intake valve to piston clearance

[Koncretekid]

In all articles I have read, especially concerning pushrod motors, it has always been recommended that we make sure we have at least .060" minimum clearance between the valve and the piston.  It is easy to see how an exhaust valve, closing only after the piston has arrived at TDC on the exhaust stroke, could contact the piston if it is not following the cam, for whatever reason.  However, I cannot see how the intake valve, which is only close to the piston at TDC on the exhaust stroke, and is only just being opened by the cam, can hit the piston.  It closes generally about 120 degrees before TDC on the combustion stroke, and it is hard to imagine that it could still be open when the piston comes up.  I know it can be lofted, but it must close in time to get sufficient compression.  Can somebody explain why we need that extra clearance on the intake valve?

Tom,

[jacksoni]

P-V clearance is minimum around TDC overlap with exhaust being closer BTDC and intake ATDC. There are a lot of factors affecting that clearance- this article mentions a bunch: www.hotrod.com/articles/predict-piston-to-valve-clearance-before-ordering-a-cam/

the exhaust valve is moving away from the piston  which is coming up but slowing. With heat expanding things, extra clearance on the EX valve is best (often recommended 0.100" or more). The intake valve is opening BTDC but only a little and starts chasing the piston so won't hit as hard LOL if it does. Valve opening is accelerating faster than the piston near TDC.  But the location varies. And the intake is opening before the piston gets to TDC, thus valve notches.  Most books say check at 10* before and after TDC. On my engine due to it's particular configuration closest approach was at 18-20* ATDC on the intake.( big cam, big valve, slow around TDC with very large rod ratio.  If things stack up on that big list mentioned in the article,  P to V clash will ruin your day. Those pushing the envelope can get away with less than the suggested but you really have to have everything right. And well measured. Advancing the cam reduces intake clearance and as that is a frequent tuning process for peak power, you have to be careful with it.

[Koncretekid]

Thanks, Jack, but if we clay the piston to valve, we will know what clearance we have between the valve and the piston, so as long as we don't change the valve timing, this shouldn't change, other than for crankshaft flex.  Obviously, if we miss a shift at full throttle, things will happen.  I had this happen when my timing chain broke at full throttle at Loring, but only my exhaust valve got bent, and only by smidgin, so no damage to anything else (except that the broken timing chain exited the cases taking out part of the cover as well as part of the cases.)

P.S. I'm enjoying playing around with my Dyno, thanks to the Performance Trends equipment you sold me.  Unfortunately, I've not been able to get back to it since October of 2019 due to the Pandemic.

Tom

[jacksoni]

HI Tom- of course some engines are not interference so if timing belt/chain breaks not much happens. Most of the stuff we play with that is not the case and it just depends where it stops. I have dropped a valve on my 4 cyl a couple of times. Though could have been fatigue (relatively small stem, titanium and 10K+ shifts) I was also running closer than that .060 clearance......DOH.

Glad the dyno stuff is working well. And with vaccines rolling out hopefully travel restrictions will ease up soon. Not yet out of woods but getting closer anyway.
Stay well!

Jack

PS. I had felt pretty good about dealing with the 4 stroke, DOHC stuff I built but have gone to a two stroke so don't worry about that valve stuff any more.  But 'nother whole kettle of fish for me to try to figure out LOL>

[Koncretekid]

Jack,

So far, I haven't heard from anyone who thinks we actually need .060" clearance to intake valve,  and your dropped valves can hardly be blamed on lack of clearance in my opinion.  The main reason I ask the question, besides all the advice that "you need .060" clearance", is that I could increase my compression ratio by going to less clearance.  Maybe only half a point, maybe more turbulence and better combustion, but as you know, every point counts.

I only have one race bike here in Nova Scotia to play with - - 1968 Bridgestone 350.  In case you don't already know, that's a twin cylinder, rotary valve two stroke, 6 speed transmission, dry clutch.  So far, 114.xx mph at Loring.  So again, no valves except the rotary ones, but no spares available, so playing with a bomb.

Tom

[RansomT]

I've run 0.040 intake and 0.060 exhaust on my "all motor" 1340cc Hayabusa engine.  Static Compression is north of 17:1.  Made a 223+ pass at Loring with OEM fairings.

[jacksoni]

I think with lightweight OHC stuff you can get away with less as RansomT suggests. It in my case both times head of valve broke off so if was kissing piston that could do it. I was pushing limits with P-V and P-H clearances.

[Rex Schimmer]

Jack,
I am sure that you are finding out that even two strokes have some inherent problem. Like proper placement of the ring anti rotation pin and not making the exhaust port so wide that the ring gets stuck in it! But I still love them!

Rex

[jacksoni]

Jack,
I am sure that you are finding out that even two strokes have some inherent problem. Like proper placement of the ring anti rotation pin and not making the exhaust port so wide that the ring gets stuck in it! But I still love them!

Rex

Yah, well that is part of the whole new kettle of fish I have to figure out. So far haven't cooked anything........ cromag   It's coming 

[johnneilson]

Rex,

Porting is an artform, getting the exhaust port right means having it wide enough to allow the piston to exit when exploding and not damaging the nikasil...

John

[Stan Back]

Love it!

[WOODY@DDLLC]

Make the piston like this! 

[RansomT]

Rex,

Porting is an artform, getting the exhaust port right means having it wide enough to allow the piston to exit when exploding and not damaging the nikasil...

John

I'm getting quite good at that on my nitrous bike.

[Old Scrambler]

Tom...........I had "0" intake to piston clearance in 2016 on my 750 Honda............evidenced by the shallow eyebrows in the very thin carbon deposits on all 4 pistons. I solved by minor relief to the piston faces. I have wide lobe-centers and have 0-adjustment potential to advance the cam from OEM setting. Intakes open at 24-degrees BTDC.

Check with Scrambler Motorcycles in Turtle Lake Wisconsin for Bridgestone spares.........Kevin stole the name from me

[ggl205]

Woody, that looks like the pistons in my old air cooled Formula Super Vee, only with the pin pulled.

John