The engine rebuild of the CB750F2 continues with the receipt of a nice used OEM crankshaft and set of stock F2 rods, courtesy of eBay sellers. After lots of cleaning and removing of the gooey remains of the Gasket Cinch sealer used to reassemble the engine cases back in the 1980s, the cases have been reassembled and pistons installed. Next up will be to install the cylinders over four very large RC pistons using rather thin liners with little in the way of a bottom taper to help ease the pistons/rings into place.
I received a set of custom cut base gaskets from Cometic Gaskets recently and did a test fitting of the cylinders over one piston which had rings removed. The base gasket was left out during the test to help determine what the squish band thickness was going to be, then adjust the cylinder height accordingly. The initial check seemed to show that the pistons were dead even with the cylinder head gasket surface even with no base gasket installed. The base gaskets are .020” thick and I was open to using a pair of them to help reduce compression and gain some squish width. The huge pistons have only slightly domed crowns with a couple of deep valve clearance cutaways, so other than the fact that they are 12mm oversize, it appears that the engine can be put together with a single new base gasket and the used copper head gasket. I didn’t ask Cometic about making a head gasket for 73mm bores, but it seems like the two fire rings would be very close together if they were to make up a special gasket like that. Hopefully, the copper gasket won’t leak and the .040” thickness will help to keep adjacent cylinders from firing through the narrow bridge between them. The head gasket thickness will also provide a 1mm squish clearance, needed for proper cylinder mixture burning. If the compression readings drop down closer to 200psi (from 225psi originally), hopefully the electric starter will be up to the task of spinning this big mill over reliably.
In a consultation with my friend Mark McGrew, famed CR750 bike/engine builder, he assured me that the F2 camshaft with its big lumpy lobes will, indeed, function when installed into an earlier K-series cylinder head. Despite the factory cam timing figures, which show the intake opening at TDC, instead of the usual 5 to 10 degrees BTDC, the combo of the head and valves in conjunction with the F2 camshaft, (which was designed to open bigger valves on a different cylinder head with revised valve angles), the piston/cam combo should yield some healthy power readings, at least below 8k rpms. I will be turning it over VERY CAREFULLY, once the cylinder head is bolted up to the cylinders and the camshaft is installed. The cam seems to work fine on the bench, using stock springs, but you can’t deny the difference in the camshaft lobe profiles is substantial. It will be most interesting to see how it all comes together… or not!
Follow-up notes from Mr. McGrew mentioned using Copper Coat gasket sealer in a VERY THIN application on the solid core gasket, so I tracked down a small can at the local auto parts store. Unfortunately, a minor hernia operation is going to get in the way for awhile. I am restricted to lifting less than 20 lbs for 6 weeks! Will lay low for a week or so and just poke at the pieces until I am feeling better. I think the head must weigh close to that with the valves/springs all installed. The rest goes back together like a puzzle once the head is bolted down on the cylinder block.