How to Build a B20 VTEC Part 7: V2V Clearance, Degreeing the Cams, and P2V Clearance
Building a custom engine is basically 80% measuring and double checking things. Every time you think your done measuring there’s another thing you need to check. I’ve got my GSR head and B20 block ready to go but they can’t be bolted together until I double check my valve to valve clearance, and piston to valve clearance.
I’m running a set of Skunk2 Tuner Stage 2 cams with a set of Supertech pistons so I could probably get away with not checking these clearances but I’d rather play it safe and check them.
V2V - Valve to Valve Clearance
Valve to valve clearance is how much space remains between the intake valves and exhaust valves when they are at their closest point to each other during the engine’s rotation. This is something you have to check when running larger cams and is especially important if you want to play around with adjusting the cam gears.
The recommended min V2V clearance is .030" - .040" depending on your personal safety tolerance. I’ve opted to aim for somewhere between .035" - .040" of clearance.
In order to check V2V, both cams must be installed in the head and ideally only 2 valves / rockers (one intake, one exhast) are installed to make turning over the camshafts easier. (less spring force to fight) To make the process even easier you can buy checking springs that are super weak and meant just for measuring but I don’t have any so I used two of the stock exhaust GSR springs as they are soft enough to be manageable.
VTEC must also be locked and valve lash should be brought to zero.
Then all you need to do is set the cam gears to various timing marks and rotate the cams in unison until the valves are at their closest so you can measure how far apart they are.
I used a sacrificial timing belt clamped around the gears.
And used feeler gauges to measure the distance between the valves.
It’s a bit tedious to check all the combinations but it’s better to spend awhile measuring clearances instead of finding out the hard way that your valves were too close.
Based on my findings the tuner 2 cams should be good as long as I don’t bring the cams closer to each other by more than 2 degrees. (ex: +2 on intake, or -2 on exhaust, or +1 on intake and -1 on exhaust)
References
Degreeing the Cam
Next up is to check piston to valve clearance but we can’t do that until the cams are degreed. Degreeing the cams is important because it lets us confirm if the cams are aligned with the crank when the crank is at TDC (top dead center) and the cams are at 0 degrees.
Cams will fall out of sync with the crank when the block or head have been machined or a head gasket with a different thickness than stock is being used because these affect the distance between the cams and the crank.
On a B series motor the cams get retarded when the distance between the crank and cams is decreased, and are advanced when the distance increases.
Since I’m running a .040" head gasket which is thicker than stock and my head has been milled (but I don’t know how much) it’s basically a guarantee that my cam timing is off so I must degree them.
I didn’t want to dish out the big bucks for a high end setup so I’m using a SPI 1" dial gauge (SPI 24-333-7) that I had on hand and a Summit cam degree wheel.
When picking out cam degree wheels you should be mindful that most engines rotate clockwise but B-series engines rotate counter clockwise. This means markings on the degree wheel for “Intake Valves Open” or “ATDC” (after top dead center) will be backwards. I couldn’t find any cheap counter clockwise degree wheels so I opted to buy a clockwise one and just cross out the incorrect info.
After degreeing my cams I found I needed to advance both the intake and exhaust by 1 degree to get them properly aligned to the crank. This tells me that my cams are further away from the crank than stock which makes perfect sense since I’m running a thicker head gasket.
Side Note: My Intake Cam Doesn’t Match The Skunk2 Specs
This was unexpected but I discovered that my intake cam doesn’t match the official Skunk2 specs. At first I thought this was an error on my part but I triple checked my measurements and each time I kept getting the same result. The exhaust cam matched up perfectly so I’m fairly confident it’s the intake cam and not my process.
The Skunk2 specs are:
| Intake @ 0.040" | |
|---|---|
| Opening | 38* BTDC |
| Peak | 94* ATDC |
| Closing | 46* ABDC |
But when my intake cam is adjusted so it opens at 38* BTDC it doesn’t close until 52* ABDC which is 6* of more duration than anticipated. I’ve found others online with similar reports so I guess it’s a bit more common than you’d think with Skunk2. Example 1, Example 2
The proper approach would be to adjust it based off the peak of the cam but I can’t measure peak lift with my setup (dial gauge can’t reach far enough into the head) so I’m taking a shortcut of just advancing the intake cam 1* more and hoping that’s good enough.
References
- Degreeing a Camshaft Correctly: Why You Can’t Just “Line Up the Dots”
- Jay’s Tech Tips #22: Adjustable Cam Sprocket 101
- Degreeing cams
P2V - Piston to Valve Clearance
With V2V good to go and the cams properly degreed all that’s left now is to check P2V clearance. Piston to valve clearance is how much distance remains between the pistons and valves when they are at their closest during the engine’s rotation. The P2V will change depending on cam timing so it’ll need to be checked at multiple cam gear settings.
Checking P2V is somewhat easier than V2V because the cam gears can be adjusted independently of each other and the main goal is to check for how far advanced / retarded each cam gear can go before you begin to run out of clearance. The min recommended P2V is 0.1" for the exhaust, and 0.08" for the intake.
Just like V2V, VTEC must be locked and valve lash should be set to 0.
I opted to check it using the clay technique (see video linked in references) because clay is cheap and easy to work with. The only part that sucked was having to torque down the head, time up the engine, and then remove the head for each combination I wanted to check.
My P2V was as follows:
| Cam Gear Setting | Exhaust | Intake |
|---|---|---|
| -3 | Plenty | Plenty |
| 1 | .17" | .13" |
| 3 | .18 | .099" |
-3 degrees for the intake and exhaust says “plenty” because the valves barely made an imprint on the clay so it felt pointless to measure the distance.
Based on these measurements I feel fairly safe running my cam gears anywhere from 3 degrees advanced to 3 degrees retarded. V2V will be the bigger limiting factor for my build.
I could have continued measuring beyond 3 degrees in either direction since I still had plenty of clearance but it didn’t feel necessary because it’s unlikely my cams are going to be ran that far advanced or retarded. Most of the other setups I’ve seen online are running somewhere around +1, or +2 on the cams.