Topic: Valve to piston clearance?

[tweakin]

I am getting ready to button up my F2 this coming week and I can find what valve to piston clearance should be to avoid tapping a piston.  1977 cb750 F2 with 836 kit.  Planning on running a Megacycle 125-65 cam.  I am going to clay everything to make sure I will hopefully have no issues.

Thanks for all the help!

[Bill/BentON Racing]

.080-.100",better to have too much,than not enough! ,Bill

[tweakin]

.080-.100",better to have too much,than not enough! ,Bill

Thanks Bill that is what I thought I remembered but wanted to make sure.

[kos]

You can be tighter than that, but if you have that and are running good valve springs along with that cam...you are good to go. What pistons? JE or Wiseco? or ?

KOS

[tweakin]

Thanks KOS, I am running JE pistons

[turboguzzi]

You can be tighter than that,
KOS

+1

060" is OK too IMHO

[tweakin]

Just to clarify, by 060"  you are saying 060 inches?

You can be tighter than that,
KOS

+1

060" is OK too IMHO

[kos]

60 thousands of an inch is what is is saying. and that is plenty of room.

KOS

[tweakin]

Thanks Kos!

60 thousands of an inch is what is is saying. and that is plenty of room.

KOS

[Bill/BentON Racing]

Hey,you can run .040,but in his case the #'s recommended were on the safe side,below .060 you rolling the dice.Bill

[mec]

very surprised that none of the HiPo gurus differs between in and out valve nor if you run your engine with or without a rev limiter.

my engines run with rev limiter, minimum in 0.04 and out 0.06.

mec

[MRieck]

very surprised that none of the HiPo gurus differs between in and out valve nor if you run your engine with or without a rev limiter.

my engines run with rev limiter, minimum in 0.04 and out 0.06.

mec

Good point mec. The exhaust is always the larger #. You rarely run into problems with aftermarket pistons....running larger cams with stock pistons can be a problem especially on the exhaust side. You can time the cam to get out of a bind. I have run the exhaust down to .040  modern 600cc roadrace engines...that is tight. Valve clearance was set at .008 if I remember correctly and HRC springs were used.

[kos]

very surprised that none of the HiPo gurus differs between in and out valve nor if you run your engine with or without a rev limiter.

my engines run with rev limiter, minimum in 0.04 and out 0.06.

mec

Hey, When your rider has the motto, "run it like a rental" a rev limiter is of no use and if your rider "backs the bike" into the corner using the rear wheel to slow it down and turn it into the corner which can over rev engine if a slipper clutch is not used..again a rev limiter is not worth much.

I think Mike and myself have given everyone interested in this subject.. ample amounts of time on this subject.

Mark @ M3Racing

[Big Jay]

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.

[z1100r]

 When Pops Yoshimura was tuning these engines he said 2mm or 0.080" clearance.

 Whos saying values less than that are OK and based on what ...0.040" is half the recommended value from anything I've ever read.

  Never heard of moving the cam timing to get piston clearance. Again, quoting Yoshimura "accurate cam timing is essential, set it precisely to the angles given with your cam at exactly 1mm or 0.040" lift. If its imposible to do this due to manufacturing tolerances split the error between inlet and exhaust, dont put it all on one.

I still have the Yoshimura literature/catalog from way back when.

Do megacycle say anything different to 0.080". Or do any other cam grinders.

Tuning an engine is painful and meticulous, if there isn't enough meat in the piston crown to achieve the correct clearances you're on a loser straight away. You have tuned this engine for more power at great expense but cant use it to its full potential because you're worried about clouting a valve if you miss a gear or overrev...crazy spending the money and wasting the effort. IMO

[paulages]

I was taught .050" intake and .080" for exhaust.

[Retro Rocket]

Whos saying values less than that are OK and based on what ...0.040" is half the recommended value from anything I've ever read.

Well there's about 2-300 years of high performance experience in this thread alone, For example,

Mike Reick {JMR drag racing  proprietor, engine building guru}
Mark McGrew {M3 racing proprietor, Dayton winning 750/4's and much more}
Big Jay, {CBR Zone and Ape engineering proprietor and engine guru}
Mec, {Engineer and racer, Owner of home built Egli racer with 1000cc monster engine with awesome home made mods, including the frame}

Now considering that these guys have multiple titles both on the strip and on the track i would say without hesitation that the know what they are talking about.... Actually, these guys are amongst the most knowledgeable people on the planet when it comes to these bikes....

I expect a discount now guys.....  ;)..J/K

[z1100r]

Well Ok, what kind of engine is being built here..? A drag bike engine, A daytona race engine, or a mega big bore special.

 As far I can detect neither of those, a limited use cafe racer is being built..well true or not. An exceptional looking bike with some really trick parts that has no need to be flaunting with catastrophe. Absolutely no point in that.

 I would Ask Mega-cycle if I were you but I bet they say 0.080".

[tweakin]

Awesome info guys!  I love this site and being able to pull from all the many years experience is second to none.  I hoping when I measure everything today clearance will not even be an issue.

Head looks great Jay!  Thanks for taking the time to do it right!

[MRieck]

When Pops Yoshimura was tuning these engines he said 2mm or 0.080" clearance.

 Whos saying values less than that are OK and based on what ...0.040" is half the recommended value from anything I've ever read.

  Never heard of moving the cam timing to get piston clearance. Again, quoting Yoshimura "accurate cam timing is essential, set it precisely to the angles given with your cam at exactly 1mm or 0.040" lift. If its imposible to do this due to manufacturing tolerances split the error between inlet and exhaust, dont put it all on one.

I still have the Yoshimura literature/catalog from way back when.

Do megacycle say anything different to 0.080". Or do any other cam grinders.

Tuning an engine is painful and meticulous, if there isn't enough meat in the piston crown to achieve the correct clearances you're on a loser straight away. You have tuned this engine for more power at great expense but cant use it to its full potential because you're worried about clouting a valve if you miss a gear or overrev...crazy spending the money and wasting the effort. IMO

 I said I have run the clearance....I did not say I recommend it. That particular roadracing engine also ran a squish of a little over .025 with good rods, HRC springs (as mentioned), tight cylinder to skirt and Web cams. The valves are light...3 exhaust valves probably weigh as much as 1 of those old, clunky 7mm Suzuki/Kawasaki OEM pieces. Valvetrain control was good.
  You can change piston to valve on the exhaust side by advancing the cam and getting it to closer earlier on the exhaust stroke. Obviously I'm talking about twin cam engines.
  There were reasons for running that clearance and I'll be the first to say it was a compromise I really didn't like. The reliefs showed kiss marks when the engine was torn down but it did run well and was strong. Like I said..it was a compromise.
 On a side note I do know of 1 guy who ran valve to valve at .030 on a dragrace CB750. That is also extremely tight too but he was using Ti valves, good springs etc.
 .080 is very safe and I'm sure Megacycle would say .100 or .120 is even better.

[bwaller]

I should stay outta this but after asking Mike for advise about setting seat pressures, I think this can be a large part of running tighter clearances. I will also mention that there was a need for 1.76mm of extra shimming to get KPMI valve springs set to 75lbs on the 570. This head had not had seats cut previously either. Maybe not a huge factor but a factor non the less.

[Tintop]

Great thread, thanks for asking the question tweakin.  Interesting point bwaller.