Combustion chamber volumes

[pae]

In response to Pinheads request for some engine details...

A couple of quick measurements over the weekend. Will get more accurate details and set up a page on my website with BIG pictures.

Combustion chambers are ~ 60mm diameter by 17mm deep, and pretty well spherical in shape. At a rough calculation that makes them about 20cc each, but I'll do that accurately later with a pippette.

The old head gasket measures 1.25mm.

I'll check the deck height for your (top of barrel to top of piston at TDC) so you know how much you might have to play with. The pistons are of course crowned so that'll reduce the volume, I'll check on that too.

An original brochure lists compression ratio (UK) at 9.0:1. So with bore/stroke of 59.8 x 55.8mm that gives 156.7cc per pot (627cc overall). If compression is 9:1 then the total combustion volume should be 17.4cc. With a head gasket of 1.25mm thick that's 3.5cc, leaving 13.9cc in the deck and combustion chamber combined.

If you want to get to 10:1 then you'll need to take about 1.73cc out, to reduce it to 15.7cc, or about half a head gasket thickness (~0.6mm).

I think that's correct. As I say, I'll do a proper check later this week.

regards, Phil

[Pinhead]

Great!! This is jut the info I was looking for!!! The only other thing I need is valve clearance... I'm guessing there is more than a half-mm clearance, so do you think there is enough clearance to shave the head that much (or get a thinner head gasket)?

[pae]

Leave it with me. May be the weekend before I can let you know.

regards, Phil

[cb650]

I took off about .o25" (inch) on a couple of mine with no probs. Some day going to make the kid play with one and do some clay to find out the valve clerance.



Terry

[Pinhead]

Cool! I haven't had my engine torn down, but by looking at the pictures it looks like the intake valve seat will be the biggest limiting factor on shaving the head. The next step would be planing the block (cylinders). That'd probably be a better bet, too, since you'd be able to get different heads if something bad were to happen and keep the same CR without having to shave each one. That would be limited by the compression height, though, since you don't want a ring to come out of the hole at 8000 rpm.

[Pinhead]

What if you were to split the difference between the head gasket and the bottom cylinder gasket? 0.3mm on both?

[pae]

Hmmmm, did a 'fairly accurate' measure of the combustion chamber volumes at the weekend and they are about 26-27cc which is a bit more than I'd expected. Having looked at the pistons though they are quite crowned so I have to assume for the moment there's about 12cc of crown on them to bring the squish volume back down around 14cc (plus gasket at 3.5cc).

Didn't get much chance to do anything else so further details will have to wait, I'm afraid.

regards, Phil

[Pinhead]

Thanks for taking the time to make these measurements. I know they'll come in handy. I wonder how thin a bottom cylinder gasket we could run...

[pae]

Hi Pinhead, removed the valves today and took a few more photos for you. Shout if you need anything else...

regards, Phil

[Pinhead]

Thanks! Those are the best pictures I've seen to date. Those will really come in handy once I get into the final planning stages of my mods. Thanks again!

[pae]

Glad you like. I've been cleaning it further and de-coking the exhaust ports today so will get some more soon. I'll use my daughters camera too as it gets much better focus on the close shots than I can.

Just seen this. http://www.sohc4.us/forums/index.php?topic=23027.msg237028#msg237028 It's a 750 I think, but the guy doing the work (MRieck) can work wonders and might give you some pointers.

regards, Phil

[Volker_P]

Seems you spend much time for internal cleaning also. I would recommend to check also the surfaces for the small O-Rings of the oil lines on both sides of the head. I remember I recogniced some machining traces at the cylinder side but I finally forgot about polishing them away last time. I guess this may help to extend the interval for the head oil leak. I'll try to remember next time.

[pae]

More (bigger) pics of the ports and chambers...













enjoy, let me know if you need more

[cb650]

I wonder what a extrude hone would cost?




Terry

[Pinhead]

Those intake ports look pretty good, but the exhaust ports look like they could use some work. Definitely much better than the 750's ports!!