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Compression and oil pressure switch CB750f1

mortyboy

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Hi Everyone

I'm still in the reassembly stage (I'll be posting on the project and my discoveries once complete) and have two questions/concerns:

1. COMPRESION TOO HIGH? I've just completed a compression test. The engine has new pistons, rings, valves, valve guides, bearings and the head was shaved by 0.06mm by an engineering co. I've reassembled using standard block and head gaskets. I've oiled up the engine and oil pressure is good. On doing the compression test my readings are:
No1 220psi
No2 245psi
No3 230psi
No4 215psi
This looks way too high (an uneven). Would a thicker head gasket bring these readings down to a reasonable level and if so, where do I source one?

2. OIL PRESSURE SWITCH: I removed the original on teardown with no problem. I ordered a replacement as a mater of course from a reputable UK supplier. However, on attempting to fit it, I had problems getting it threaded and have now stripped the hole. On comparing the original, the thread pitch is tapered 0.9, whereas pitch on the replacement is tapered 1.0. Not a huge difference but obviously enough to strip out the aluminium hole. I now have a hole too big for either the new or old. Any suggestions for a work-around? I really don't want to disassemble the engine again to drill out and fit a coil.
 
First is it a good quality compression gauge? Has the engine started yet since the rebuild? I have never checked compression on a never run engine before, it normally takes a bit of run time for reading to normallize and stabilize as the rings seat in. If the compression gauge is accurate then these are on the high side...215psi is fairly close, the K model engine spec is 187psi. .06mm or .002" is basically nothing and will not cause a major increase in compression, I have milled F2 heads .060" to bump compression. Stock bore size? Stock honda pistons? What type/material and brand of gaskets? Base gasket thickness? Head gasket thickness? You can use thicker base gasket or thicker head gasket.

If you try to take a short cut now to fix the threads you risk getting metal in the main oil gallery of the engine, which also feeds direct to the heads with very small easy to plug oil orfices going to the camshaft. It would be very difficult to 100%make sure you dont get any metal in the gallery trying to fix the treads.....there probably already is metal in there now if its stripped that bad. Its a tapered pipe thread so dont know if they even make a heli coil for that??
 
Thankyou. Compression gauge is new. I haven't started the engine yet, just cranking it off the starter. Bore size is standard. Three of the original sleeves were in good condition (after honing) but one sleeve was replaced with an n.o.s. one I found. The pistons are new standard size but not Honda. Base and head gasket size I'm not sure of, although they came out of a generic rebuild kit from David Silver Spares, so I assume standard size. I'm tempted to remove the head and replace head gasket with a thicker one but can's seem to find one. I assume psi will drop and stabilise somewhat after a period of running in but if I leave them as they are, could they damage the engine?

The Oil pressure switch hole is a worry. I've managed to refit the original after wrapping the thread in a very thin strip of aluminium. It's holding and not leaking now after cranking the engine again. My thoughts are either to leave it a see how it holds up or to suction out the gallery from the side, insert something to block the bottom of the pressure switch hole to catch swarf, ream the hole with the correct size for a 1.0 coil (I found a tapered one on line but the cost is over £400!) to accommodate the new switch thread (at least the part before the taper), suction it all out again, change the oil and filter.

This is my first engine build & restoration. Quite a challenge but very enjoyable. Really appreciate your help.
 
How does the top of the piston compare between the originals and the new ones? If dome shape is different it could have affected compression. If you have good premium fuel the compression wont hurt anything. Dynoman.net used to have different thickness head and base gaskets but havent checked in a while for stock bores, everything I do is large bore stuff.
 
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