Need U20 Cam and Piston Info

dave · Post by **dave** » Sun Jul 27, 2003 9:08 pm

Hello all, I'm brand new to the world of Roadsters and have a couple of rookie questions. I pulled the engine and trans out of my 68 2000 to have a local Datsun guy diagnose some shifting problems. As long as the engine was out and on the stand, I decided to pull it apart and at the very least fix the numerous oil leaks. I also took apart the spare U20 that came with the car to see if any of its parts were worth swapping over to the running engine. I discovered a couple of interesting differences that I'm not sure how to interpret:

The engine that came out of the car has a cam with a "B" stamped on the rear end of it. Its pistons are dished and stamped with a "STD", short for "standard" I'm guessing.

The spare engine has a cam with an "A" stamped on the rear end of it. The lobes appear to be as tall as the B cam but not as wide (less duration?). Also, the orientation of the lobes between the two cams is slightly different, allowing for different valve overlap. The pistons, instead of being dished are flat tops with no discernable stampings.

Can someone tell me what I've got on my hands here? I'm looking for a healthy engine, so my plan is top use the flat top pistons. As for the cam, I can tell they are different, but I have no clue as to which one to go with!

Thanks everyone, love the site!!!

Dave

Post by **S Allen** » Mon Jul 28, 2003 9:22 am

Hey Dave,

I will take a stab at this one but I am more of an R16/Stroker kind of guy so if I lead you astray someone else is sure to pipe up. The U20 pistons are flat top. The dished pistons are most likely for an H20 forklift application. The dished pistons work great on an R16 that has a head that has been shaved one too many times. To keep your compression up-go with the U20 flatops. The "A" series cam is for the SU carb setup and the "B" cam is a little hotter and for the Solex setup. What kind of carbs do you have for the car? You can see a picture of the SUs in the engine section of this site. If you plan to run the Solexes go with the "B" cam-SUs the "A" cam is fine. Glad you like the site. Good luck!

Steve

Dave · Post by **Dave** » Mon Jul 28, 2003 10:11 am

Thanks Steve!

I'm running the SUs right now. I'll go with the B cam and the flat top pistons and see how it does. You bring up an interesting point about running the dished pistons in head that's been shaved a few too many times. Perhaps the one I was planning on running has been shaved, thus requiring those dished pistons. Switching over to the flat tops could be trouble. Time to get out the ruler and do some checking!

Dave

SLOroadster · Post by **SLOroadster** » Mon Jul 28, 2003 1:43 pm

Dave,
Where are you? The B cam is the solex cam, it is a .456 lift with a 280 degree duration. The A cam is a .42 lift at 256 degree. The B cam will work great with SUs. The flat top pistons with STD are the correct pistons, and your guess at standard bore is correct. In regaurds to the dished pistons, I would go with the correct flat tops, and run a thicker head gasket (I can't remember who makes one, I do know they are out there.)
Have fun,
Will[/b]

Minh · Post by **Minh** » Mon Jul 28, 2003 2:33 pm

Will is right....

As far as the thicker head gasket, get it if the head has been shaved to many times because alluminum headers and blocks are known to warping (but, not all the time).

If not, a standard head gasket will do.

If yes, see if the spare engine has header that can be cleaned up resused.

You should read some material on stock and racing optioned motors. It looks like you lucked out on the B-Cam since you need the B-Cam rockers and what not to even use it, just to let you know.

Dave · Post by **Dave** » Mon Jul 28, 2003 2:35 pm

Will,

I'm up north in sunny (for the next month at least) Portland, OR. I plan to register with this site just as soon as I get a non-work e-mail account set up. Hopefully tonight.

Thanks for the specs on the cams! I'll use the B cam. As for the pistons, the dished pistons were marked STD. I could see no markings on the flat top pistons but they were admittedly carboned up and I didn't look real hard. I have both heads at the machine shop being checked out. I'll use whichever one turns out to be the better of the two. If that happens to be a shaved head, I'll look into the thicker head gasket so I can run the correct flat tops. Great tip, never would of thought of that!

Dave

Post by **spl310** » Mon Jul 28, 2003 2:47 pm

B cam rockers? There are two rocker types for the U-20, but it has nothing to do with A versus B cam. It is the design type. The early rockers had chrome plating for the wear area (not the best idea), while the later ones had a hardened insert brazed into them. They interchange fully - you can use either type rocker on either type cam.

Minh · Post by **Minh** » Mon Jul 28, 2003 3:01 pm

spl310 wrote:B cam rockers? There are two rocker types for the U-20, but it has nothing to do with A versus B cam. It is the design type. The early rockers had chrome plating for the wear area (not the best idea), while the later ones had a hardened insert brazed into them. They interchange fully - you can use either type rocker on either type cam.

Sid's right about the Rockers.. by bad

Minh · Post by **Minh** » Mon Jul 28, 2003 3:35 pm

Regarding the head...

Becuase the upper timing chain's amount of slack is affected by the head being shaved. The head can not be shaved to much too.

You will get the dreaded "death rattle" if it did.

Post by **spl310** » Mon Jul 28, 2003 3:57 pm

Minh is right on about the rattle.

I would use the bottom end that was known to be in good shape. If it is the one with the flat top pistons, use the thickest head, if it is the dished pistons, use the thinnest head. Keep the cam followers with the appropriate lobe of the cam and move the B cam (if that is the one you want to use) to the head you want to keep.

Flat top pistons are the preferred design to promote proper flame travel, but the dished are OK for the street. Lots of folks will disagree there, but the flame travel issue is really only a burning issue if you are going for all out performance - not street performance.

Just my opinion - yours may vary.

Dave · Post by **Dave** » Mon Jul 28, 2003 11:53 pm

Guys, thank you all very much for the input!

No chain rattle here so if that head was shaved, it was minimal. I'll know more on that tomorrow when the machine shop calls.

To help me come closer to a decision of which head/block/piston combo to go with, can someone tell me what the compression numbers are for a "fresh" U20? I ran a test on mine just before pulling the engine: 134, 145, 136, 137. Hole number 2 had an obvious intake valve seal leak. That extra oil in the cylinder probably accounted for the slightly higher value. Aside from that, the values are nicely grouped indicating even wear. However, they all seem a bit on the low side. I would guess a freshly rebuilt U20 would be pulling in the 150 to 160 range. Am I off base here?

Thanks!

Dave

Post by **S Allen** » Tue Jul 29, 2003 9:18 am

A freshly rebuilt motor should run some where close to 175 lbs per cylinder plus or minus a few lbs.

Steve

SLOroadster · Post by **SLOroadster** » Tue Jul 29, 2003 3:12 pm

Dave,
Your compression is way soft. A standard bore U20 should be 166. I'm showing 180, 185, 190,180 on mine, however mine is bored .040 over, and my rings could be better due to bad jetting on my carbs for the last 1000 miles. (That issue has been taken care of, I no longer burn oil due to ring wash out.)
Good luck,
Will

Dave · Post by **Dave** » Wed Jul 30, 2003 6:54 pm

Thanks for all the great info guys!! The deeper I dig into this engine, the more problems I find. With a great group of enthusiasts out there though, I'm sure I'll have it sorted out soon.

Dave

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Need U20 Cam and Piston Info

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