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Author Topic:   Valve to Piston clearance problem
tlkdlw
Dirt Full Roller

Total posts: 42
posted May 25, 2005 11:10 PM  
Exaust valves hit pistons broke valve rest not good.Guy that asembled motor must not have checked clearance. I need help understanding
what efect cam advanceing has on clearance.Have
525 lift 252/261 dur. @50 degread 991/2
mahle flat top pistons angle milled 492 heads.
Pistons are .020 in hole. can't understand how to get needed clearance. had .015 head gasket
but don't think thick gasket will get needed clearance. need help please!!

Thanks Terry

NJantz
Dirt Forum Champ
Total posts: 755
posted May 26, 2005 08:32 AM  
Sounds like you need to take the pistons out and send to the machinist to have some material removed. First, figure out how much material you'll need removed.

[This message has been edited by NJantz (edited May 26, 2005).]

leapinlizard
Dirt Freak

Total posts: 402
posted May 26, 2005 09:39 AM  
On intake stroke, the piston is moving away from the valve. If the valve were to open to early, the piston would be in the way. Advanced cam.

On exhaust, the piston is moving toward the valve as the valve is being opened. If the valve doesn't close early enough, the piston reaches TDC and the valve is agian, in the way. Retarded cam.

leapinlizard
Dirt Freak

Total posts: 402
posted May 26, 2005 09:40 AM  
Geez that guy's big.

Monster
Dirt Maniac

Total posts: 155
posted May 26, 2005 10:27 AM  
Advancing the cam will decrease Exh v/p, and increase Int v/p. Retarding will have the opposite effect. Angle milling can cause decrease in both areas due to incompatible angles. You need to figure out if you need notches deepened, re-angled, or moved before you do any cutting.

tlkdlw
Dirt Full Roller

Total posts: 42
posted May 26, 2005 10:23 PM  
How do you check all that the proper way.

Thanks for the help. Terry

IowaFuzzy1
Dirt Maniac

Total posts: 136
posted May 26, 2005 11:11 PM  
Couple of ways depending on what you have for equipment.1(no equipment)Get some modeling clay,apply a thin (1/8 inch or so)layer to the top of the piston.applypowdered graphite to the valve heads for that cylinder install head with a used gasket and snug it down,install pushrods and setr valves for that cylinder and rotate engine twice.Remove head and check the thickness of the clay where the valves hit it.2(with equipment)remove valve springs form one set of valves and replace them with some light coils.install head and set valves as above.rotate engine untill valve is full open.install dial indecater so that it is on the valve retainer.Push the valve down untill it touches the piston, read the indecater.Good Luck

tlkdlw
Dirt Full Roller

Total posts: 42
posted May 27, 2005 12:09 AM  
Thanks for the help. waiting to get block back from machine shop,had to have it sleaved. It made a mess of piston and cracked clyinder in two places.With the help I will check clearance myself.

Terry

juliaferrell
Dirt Freak

Total posts: 370
posted May 27, 2005 05:44 AM  
The easiest way is to take light light valve springs and install them on #1 intake and exhaust.**You can do this with the head installed** Take a dial indicator place it on the valve stem and zero it....Put the engine on TDC during overlap phase of cam and record the reading, zero the dial indicator, push down on lifter until you have contact look at dial indicator...this will be your PV clearance. You could check other cylinders ect.......You could also move on each side of TDC to make sure you have the closest reading.

Hope that helps.

Monster
Dirt Maniac

Total posts: 155
posted May 27, 2005 06:03 AM  
The light spring and indicator method will tell you exactly, to the thousandth, how much clearance you have, but won't show you where you need it, if you need more. The clay method, done carefully and dissected with a sharp razor blade, will give you a real good picture of clearances in 3 dimensions. FWIW, minimum v/p clearance does not occur when valves are full open, it is during the overlap period, when the intake is opening and the exhaust is closing. The Int valve is chasing the piston down the bore, and the piston is chasing the exhaust valve back to it's seat. That's why minimum clearance for the exh valve is greater than the int valve... the exh valve train is more likely to be separated(float) at the point of minimum clearance. If you use the indicator method, you need to move the crank a degree at a time, until you find the minimum clearance. If I remember right, one will be a few deg BTDC, other one will be a few deg ATDC. This is important, because the valves are moving pretty fast at this point in their cycle.

[This message has been edited by Monster (edited May 27, 2005).]

dgb
Dirt Full Roller

Total posts: 70
posted May 27, 2005 06:41 AM  
Okay, while we're on the subject, maybe someone can answer this question for me: I've alway used the clay method and always tried to maintain the .080 intake and .100 exhaust that everyone recommends- is this required in valve relief DEPTH and RADIUS? I know this measurement applied to depth, but I've always had the most trouble radially- IE, I need to made the intake relief bigger around but not deeper usually. Is this necessary?? Is that much clearance required in that direction or am I making this more work than I need to? I know that by using the dial indicator method, I would not even be able to find that problem which makes me think I'm making this too hard- how much 'radial' clearance is required 'around' the intake valve? Thanks, dgb

Ells949
Dirt Full Roller

Total posts: 28
posted May 27, 2005 08:13 AM  
quote:
Originally posted by Monster:
Advancing the cam will decrease Exh v/p, and increase Int v/p. Retarding will have the opposite effect. Angle milling can cause decrease in both areas due to incompatible angles. You need to figure out if you need notches deepened, re-angled, or moved before you do any cutting.

Monster, you must have meant to say advancing the cam increases exhaust ptv clearance and decreases intake ptv.

Kromulous
Dirt Forum Champ
Total posts: 796
posted May 27, 2005 08:24 AM  
DGB, your on the right track, the radius is important as well.

tlkdlw
Dirt Full Roller

Total posts: 42
posted May 27, 2005 08:48 AM  
I am getting a better understanging from all the
input,don't fully know about valve train but getting smarter THANKS One more unknowen,what
is valve float with solid lift cam? Know what
Lifter float is with hydraulic but not valve float.

Thanks for all the replys Terry

Kromulous
Dirt Forum Champ
Total posts: 796
posted May 27, 2005 10:31 AM  
Well you can float alot of things on you, lifters, its when the lifter cannot follow the cam, and it lifts up off the cam and then crashes back into it latter down the ramp. Bad thing.

Valve float is when the valves are bouncing on the seat or over extending causing coil bind, or the spring bottoming out, to many rpm for the valve spring to keep the valve under control basicly.

Thus the reason for lightweight parts, stud girdles, shaft rockers and stiff valve springs.

The object in the valve train is to keep precise control over the cam timing events (duration @ .xx lift etc), at the max RPM you can run, plus maintain durabilty. A very hard thing to do somethings, especially with higher RPM's and radical cam profiles.


tlkdlw
Dirt Full Roller

Total posts: 42
posted June 01, 2005 09:37 PM  
Well Thanks for all the help and replys this forum is #1. Put engine back together checked clearance with clay and disected it. with .041
gasket know have .120 clearance and at lease
that much radius

Hope to find out how things go this weak end

Terry

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