. The 0.01" version.
Stock base gasket is thicker.
RCS base gasket can be reused many times . Add a very very thin layer of high temp  RTV on both sides on the gasket next time(s). Let it cure 1/2  hour before assembly.

EDIT: Use a cheap compression tester.
My K2 with fresh stock pistons had 150-160 PSI.
392 +0.50mm got 175 +/- 5PSI.
Thick gaskets.

Thinner gaskets more fun, probably in or closer to the 180PSI area if chambers have stock volume.

My K6 with bigger bore and hotter cam has 220 PSI +/-5 PSI.
+200 PSI needed.

Photo below when testing the 392 pistons in the stock K2.

[HondaMan]

I think that 410 pistons (F2/3) will get valve contact in a K0-K6 engine with stock cam.
Probably with F1 K7/8  head too.

Exactly.

The dome heights (and shapes) for the -392- pistons have not been consistent, depending on the piston manufacturer, even with ART for Honda in the 1970s. I have some old Honda -392- pistons, some replacement -392- marked pistons, and some aftermarket (CI) -392- pistons, and all 3 are different heights and areas. The quench band in your particular head determines if they will fit without contact, the size (and velocity) of the quench, and whether they will fit. I have used the DOHC 750 pistons in the -392- heads, but they had to have the diameter of their domes cut back and then they had to be weight-matched (I still have a set of those somewhere) which resulted in 10.55:1 CR (IIRC) in the -392- head of the engine I built for someone here. The pressures resembled PeWe's numbers above when the engine was broken in. I had to widen the quench band edges in the head to fit them, too. It's a hotrod bike, though!

[Pbeattie89]

I think that 410 pistons (F2/3) will get valve contact in a K0-K6 engine with stock cam.
Probably with F1 K7/8  head too.

Exactly.

The dome heights (and shapes) for the -392- pistons have not been consistent, depending on the piston manufacturer, even with ART for Honda in the 1970s. I have some old Honda -392- pistons, some replacement -392- marked pistons, and some aftermarket (CI) -392- pistons, and all 3 are different heights and areas. The quench band in your particular head determines if they will fit without contact, the size (and velocity) of the quench, and whether they will fit. I have used the DOHC 750 pistons in the -392- heads, but they had to have the diameter of their domes cut back and then they had to be weight-matched (I still have a set of those somewhere) which resulted in 10.55:1 CR (IIRC) in the -392- head of the engine I built for someone here. The pressures resembled PeWe's numbers above when the engine was broken in. I had to widen the quench band edges in the head to fit them, too. It's a hotrod bike, though!

So what you are saying is that the dome size of the various piston for the 750 aren't well documented because they were quite variable from factory? I guess I will have to measure mine.

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[HondaMan]

I think that 410 pistons (F2/3) will get valve contact in a K0-K6 engine with stock cam.
Probably with F1 K7/8  head too.

Exactly.

The dome heights (and shapes) for the -392- pistons have not been consistent, depending on the piston manufacturer, even with ART for Honda in the 1970s. I have some old Honda -392- pistons, some replacement -392- marked pistons, and some aftermarket (CI) -392- pistons, and all 3 are different heights and areas. The quench band in your particular head determines if they will fit without contact, the size (and velocity) of the quench, and whether they will fit. I have used the DOHC 750 pistons in the -392- heads, but they had to have the diameter of their domes cut back and then they had to be weight-matched (I still have a set of those somewhere) which resulted in 10.55:1 CR (IIRC) in the -392- head of the engine I built for someone here. The pressures resembled PeWe's numbers above when the engine was broken in. I had to widen the quench band edges in the head to fit them, too. It's a hotrod bike, though!

So what you are saying is that the dome size of the various piston for the 750 aren't well documented because they were quite variable from factory? I guess I will have to measure mine.

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What I have seen is:
the ones I call the "early" type, which have a fairly coarsely-cast appearance on the tops with a divot indicating the intake side, and the whole finish is pebbly-looking. The next type have nicer finish on the domes themselves while the rest of the piston looks very machine-finished: these domes are slightly smaller in diameter, but also slightly taller. A 3rd type I've seen appeared to be modified versions of the -410- pistons, which have an even, smooth dome across the whole top, but they were 'stepped' around the outer edges about 5mm in, leaving a slight step-up edge at a steep angle. The center of these domes was taller than the edges: on the others the top of the domes was consistent across them.

This last version led me to make the real high-compression types from some of the CB750 DOHC STD size pistons (+1mm for the SOHC4 bore) back in (about) 2008 when, for a while, we were having a hard time getting -392- pistons at all. CI didn't make them yet and Honda had quit them.

[scottly]

I measured the dome volume of a stock K7 392 piston at about 3cc, by forming modeling clay over the piston top, then filling the cavity with a syringe. However, the outer edge of the piston sits .030" below the deck at TDC, so this volume would be subtracted from the dome; my scribbled notes from 11 years ago show a calculated .7cc? Some of the 392 pistons, like Jerry's F0, had a full dome instead of the partial dome like mine.
What head are you using? The early heads had 22cc chambers, the 392 head had 24cc, and the 410 had 28cc.

[Jerry Rxman Griffin aka MuthaF'er]

Ky K8 engine had the same piston dome as Scottly. My early F0, as Scottly says, had a "full" dome. The interesting part is BOTH show 392 near the pin holes. The F0 ran much stronger than other 750s. I also measured the stock cam exhaust lobes at 0.1mm taller too.

[PeWe]

Cruzinimage "392" pistons.

https://www.cruzinimage.net/2017/08/17/77-78-honda-cb750k-k7-k8-0-5mm-over-size-pistons-set/

My K2 got this

[HondaMan]

Here's my last set of the ones I made from the 750DOHC pistons. They correspond to +1.00mm overbore in the SOHC750 engine. The domes require at least the -392- head (>10:1 CR), but the -410- head will also work at slightly less compression, around 9.4:1 or so. You can still see the flycuts for the 4-valve head design.