Bongo Fury

Lovely pics there Simon. Phew - makes you realise what a fancy piece of casting it all is

So it defintely is a flat faced head - previous comments made about skimming not affecting the compression ratio still stand

dandywarhol wrote:So it defintely is a flat faced head - previous comments made about skimming not affecting the compression ratio still stand

Yup, I noticed that and thought of you. Still trying to get my head around it though as if it is flat, how do you grind/skim/machine anything off it without making valve seats more shallow, plus with the valves ending up 'lowered' into the bores as a result (and that in turn decreases internal volume of combustion chambers thus raising compression ratio). Once I can get my head around all that then I'll understand why g/s/m'ing a flat head makes no diffrence to compression ratio

Jeepers, are these the pistons flush with the top in the second picture?

Are the 'combustion chambers' just these little recesses in the pistons?!!

I know that diesels have high compression ratios, but I never realised it was like this...

Trouble at t'Mill wrote:Jeepers, are these the pistons flush with the top in the second picture?

Are the 'combustion chambers' just these little recesses in the pistons?!!

I know that diesels have high compression ratios, but I never realised it was like this...

Because it's an "indirect injection" diesel there is also the volume of the pre-combustion chamber in the head (where the injector nozzle and glow plug are) it's behind the circular plate with an oval opening in it.
The oval opening is where the air goes in and the bang comes out.

mikeonb4c wrote:

dandywarhol wrote:So it defintely is a flat faced head - previous comments made about skimming not affecting the compression ratio still stand

Yup, I noticed that and thought of you. Still trying to get my head around it though as if it is flat, how do you grind/skim/machine anything off it without making valve seats more shallow, plus with the valves ending up 'lowered' into the bores as a result (and that in turn decreases internal volume of combustion chambers thus raising compression ratio). Once I can get my head around all that then I'll understand why g/s/m'ing a flat head makes no diffrence to compression ratio

As scanner says, the precombustion chamber takes some of the compression volume. These cutouts on the piston crowns are more valve recesses - there's probably no more than about 0.5mm clearance to allow for conrod "stretch" at high revs and some overlap at TDC. The Bongo engine will be running around 20/22 :1 compression ratio I guess - around 430 PSI of pressure.

A lot of manufacturers say NOT to skim flat faced heads for that reason but a light skim of say 0.10/0.20 mm usually is ok - you can even take if off the face of the valve too. The problems start when you make the pre combustion chamber plates too thin.

If 0.20 won't take off the damage/warp then a new head is usually needed IMO (although different engines have different tolerances). Another problem which can arise (and the Bongo head is one of that ilk) is the camshaft runs directly in the head with matching machined caps - skimming can cause these camshaft bearings to distort when the head is torqued down.

I wonder if thats what happened to Hoggggggggmans