Topic: Dynoman 836 Turbo Pistons for a 78 CB750A

[LiveeviL81]

Hello,

I recently purchased Dynoman's 836 8.5:1CR Turbo Pistons and they just showed up today. The pistons look great, and no complaints on the manufacturing, but there are a few differences on these pistons from the stock ones that bring up some questions.

First, my stock 78' CB750A pistons have the valve relief cuts in line with front and back of the cyl and with each other, while on dynoman's they are cut more deep and slightly offset. This seems to match the valve location on the head, so I am not super worried here. The top is also FLAT. I know on the stock 'matic pistons they call them flat tops, but there is a slight curve, these are super flat. Assuming this has something to do with machining and the next few points to maintain 8.5:1 CR.

Another thing I noticed, is that the skirt's are shorter than the stock piston by 7mm, the wrist pin to top of piston is also a smaller distance on the 836 by about 1 mm.

I also noticed the oil ports are in slightly different locations, along with the ring locations.

None of these stand out as super important except maybe the skirt length. Thoughts?

Thanks,
Kaleb

[PeWe]

Check their tech page where the pistons are shown. 2 types.
How the ring gaps should be orientated, end gaps, bore clearance.
http://dynoman.net/engine/pistons/piston-install/65mmSOHC.pdf

You will see they match how the chambers look like with valve placements.

These are newer pistons, stock is old tech, very old tall pistons.

Here a photo of my engine that show the orientation.
Widest pocket for inlet valve.
 

Dent in dome for sparkplug.

[LiveeviL81]

PeWe,
I looked at the pages you were talking about, and feel a lot better. I think the biggest difference that scared me was the valve clearance pockets are in different locations, but after looking at my head. I am not worried anymore. Though I am surprised they are able to get away with shorter skirt lengths.

Kaleb

[PeWe]

Here 3 pistons connected together via wrist pins.
Old RC 836 cast       65mm
Dynoman JE 890cc  67mm
Dynoman JE 1005cc  71.25mm


All of them have the same valve pocket design. Pocket where valve opens. I verified valve matched pocket with clay. I have oversized in valves.

The older  RC needed some massage with a Dremel to get 1mm clearance around valve disc vs pocket circular shape.

[MRieck]

 The old piston is an ancient cast piece.....technology has improved over the past 50 years. All this improvements are a good thing. The only thing I don't like is the fact those new turbo pistons use an XA ring pack......I really believe an XC would be more appropriate.

[PeWe]

XA as the 71.25mm piston above.
Oil rails not fun.
Good idea to file the inner end corners less sharp, more rounded.
Mine hooked into the expander ring so the thin ring could not compress all the way, one flimsy oil ring pinched during cylinder assembly.
I had to order a new set of rings from Dynoman. Shipping not cheap for obe tiny box of rings so I ordered a full set foe all 4.

Good for next time use when pistons now are gas ported. Increase chances to reach 110whp

A turbo engine might not need that


The 890 piston has the XC ring set if I remember correctly. Much easier to install. I filed the inner corners anyway.

My older JE 970cc pistons have XC rings too. Bought NOS 2021, had been part of a build never finalized. Probably over 15-20  years old.

I wonder if XA rings handle oil better despite the thin oil rings are softer and must have less forces against the bore?

970cc XC rings  vs 1005cc XA