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Cam/Valve pockets

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MotorHead67

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Has anyone out there installed a high(er) lift cam in a BJ8?
I like the looks of the Denis Welch DWR-8 cam (.401 inch valve lift). They say it requires that valve pockets be cut. I assume this is a relief in the block so that the valves don't contact the top edge of the cylinder. I wonder if this cut is standard procedure for a good engine shop? Should I be concerned about cutting into water jackets? Also it seems like a relief in the block could make an edge in the combustion chamber which, unless I'm wrong, could cause a hot spot and detonation?
Any thoughts?
 
Hello MH,

this is a modification that is done on 'A' series small bore blocks when fitting a large bore head as the larger exhaust valve may contact the block. I presume that the 'C' series engine is of a similar layout.
I would imagine that the overlap is relatively small and the recess would be in the cylinder wall and also shallow. (certainly less than the distance from the piston top ring groove to the piston top otherwise the ring will be damaged with combustion gases)
The edges of the recess need to be rounded off to avoid hot spots.

Alec
 
Hi Motorhead,
I've got a little more lift than that because I went with higher ratio rocker arms. There was no problem relieving the block. (didn't cut into anything bad) I brought my engine to a machine shop so they could figure out where to locate the pocket and how deep it needed to be cut. Once the math is done, it's pretty straight forward. The exhaust valve is what needs to be ported. Here's a shot of what the work looks like.
 
Thank-you, both, for the info. Where else on earth could I have found the answer to that question?
GregW- a picture is worth a thousand words.
What lift and duration did you use? How's the motor run, strong?
Again, thank-you.
 
Looking at the pictures my question is how does this work with the head gasket ? My frame of refference is my m spec 100 where I needed to use the DW steel head gasket to resolve a blown head gasket issue. The pockets would seem to be a risk given the reduction in spacing. Perhaps this is just a difference between the 4 and the 6. Any input on this point ? thanks jtb
 
Hi JT,
The four is the same situation. The parts don't line up as you would first suspect.

The thing that isn't obvious is that, at least on the head gaskets I've seen, the gasket bore is about .149" larger than the cylinder bore plus an additional .088" cutout around the exhaust valve. The exhaust valve relief in the combustion chamber also overlaps the cylinder bore. The stock cam lift only opened the valve into this combustion chamber & head gasket relief area. We are simply extending this chamber & gasket relief deeper into the cylinder face to make room for more valve lift. Since there isn't any gasket or cylinder head in this area in the first place, we aren't compromising the sealing.

Also, the exhaust valves on # 1 &4 are located toward the outsides (front & back) where the gasket sealing surface is wider. Same with cylinders 2 & 3 which have their exhaust valves located toward the center where the sealing surface is wider.

A stock 100-4 head has about .455" clearance between the exhaust valve head & the block with a .055" thick headgasket. Assuming a working clearance (margin) of .062" for valve to block clearance, this would limit valve lift to .393".

Obviously, not all engine designs have the edge of the valves overlapping the bore. It's just a way to get larger valves with a small bore.
D
 
Hi MH,
No I don't know. If an engine was assembled, you could accurately measure how far a valve is depressed against the spring until it bottoms out & compare this number to the actual valve lift. Or turn the engine over until a valve is at full lift & then measure how much additional travel there is until the valve bottoms on the block. If the head was off the engine, measure from gasket surface to valve head & add the gasket thickness. It may depend on whether the head has been milled etc. You need to have about .060" clearance between valve & block with the valve at full lift.
D
 
Thank-you sir.
I ask because I've found different opinions regarding how much lift will cause interference.
In the end, though, it'll just have to be checked.
I'll decide on the cam specs first and then check and machine if neccesary.
 
Hi MH67,
I got an Isky 268 deg regrind of my original BJ8 cam. With a 1:1.55 rocker, lift is about .43". Engine hasn't been fired up yet, waiting on carbs and finishing bodywork. I hope "strong" is an understatement. I can hope, can't I?
 
Hi Greg,
I strongly advise checking for valve to block clearance as I mentioned earlier. The definitive check is to put clay on the block areas under the valves, assemble head & pushrods temporarily, set valve lash, crank engine over carefully, remove head, & measure the valve impression thickness in the clay.

Since the engine is already together, measure the additional valve travel that is available from the full lift position. If it is less than .030", don't run it. If it is .050" or more you are OK, if .060 or more, very good. You could do this by turning the valve lash adjuster until the valve makes contact with the block & determining the additional travel available. I would suggest using a dial indicator, but feeler gages & some head scratching (yours) could do the job. A usable dial travel indicator with magnetic base stand can be had for about $15.

Under running conditions, clearances can be reduced due to slight valve float & carbon build up. If there is valve to block contact you will be in for some expensive rework.

The other advantage here is that there is room for some gas flow at full lift. Otherwise part of the valves effective opening area is blocked off.

Rocker arm ratios are a "nominal" number. The actual ratio changes as the valve & rocker move through their travel. The rocker to valve stem contact area moves across the valve stem as the rocker moves through it's arc. This changes the leverage & effective ratio of the arm. A simple change of rocker arm stand height or valve seat depth can also change this geometry. Ideally, the valve to rocker contact patch will move from slightly inboard, through valve stem center to slightly past center, & back to it's slightly inboard position as the valve moves through it's cycle. With a correctly set up engine, (valve stem length, rocker stand height, valve seating depth, & rocker tip radius) this side to side motion will be mostly rolling across the valve stem tip instead of sliding across it. The actual rocker arm ratio still varies though. With very high lift cams, the side to side scrubbing action of rocker to valve stem becomes so great the roller rocker tips are needed to get reasonable rocker, valve stem, & valve guide life.

A simple check is to put a very thin piece of paper between the rocker & valve stem, turn the engine over, & see if the paper is cut. If not, you have good rocker arm geometry.
D
 
Hi Dave,
My machinist hinted that I could do the pocket porting at home, but I didn't want to learn on an increasingly rare engine. I had him do the porting and set up the geometry. He made a new set of push rods once the engine was mocked up.(I needed a new set anyway) I can't remember the exact valve to block clearance, but it was around .060".

Thanks for the concern,
Greg
 
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