installing a new head on 72 combat

I found a new old stock head for my 72 combat.It is an rh6 I believe.The valves and their gear installed well,I had the block bored to 60 over,bought pistons,installed pistons,put head on and tried to kick it through without the pushrods installed and the pistons seem to hit the head.If I put on2 copper head gaskets it is o.k. I checked piston height seems o.k. compared to the old pistons. When the head is off, the pistons at TDC are about a 16th of an inch above the top of the block. Does anyone have an idea of what is wrong?

What are you doing for a base gasket ?

Don't suppose you noticed if the old pistons came up above the cylinder before taking it down ?

Are you sure you have the right head? Should have a relief in the head, a "squish" band, about 1/8" deep, then the hemishpere. You might have a 650 head that someone in the "wanted" section is looking for, no squish band...

Old Brits sells a spacer plate that goes below the cylinders, on the crankcase mouth. Better than two head gaskets.

Define hitting the head. You have the spark plugs out I PRESUME for such a test (otherwise the head would tend to lift from the compression factor ,confusing you.). My chart states P/N 064097 for RH 6 ,750 Model , C.R. 9.6:1, 32 mm. intake ports,application 73 Stock Commando.

There is a possiblity that the cylinders were previously skimmed across the deck in order to raise compression (this has been done many times, never mind the naysayers).

If the pistons then hit the head, the naysayers may have a point !!!

P.S. Have you tried refitting the old head, to see if it makes any difference.
That squish band, as mentioned.....

Standard 750 head should fit a real Combat just fine w/o base gasket and just a thin flamering head gasket. I would first suspect cam not installed at 0 degrees and the valves are hitting piston not piston hitting head. You can look in plug hole to double check that. Combat pistons stick up proud of barrel surface .050".

Steve I believe he said there are no pushrods installed for this test so we can rethink valve interference/action.

G'day Paul,
Do you still have the old pistons, if so, measure the distance from the top of the gudgeon pin hole to
the top of the piston and check that measurement against the new ones.
Are the new ones flat top pistons.
Compare that squish band to see if the heads are identical.
Some one here will be able to tell you how much space is required between the pistons and the head for various compressions.
So I guess you could check by placing some putty or plasticine on top of the pistons, (20 cent piece size), then some shims (washers)
in between the Head and the Barrells and turn the engine over once. Then you can calculate that space.
I don't know if you can use that head or not, but the boys here will know, but I do know this...
My Combat motor is running .040 over, which increases the compression ratio even more, so to be safe I removed
.020" from the top of the pistons, which was about one third the thickness of the top of the pistons.
So if it comes down to having to use the new head, then there's that option if needed.
AC.

Some 750 heads are not bored large enough to clear .060 OS pistons. The pistons should be able to go up into the counterbore in the head with extra clearance. The pistons will normally come out of the top of the cylinders around .080 inch. Also check the head gasket. Many head gaskets do not have a large enough hole to allow a .060 overbore. I normally turn around .020 from the piston above the top ring to avoid this problem when I am using .060OS 750 pistons. Just go down about .100 from the top of the piston with the cut. Jim

Rohan said:

If the pistons then hit the head, the naysayers may have a point .....

Click to expand...

Certainly not the naysayers who claim that cylinders don't get skimmed. THEY DO. That is my point. And skimming them produces bad results if it's not accounted for.

hobot said:

Standard 750 head should fit a real Combat just fine w/o base gasket and just a thin flamering head gasket. I would first suspect cam not installed at 0 degrees and the valves are hitting piston not piston hitting head. You can look in plug hole to double check that. Combat pistons stick up proud of barrel surface .050".

Click to expand...

He said he was having contact without pushrods installed. Can't be the valves in that case.

Somebody would have had to do some serious metal removal before the piston would bottom out on the squish band . The couterbore in a rh6 head is .100 in. deep plus a .040 gasket which means the piston would have to come out of the top of the cylinders by .140 inch before it would stop on the squish area. More than likely either the counterbore is not large enough for .060 pistons or the counterbore is not quite lined up with the cylinder bore. Jim

You could also check that it isnt the gasket that the piston is hitting, when I put my head on I had to 'adjust ' a couple of holes in the gasket to get it concentric with the bores