Author Topic: No clearance between intake valve and piston  (Read 11189 times)

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Offline RSchaefer

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Re: No clearance between intake valve and piston
« Reply #25 on: January 28, 2013, 12:44:21 PM »
Wish I could answer your question, unfortunately I am still building.  That said I finally ordered the tires, which to me means I NEED to git'er done, don't want the tires sitting around getting old.  Sorta a personal deadline I am putting myself under.

So I did most of Marks "low-cost" suggestions in his book.  I took out all of the obstructions in the intake, chamber and exhaust.  Created a bowl at the intake under the valve.  Purchased Don's low back-pressure (glass pack) muffler.  Removed .010 from the head, did a valve job and honed the cylinders - and advanced the timing 5 degrees.

We will see how it goes!!
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Offline Kickstart

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Re: No clearance between intake valve and piston
« Reply #26 on: January 30, 2013, 06:02:01 PM »
Wish I could answer your question, unfortunately I am still building.  That said I finally ordered the tires, which to me means I NEED to git'er done, don't want the tires sitting around getting old.  Sorta a personal deadline I am putting myself under.

So I did most of Marks "low-cost" suggestions in his book.  I took out all of the obstructions in the intake, chamber and exhaust.  Created a bowl at the intake under the valve.  Purchased Don's low back-pressure (glass pack) muffler.  Removed .010 from the head, did a valve job and honed the cylinders - and advanced the timing 5 degrees.

We will see how it goes!!

Good luck.... do you have a build thread?  I would like to follow it.

I should have done more head work.  I was originally trying the keep the costs down on the rebuild and since I heard that the F0/F1 heads were usually better machined/finished than the later K's (don't really have any obstructions), I decided to skip this step.  I did have new valve guides installed (and therefor the seats re-cut), and I did do some de-shrouding work around the valves... but I didn't create that bowl/pocket above the intake valves.

I'm also keeping the stock 4-1 exhaust (because I like the way it looks), which probably is significant restriction.
- Chris
75' CB750F Orange flake (rider)
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Offline Kickstart

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Re: No clearance between intake valve and piston
« Reply #27 on: January 30, 2013, 06:06:13 PM »
OK... so I just saw this post:

http://forums.sohc4.net/index.php?topic=117404.msg1325914#msg1325914

And I'm thinking that may explain why my stock set-up has the intakes opening 4 degrees After TDC.  I did have 0.01" milled off both the head and deck (0.02" in total).  Hmmm, I wonder how much later this actually makes the cam timing.

.... off to the garage to measure the diameter of my cam sprocket.
- Chris
75' CB750F Orange flake (rider)
75' CB750F Blue (Project)
75' CB750F Painted black (Project)
No Reserve Racing #171 AHRMA

Offline Kickstart

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Re: No clearance between intake valve and piston
« Reply #28 on: January 30, 2013, 06:49:48 PM »
OK... so I just saw this post:

http://forums.sohc4.net/index.php?topic=117404.msg1325914#msg1325914

And I'm thinking that may explain why my stock set-up has the intakes opening 4 degrees After TDC.  I did have 0.01" milled off both the head and deck (0.02" in total).  Hmmm, I wonder how much later this actually makes the cam timing.

.... off to the garage to measure the diameter of my cam sprocket.

Replying to myself here :)

So, the circumference of the cam sprocket is around 110mm 264mm (circumference where the cam chain rollers would sit).
Based on my back of envelope calculations:

.508mm removed from head and deck / 264mm circumference X 360 degrees X 2 crank rotations (because I measured the circumference of the cam sprocket instead of the crank sprocket) =  1.39 degrees later cam timing. 

I guess if you assume the front cam slipper is also worn, 4 degrees late from spec starts to make sense.

Thoughts on my logic here?
« Last Edit: January 30, 2013, 06:54:56 PM by Kickstart »
- Chris
75' CB750F Orange flake (rider)
75' CB750F Blue (Project)
75' CB750F Painted black (Project)
No Reserve Racing #171 AHRMA

Offline RSchaefer

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Re: No clearance between intake valve and piston
« Reply #29 on: January 31, 2013, 10:58:17 AM »
Well, here is my thread http://forums.sohc4.net/index.php?topic=82270.0

The following is NOT going to answer your question - maybe just give you food for thought.

I started it a couple of years ago and then realized I was not going to be able to post in any kind of timely fashion like others.  I did take off on the build, and did consult with the forum frequently, bought Hondaman's (Mark Paris) book, etc., etc., etc.

I thought I would re-start the thread and then gradually add in the process steps to see what people thought, answer questions, etc., but I did not really get to many responses and most of the guys on the form who are running "F's" I have already consulted with in one way or another anyway, like Jerry in Colorado, Don in New Jersey, Mark in Colorado, etc.

I got cash again for Christmas like I did for the last two Christmas's for my project, and think I am closing in on the home stretch.  I joined the VJMC and started mixing with other VJMC members, went to the west coast rally, etc., and am kinda hoping I can get it together for the Classic Motorcycle Show at Barber this year in, I think, 1st of Oct.  1st 100 registered classic display bikes for the VJMC get into the Barber event free.

I wanted to stay with the stock muffler because I liked the look as the muffler is LONG, and just looks right on the F.  My original muffler "looked" good from above but had several rusted out holes underneath.  I ended up buying a stock replacement from eBay that looks really good.  All that said, I also purchased Don's glass-pack muffler as it was reasonably priced, this was before I found the one on eBay.

So after all that I think I'm going to start out with Don's muffler and see how she goes.  There is talk about one of the problems with the "F" is all of that additional weight on the right side i.e. 4into1 exhaust and the huge muffler.  I was never going for EXACTLY period correct (100 point restoration), I was going for obviously the CB750 look but wanted to make it ridable, maintenance free (more free) and if possible a little faster then stock.

So enter the Hondaman low cost improvements, timing advance, etc.  And if I'm going to minimize the asthma, why burden the exhaust with so much back-pressure (stock exhaust).  OK so that the current thinking, but its not running so I still have the stock muffler to fall back on.  I'm hoping per Don that his muffler sounds good at basic idle, normal cruise, but starts to pick-up the pitch when you crank her on, we will see.

So sorry for all of the dialogue and getting back to your issue with the timing.  This WAS the most complicated process/change I undertook.  It took me awhile to understand there are/were manufacturing variations "back in the day" and how that impacted this issue of advancing the timing in particular.  You could rotate/reverse the cam sprocket and get a different reading, so for instance there is a front and back.  I don't think the factory intended that, but there is due to variations.  The notch on the end of the cam is not exact, etc., etc., etc.

The other problem I got into was the kinda of double speak, in other words was I advancing or retarding, how was I changing duration, what about the variations between 1-4 and 2-3, it started driving me crazy.  I think I went back and forth with Mark for over two weeks before I finally FULL understood exactly what I was doing, were I should be measuring, etc.

What I finally did, once I understood the measurement issues, direction I wanted to go, etc., I tried to duplicate stock, what I had when I took it apart.  I think even some of the questions I had about direction also got resolved once I went back to stock and tried to duplicate "stock."

I also had to resolve the 10 thousands as that's what I had removed from the head.  For what its worth I was thinking it was Mark who recommended I not remove anything from the cylinder's just buff it up.

Somewhere along the way I finally got it, took me weeks to fully wrap my head around what was going on.  The forum kinda helped but most guys were/are not getting into this kind of detail.  I did find several other generic web sites that helped me full grasp the details of spec'ing the cam and advancing the timing.

So 1st you have got to find TDC and that may not, is probably not with the point plate 1-4 on the "T" and the end of the cam with the horizontal line, lined up (slightly off).  Once you got that figured out, and you understand what your original motor had in terms of advance, assuming like it was in my case, stock from the factory, then you can start working the slotted cam sprocket to advance, in my case 5 degrees.

So for what its worth, at my advanced age, I cannot remember the detail!!  I would have to just get back into it again and start trying to re-wrap my head around it.  What I THINK I did was to adjust to the correct TDC and advance by 5 degrees.  For sure nothing is hitting, BUT its not running yet!
'66 CB77, 305 Superhawk (Project Bike)
'72 CL 175, (Project Bike)
'75 CB750F (Project Bike, Complete)
'05 GL1800 ABS Black Cherry (Current Ride)
'87 GL1200 Wineberry Aspencade
'83 GL1100 Wineberry Aspencade
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Offline Kickstart

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Re: No clearance between intake valve and piston
« Reply #30 on: February 03, 2013, 11:39:28 PM »
Using only the inner valve spring, I was able to push the valve down with two screwdrivers.  I don't think this is the right way to do it, but it worked for me (picture attached).  I just now read a post by 754 where he says he prys up on the cam follower, but I'm not sure how I would do that without damaging the cam lobe or follower.

Anyway, my brute force "dueling screwdrivers" method seem to be accurate enough.  ... and by doing this I found out that I might have had too much milled off the head and deck. (I didn't think 0.02" would be such a problem... or maybe my machinist took off more than I thought?)

I was hoping to get 0.065" intake and 0.080" exhaust clearances, but there is no cam opening degree that provides this. 

Cam timing was measured at 0.04" intake lift with 0.002" of lash:

Cam timing at 5 degrees BTDC - Intakes around 0.050" clearance, Exhausts at 0.085":
  • Intake clearances were between 0.046" - 0.055" (measured at 6 degrees ATDC) - #1 was 0.0455, #2 and #4 were 0.055, and # 3 was 0.048"...  I wasn't expecting this much deviation.
  • Exhaust clearances were between 0.0845" - 0.099"  (measured at 5 degrees BTDC) - #1,2, and 3 were all 0.0845"-0.085"... only # 4 was 0.099" (which I don't quite understand why that one was so large - I need to go back and take some more measurements to double check this.)

Cam timing at 0 degrees TDC - Intakes around 0.070" and Exhaust 0.074":
  • Intake clearances were between 0.066" - 0.0725" (measured at 5 degrees ATDC) - #1 was 0.067, #2 was 0.0725, #3 was 0.07, and #4 was 0.066 - so not quite the same deviations as the 5 degree measurements, which tells me I've got around +/- 0.006" error range... or something (I'm too tired right now to calculate the standard deviation)
  • I only measured the #1 exhaust clearance (hey it was getting late) - 0.0735"

Realizing that neither 5 degrees advance nor 0 degrees (stock cam advance for my bike) would work, I took a couple measurements at 3 degrees advanced:

Cam timing at 3 degrees BTDC - Intake 0.060", Exhaust 0.082":
  • #1 Intake clearance = 0.0585" (measured at 8 degrees ATDC)
  • #1 Exhaust clearance = 0.0815 (measured at 5 degrees BTDC)

So, I guessing 3 degrees BTDC is my best bet... I pretty much don't have many other options if I want to maintain at least 0.08" clearance on the exhaust and still have something I'm comfortable with on the intake clearance.

Is 0.0585" enough for the intake?  I'll be using HD valve springs from APE, so I'm thinking that will give me a little insurance.


In case anyone is interested, here are my cam measurements (stock 1975 F0 cam):
(all measurements with 0.002" intake lash and 0.003" exhaust lash - 0.04" lift for opening and closing)
#1#2#3#4
Intake opens0 degrees0 degrees0 degrees0 degrees
Intake closes39 ABDC39 ABDC40 ABDC39 ABDC
Intake lift.307".309".310".305"
Exhaust opens42 BBDC42 BBDC42 BBDC41 BBDC
Exhaust closes2 BTDC1 BTDC2 BTDC2 BTDC
Exhaust lift.290".292".288".291"
- the ? above is because I wrote down 52 ATDC, which was obviously a mistake EDIT: I went back and measured this... 39 degrees ABDC... also correct the intake closings from ATDC to ABDC
« Last Edit: February 04, 2013, 07:22:45 PM by Kickstart »
- Chris
75' CB750F Orange flake (rider)
75' CB750F Blue (Project)
75' CB750F Painted black (Project)
No Reserve Racing #171 AHRMA

Offline Kickstart

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Re: No clearance between intake valve and piston
« Reply #31 on: February 04, 2013, 07:18:36 PM »
So I was looking around my garage for the new tappet screws I bought a while back, and I came across one of my tire irons...  I was like "ooh, that's how you do it"

This was much easier than what I was doing, which did slip off a couple times, thus the term "dueling screwdrivers".  Still it makes me nervous I could mar the cam lobe or follower.  Maybe I'll find an aluminum tire iron for future measurements.
- Chris
75' CB750F Orange flake (rider)
75' CB750F Blue (Project)
75' CB750F Painted black (Project)
No Reserve Racing #171 AHRMA

Offline Kickstart

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Re: No clearance between intake valve and piston
« Reply #32 on: February 04, 2013, 07:35:03 PM »
I went back and measured the #4 exhaust clearance again.  This time with a new tappet/lifter adjustment screw.  I thought maybe the old screw was worn at an angle and the reason for the much larger valve to piston clearance measurements (compared to the other exhaust valves).

Unfortunately, I measured 0.098" clearance "again" (with the cam at 3 degree advanced).  I just can't understand why this valve has around 0.014 - 0.016" additional valve to piston clearance than the other exhaust valves. 

Does this make any sense?  I'm having trouble understanding what would cause this.

- Chris
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Offline lucky

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Re: No clearance between intake valve and piston
« Reply #33 on: February 05, 2013, 03:46:49 PM »
.01 thousands???

.001 is ONE thousandths.
.01 is 10 thousands unless you meant millimeters.

Many times an engine only has .010 thousands clearance to start with.
You may just need a base cylinder shim.
Of course then the cam chain could need "adjustment"
Thicker head gasket?
Lots of options.

Offline lucky

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Re: No clearance between intake valve and piston
« Reply #34 on: February 05, 2013, 03:50:04 PM »
With the cam advanced 8 degrees the intake valves don't contact the pistons.  However, it doesn't look like I have enough clearance.

As best as I can measure, my clearance is only around 1.25mm.  I got one measurement of 1.2mm but I might have squeezed the clay with my calipers on that measurement.

Pictures attached.  I think I read somewhere that you should have 0.08" clearance (about 2mm) so I'm thinking the 8 degrees advanced isn't going to work for my set-up.

I think I'll try this again at 5 degrees, and this time use a degree wheel to make sure I'm super accurate.  While I was anal about advancing the cam, I still based it on the location of the stock cam sprocket.  So I might still be off by a couple degrees.

Oh, also, all measurements were done with the valve clearances set at stock specs (0.002mm intake and 0.003mm exhaust).  Not that 1 or 2 thousandths of a mm would make a significant difference.

1.25mm is .049 thousandths.

You need to stick with one or the other. mm's or thousands of an inch.
Very confusing reading.

Offline Kickstart

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Re: No clearance between intake valve and piston
« Reply #35 on: February 05, 2013, 04:11:17 PM »
Fair enough on the comment about switching between Metric and US Customary/English Units... I even confused myself...  the "0.002mm and 0.003mm" statements were typos... I meant 0.002" and 0.003"


My more recent posts regarding clearances are in inches.
- Chris
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75' CB750F Blue (Project)
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Offline bwaller

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Re: No clearance between intake valve and piston
« Reply #36 on: February 05, 2013, 05:09:04 PM »
The extra clearance for that valve may simply be from a previous valve job where there was a little more ground from that seat. We used to use manual cutters which certainly weren't as accurate.

Offline Kickstart

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Re: No clearance between intake valve and piston
« Reply #37 on: February 05, 2013, 07:54:31 PM »
The extra clearance for that valve may simply be from a previous valve job where there was a little more ground from that seat. We used to use manual cutters which certainly weren't as accurate.

As I was going to sleep last night I was thinking a deeper cut valve seat could be the only explanation... thanks for confirming my suspicion. 

In your experience, does that make that cylinder run more restricted or is it not enough of a difference to cause any significant change in air flow?

Thanks 
- Chris
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Offline scottly

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Re: No clearance between intake valve and piston
« Reply #38 on: February 05, 2013, 08:13:01 PM »
.050"+ of clearance is probably adequate. You started this thread stating a negative clearance, as in the valves were hitting the pistons. 
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Offline 754

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Re: No clearance between intake valve and piston
« Reply #39 on: February 05, 2013, 08:18:35 PM »
 You can clearance the  stock pistons in a few minutes with a die grinder if it is a small amount..
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Offline Kickstart

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Re: No clearance between intake valve and piston
« Reply #40 on: February 05, 2013, 08:39:17 PM »
.050"+ of clearance is probably adequate. You started this thread stating a negative clearance, as in the valves were hitting the pistons.

Yeah... I've come a long way :)    I realized that using a dial indicator degree wheel is the only way to do this properly.
I've seen other posts also saying .050" is OK for the intake, but I wanted to go with Big Jay's suggestion:

We advise .065" intake and .080" exhaust.  With the exhaust, the piston is chasing the exhaust home on the exhaust stroke. If the valve gets "late" (float), the piston can catch it.

Float is not that big of an issue on the intake.

I was thinking about this:
You can clearance the  stock pistons in a few minutes with a die grinder if it is a small amount..
But I've never done this before, and I'm not sure if the risk of screwing something up is worth the extra 0.010" I'm chasing on the intake.  Maybe I'll just run at 3 degrees advance and be content with the extra 2 HP at highways speeds in lieu of more torque at lower RPMs.
« Last Edit: February 06, 2013, 06:51:05 AM by Kickstart »
- Chris
75' CB750F Orange flake (rider)
75' CB750F Blue (Project)
75' CB750F Painted black (Project)
No Reserve Racing #171 AHRMA

Offline scottly

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Re: No clearance between intake valve and piston
« Reply #41 on: February 05, 2013, 08:47:18 PM »
Maybe I'll just run at 3 degrees advance and be content with the extra 2 HP at highways speeds in lieu of more torque at lower RPMs.
Sounds like a good plan to me... ;)
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Offline 754

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Re: No clearance between intake valve and piston
« Reply #42 on: February 05, 2013, 09:04:05 PM »
 10 thou is the thickness of three hairs..
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Offline Kickstart

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Re: No clearance between intake valve and piston
« Reply #43 on: February 06, 2013, 04:30:17 PM »
10 thou is the thickness of three hairs..

Hmmm... and I'm not one to leave good enough alone :)

Do you just do this by hand, what type of bit do you use, and how fine of a grit do you use for the final surface finish?
- Chris
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Offline lucky

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Re: No clearance between intake valve and piston
« Reply #44 on: February 06, 2013, 05:33:01 PM »
Fair enough on the comment about switching between Metric and US Customary/English Units... I even confused myself...  the "0.002mm and 0.003mm" statements were typos... I meant 0.002" and 0.003"


My more recent posts regarding clearances are in inches.

Thanks Kickstart.

Offline scottly

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Re: No clearance between intake valve and piston
« Reply #45 on: February 06, 2013, 06:45:33 PM »
I'm not sure if the risk of screwing something up is worth the extra 0.010" I'm chasing on the intake.
I'm not sure if the risk is worth it either.
Don't fix it if it ain't broke!
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