Small valve runout

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pmoose
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Small valve runout

Post by pmoose » Thu Dec 24, 2015 6:33 pm

Hello everyone and merry christmas. This question may be in the wrong section but here it is: Im building a small IC engine (around 40cc) and Im having problems with my valves sealing. I have access to an older craftsman valve grinding machine to true the sealing surface up but you have to fiddle with its collet to get the runout, measured on the valve stem close to the head, acceptable. But, what is acceptable runout on a valve, stem vs sealing surface on head? The valve in question has a 5/8" dia head, 3/16" dia stem, 45 deg face, and the stem is roughly 2.25" long.

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wsippola
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Re: Small valve runout

Post by wsippola » Thu Dec 24, 2015 8:30 pm

Can't say as I know, but I would think the runout needs to be less than the clearance between valve guide and stem. The closer you can get it, the better!

Tom - AMS
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Re: Small valve runout

Post by Tom - AMS » Fri Dec 25, 2015 4:45 am

for many Years Kwik-Way was leading US manufacturer of valve grinders and seat grinding equipment. Today leading edge shops. like ours, machine valve seats with Serdi and Newen CNC controlled machines. In the '50, '60, '70's K-W advertised their seat run-out on was less than the 0.002" on automotive engines. And from 50+ yrs experience grinding seats with KW Seat GTrinders, I can say it was even less than 0.001", depending on operator's skill.
On today's auto & motorcycle engines with valve stems in the 4mm to 5mm diameter range, the seats must have less than 0.0005 run-out, if one is going to produce a quality hi-performance cylinder head, with rock-solid reliability & longevity. With our CNC seat machines, TIR is typically 0.0002 or less.
On the valve itself, K-W's 6 ball valve stem chuck on their valve grinder will face valves with virtually zero run-out in regards to stem / face. For this reason, we still use hi-production-style KW valve grinders made in the mid 80's in our shop. Very precise and robust machines.

From a theoretical point of view, stem-to-face TIR should be less than the guide-to-stem clearance... maybe no greater than 1/2 the stem/guide clearance. For the multi-valve heads with small 4mm or 5mm stems, we won't use new valves with anything greater than 0.0005" run-out.

One method we use to check valve to seat contact location and fit, is to "paint" the face with a black felt tip marker. Then place the valve in the guide, hold it about a 1/8" off the seat. Then use your index finger to quickly slap the valve closed, against the seat. remove the valve and inspect the witness mark on the face - it should have a fine contact line all the way round. In production, to test the seal/fit of valve to seat contact, we simply place the valve in the head, sitting on the seat ( with no spring installed) and pull a vacuum on the port. It must register at least 25"Hg vacuum.
On scale model engine, I've had better luck turning them between centers, and while still set up in lathe, grind the face 44.5° ( for use with a 45° seat.) Hope this helps.
Tom-AMS

pmoose
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Re: Small valve runout

Post by pmoose » Fri Dec 25, 2015 12:54 pm

Thanks Tom for all the information. Ive never been impressed with how the collet on the craftsman grinder worked. It closes at the front and back as its compressed length wise. It grips a wide range (1/4"-1/2") but its not nearly as accurate as i would assume a valve face would need to be. I guess ill set my valves back up in the lathe and true the faces between centers.

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liveaboard
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Re: Small valve runout

Post by liveaboard » Sat Dec 26, 2015 4:34 am

I enjoyed Tom's comprehensive review on the science of valve grinding.
I sometimes used to operate a simple valve grinding machine in the garage where I worked as a mechanic in the 80's. Things have clearly moved along!

Regarding the original query; the old way of seating an engine valve might be suitable for you. grinding paste and a hand lapping tool; a little suction cup to hold the valve while you spin it between your palms it the seat.

spro
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Re: Small valve runout

Post by spro » Sat Dec 26, 2015 7:25 pm

I agree and have been there with many valves. Remember the dual compound cans and the different tools to produce the back and forth lap. In the end it was pressure sensitive and once we got that seal.. Using Prussian blue, Dykem , pencil lead marks, whatever we find the valve is sealed at the top. It is supposed to be sealed of 1/3 width in the center. So it is new valve day. In fact the last time I lapped valves (into a Toyota 22r) I had to make sure about the counter angles below and above. Engineered approximation of heat distorts the valves so, only one area applies for that engine. The end of that was perfect sealing valves and the head gasket was too thin. The engine is over compression forever(requiring higher octane) and the valves won't leak. Old stuff.

pmoose
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Re: Small valve runout

Post by pmoose » Sat Dec 26, 2015 7:49 pm

Thanks for all the replies. I do have one of those suction cup-on-a-stick tools and grinding and lapping compound. The problem is still that my valve is either bent, ground with too much runout, or the seat is not concentric with the valve stem guide because im not even to the point of lapping due to the lack of concentric contact between my valve face and seat. I didnt think it would be that hard to make valves seal but its proving to be a huge problem for me :shock:

Tom - AMS
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Re: Small valve runout

Post by Tom - AMS » Tue Dec 29, 2015 3:58 am

pmoose: If the valves are not bent, check guide to seat concentricity. With small guides/seat concentricity is critical. If you machined the seat and guide from the same datum surface, use a DTI in your mill spindle to center on the guide, note the clock position of max TIR, if any. Max TIR should be less than 0.0003". If the guide and seat are aligned, then "paint" the seat with black felt-tip marker. Install the valve; use your fingers from the stem end to gently pull and slightly twist the valve back & forth against the seat, just enough to wear the maker off. Look at the witness marks to see where it's touching the seat. Often top edge of the seat is not "square" to the guide's axis, and without a "clearance" angle, it can cause the valve to seat as if bent. Machining a narrow 15° angle, square to the guide's axis, at the top at the of the seat so the valve's 45° face sits evenly on the seat will correct this. Same thing can have with the "bottom" of the seat, if the valve & seat are both 45°. Machine a narrow 60° - 75° angle below the 45° seat. Machining the valve at 44.5° will also help. Note: the width of the valve seat controls two important parameters: too wide and the valve seating psi will not be enough to seal. Too narrow and the valve will not run cool enough and could suffer excess stem / guide wear or even galling / seizing, as well as face burning or "pie" cracks. On small scale model engines, I use a seat width of 0.020" - 0.030".
On engines with small valve stem diameters, IF you decide to lap the valve & seat, its best to use a piece of rubber hose that fits snug over the stem to pull/twist the valve. Using suction cups can actually flex the small valve enough to seem to lap OK, but not seal in when in use.
Tom - AMS

edit: I'm not a big fan of lapping valves...if the seat and valve are machined correctly, no lapping is req'd. Lapping is typically used to correct a poor surface finish, but it can also make the situation worse in some cases. If you can machine a better seat or valve, instead of resorting to lapping, IMHO, that's the best solution. Best regards - Tom
Last edited by JackF on Tue Dec 29, 2015 11:49 am, edited 1 time in total.
Reason: Removed link to commercial site. Jack

JackF
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Re: Small valve runout

Post by JackF » Tue Dec 29, 2015 11:47 am

Tom,
Thank you for the added info on on working with small valves, I'm sure many here, including I , can make good use of it. :) :)

As a side note, linking your post to your commercial web site is not permitted. There is a Commercial Vendor section here on this site for that purpose.

BTW, Welcome to the site and enjoy your stay. :D :D


Jack.

Tom - AMS
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Re: Small valve runout

Post by Tom - AMS » Wed Dec 30, 2015 3:44 am

Jack, thanks for the tip about the link in my sig. ...and for the welcome :-) Tom - AMS

tornitore45
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Re: Small valve runout

Post by tornitore45 » Thu Dec 31, 2015 8:21 am

Model engines valve sealing is completely different from full size engines.
The seat should be no more than 0.010", just a hand twist with a zero flute countersink or a homemade tool with a centering tail fitting into the stem guide.
Half a dozen twist using a very mild abrasive like Mon Amy would do it. The rest of the sealing is done by running the engine.
My first mistake was to have a 0.050" seat, it looked good but there was always an escape path somewhere around, since is difficult to have two large areas mate perfectly all around.
Mauro Gaetano
in Austin TX

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