Q: More OHC6 Rebuild Questions
OK, I think I have an idea of what to check, but still have some questions:
1.) The valve height should be from the top of the head to the tip? I would think that this is done without the valve springs. Is that the best way to measure?
2.) What is the oil restrictor? Where is it located? How can you tell if it is bad? I lost two camshafts (worn lobes) due to low oil pressure, so I want to make sure that my 230 HP Sprint cam will survive.
3.) Where is the 30 vs. 45 mentioned earlier? Valve angle doesn’t sound right, since they are perpendicular to the head. I have one ’66 head, two ’67’s, and one ’68; are they all the same in terms of performance and oil flow? Numbers don’t matter much, since very few people even know what the numbers are to be.
4.) What type of work do the cam followers require?
5.) What type of prep work should be done to the lifters? I know they can be disassembled and cleaned, and was planning on doing that, but is there an easy way to check the leak-down rate?
6.) Are rings and pistons still available? I know that one of the three engines that I have needs new rings. I was going to use the best parts of the three to make this new engine. What is a safe overbore? Does this cause any overheating problems like the V8’s?
7.) I am converting from an automatic to a four-speed. I have found the correct transmission (3.11:1 first) and all of the clutch parts. Someone asked about the Z-bar being a different length, that is correct, to compensate for the different width of the engine. Will the driveshaft have to be shortened? The manual for 1967 shows that they have the same “stripe”, so I would think they are the same. How about the yoke. Is there anything I have to worry about here? The transmission I have doesn’t have the yoke, so any ideas on where to find one?
A: Okay, I’m now back to the top of your letter after answering all your questions, and one theme runs through all those answers: INFORMATION!! Our experiences are always mixed with good documentation from Service Manuals and other forms of the printed word and number! We would not likely have learned so much without finding way more info than we could just looking at parts, so EQUIP yourselvles with a small personal LIBRARY regarding your car and automotive knowledge in general!
Albert Einstein said something like “Why bother remembering that which you can alway look up?”
1) Yes, from the top machined surface that the cam cover gasket sits on to the tip of the valve, but WITH the valve springs installed at the completion of the valve job. This info MUST go to the machinist who does this work!! If he is not willing to listen to your input regarding this stuff due to pride (“I’m not having some stupid customer tell me how to do MY job!”), FIND ANOTHER MACHINIST!
2) This is where the 1967 Service Manual becomes valuable! It describes, in words and pictures, how this system works. There’s a passage through the head with the restrictor, it’s between cylinders 2 and 3, and it is a piece of tubing about 2 inches long with an hourglass-shaped crimp in the middle of it that is about 1/8″ internal diameter. It then has an .080″ hole drilled in the side of the crimp, and the sizing of both the drilled hole and the hourglass crimp are critical to the flow volume and the pressure of the oil being fed to both the camshaft AND to the lash adjusters (lifters). Sometimes when poking around the holes and passages of a head being cleaned, these are damaged. They are also a bit fragile to try to remove from a head that hasn’t been hot-tanked enough to let it come loose.
3) Numbers MATTER ABSOLUTELY! The cylinder head numbers (to I.D. heads) are listed in Pete McCarthy’s first book, written for drag racers around 1981, including the NHRA minumum cc’s for racing, gives a good idea of which heads are which for mix-n-match engine building! I’m sure they’re in the parts books as well, send me your numbers, I’ll help you choose a head. The 30 degrees and 45 degrees refers to the valve face and the valve seat; when the machinist actually grinds on your head and valves, he’s cutting a surface that is angled. The only heads that are 45 degrees are the second design for 1969. All previous heads used the 30 degree valves. If there’s any doubt, give me your head casting dates as well. PS Ever heard of a 3-angle or 5-angle valve job? That helps shape the valve seat area for a smoother flow from the port through the valve. You cannot see the angle with head assembled, the valve need to be removed!
4) The current replacement followers have a rough and uneven surface that rides against the camshaft, and need to be machined smooth. I did mine on a veryfine knife sharpening stone, one-by-one, by hand. That takes a lot of cutting oil and from 1/2 hour to 45 minutes per follower. Jerry Woodlad describes this in more detail in his literature.
5) I use new lifters, they are a slightly different design, meaning they don’t have that little pin in the top hole, and you can’t mix-n-match with these.
6) Pistons for a 230 I believe are still available, but for 250’s have been difficult to find for some time. Safe overbore depends on the casting quality of your block, this applies to ANY engine! Usually try to stay at .030″, if you can find them, you might be able to do .040″, and I have done .060″. The rings are standard sixes to the industry, even the 6-cylinder Chevy used the same ring size, and even the same rod and main bearings! In my 250 inch engines I have also used modified pistons from a 307 Chevy V8, and there’s a piston in the 4.0Liter modern Jeep engine that may be usable. Its standard size is .005″ larger than our stock piston, 3.88″ instead of 3.875″. I haven’t got one in my hands for comparison yet.
7) Your driveshaft and crossmember should be the same. The only different crossmember was for the THM400, all others used one member, that covers all 3 years, 67-69.
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Content last modified: January 16, 2014 at 3:10 pm