valve clearance puzzle


Post Reply
divan

valve clearance puzzle

Post by divan »

I just went through checking and adjusting valve clearances on my 79. I have always used the method for positioning the cams that is in the Honda manual. This year I decided to try the method described by Mike Nixon ("Valve and Cam Chain Adjustment the Right Way," Tech Chronicles, Engine section), which is essentially the same as the Honda manual except for positioning the cams. The manual, as Mike points out, has a "quick and dirty" method, that allows you to do the whole procedure in 4 steps: with #2 exhaust valves fully open, you can check 3 sets of valves, etc. Mike's article has you set each piston at TDC on the compression stroke, so that the lobe flanks of the cams for that piston are perpendicular to the lifters and the lobe tips are pointing "out" on each cam -- same position you use to insert the valve depressing/lifter holding tools to change shims. This of course requires six steps -- you do the TDC compression setup and check all 4 valves for each cylinder in sequence. But Mike says this is the more accurate test point. Being the worrier that I am I decided to compare the results from each method. And that led to the puzzle. I can't go through all of the details here, but using the TDC method vs. the Honda method, you end up measuring clearances at DIFFERENT points on the cams for all but 4 valve sets (#1 exh, #3 int, #5 exh, #6 int, which are the same as the TDC method). Basically, with some variation, the Honda manual method measures clearances on all other valves sets around 180 degrees different (3#exhs and #5 intakes are measures at 90 degrees difference). It wasn't surprizing to find that the clearance measurements were different -- cam lobe wear, bearing wear, etc. varying at different points by a few hundreths of a mm (e.g., .09 vs .11 mm). But here's the puzzle: one valve --one of my #4 intakes -- measured .11mm clearance on the TDC setup and .07mm on the Honda manual setup. If I adjusted the shim to get close to the desired .08mm, and I took the TDC setup to be the "right" reading, I would have to close the gap .03mm (with a larger shim). But then the reading at the Honda manual seup position would be .04mm, .02mm narrower than the minimum tolerance of .06mm (and WAY too tight -- burned valve time). The obvious solution would be to leave it alone, since both .11mm and .07mm are within the tolerances -- and that is what I did. But I worry about such things -- the old saw, "a little knowledge is a dangerous thing," is true. Which check point is in fact "correct" and why? I'd guess that all the clearances should be measured with the cams oriented so that the cam flanks are perpendicular to the lifters, with the cam rotating towards (just before) the opening cycle begins (the way they are for all of the exhaust cams using the TDC method). With my #4 intake, for example, the Honda method has the cam oriented close to this position (with flank perp., cam appraoching opening) while the TDC setup orients the cam just the opposite, with the flank perp. but cam rotating just after closing the valve. Does anyone know the answer to the puzzle? I'd imagine that people using only one of these techniques might end up (as with my #4 example) with some potentially serious valve clearance problems. But maybe I worry too much.
Ride safely -- DS
[/quote]

User avatar
Don
Amazing Poster
Amazing Poster
Posts: 806
Joined: Wed Apr 02, 2003 3:13 pm
Location: Biloxi, Mississippi, USA
Location: Biloxi, Mississippi, USA

Post by Don »

Not the answer you're looking for I know, but doing the checks BOTH ways, and then accepting the looser of the two seems like the "correct" method to me.

If I was doing it on my bike, I'd use the "Honda method" called for in my service manual - Especially now that you've pointed out some differences in the two procedures. Seems as all you'd be saving using "Mike's method" is a little time, and what's a little time savings, especially if you're then worried about the resluts :roll:

Don

cbxchris

Post by cbxchris »

Well I couldn`t stand it I`ll wade in. From doing many engines of all kinds and especially after market camshafts I DO NOT like the honda method. Why would you want to measure cam lobs at different places? I would guess the one lob you have a problem with is ground a little off. Its no big deal but honda was not perfect with thier grinding....you ought see some aftermarket grinding. Guess thats why there`s basically only 2 aftermarket cam manufactors left. I wouldn`t lose any sleep over this.

broook
Power Poster
Power Poster
Posts: 514
Joined: Tue Mar 11, 2003 9:14 pm
Location: Newfoundland
Contact:

cams

Post by broook »

I always go with the heel of the cam and the toe perpendicular to the shim. Might take a little longer but at least you know everything is right. :D

User avatar
sr71cbx
Power Poster
Power Poster
Posts: 374
Joined: Sat Feb 01, 2003 1:00 am
Location: Lake Havasu City, AZ
Location: Lake Havasu City, AZ

valve clearances

Post by sr71cbx »

Hi all,
broook's method is good,I use the same on mine,also,I set my valves at .004 in and .005 out,this gives better static compression and allows for plenty of operating room without being horribly noisy. 8)

andy

valve check

Post by andy »

To save a lot of handcranking, you can rotate motor either direction to find desired lobe position, just be VERY CAREFUL intake and exhaust valves don't bang into each other if valve depressing tool in place.

George

valve clearance puzzle

Post by George »

How important is it to have all values/shims set to a .08 clearance? Checked mine recently and all but two values were out of spec, between .06-.13. In order to get all the value clearences set to .08 I'd need to replace about 14 shims, swapping a few helps. Assuming I can find exact size I'm looking for that is. If I don't stirve for .08 on all the values I can get away with replacing only 5 or 6. Keeping the rest at about .07-.11. Is that good enough?

Thanks

George

valve clearance puzzle

Post by George »

Corection: All but two were in spec.

George

valve clearance puzzle

Post by George »

Corection: All but two were in spec.

Post Reply

Return to “ENGINES: Maintenance, Performance, Parts Sources”