How can I tell what cam is in my old 86 4.2?

They're sold by lift and duration. Check the lift at the rocker arm. If it run lumpy, it probably has a long duration - good luck measuring that!

-Brian

Mine is supposed to be a stock 1986 258 USA built engine modified for the Canadian market. No cat needed. No air pumps, etc....

No clue where the Jeep actually came from. It was hot purple/pink for it's first coat of paint. You know, a 'Barbie' Jeep.

None of the engines you list match mine, thanks anyway.

Mike

"L.W.(ßill) Hughes III" wrote:

I am misleading there a bit.

It is a US Jeep with the speedo in miles. The cat isn't needed up here, don't know what it had original.

Mike

Mike Roma>

You can do it and get pretty close Mike. The important numbers are how many degrees the LIFTER is above .050 lift. This is the industry standard for measuring camshaft duration. Advertised duration measurement methods vary between manufacturers, so I would be leery about measuring how many degrees the lifter is above zero lift. Some cam companies use a small amount of the lift to act as lifter preload, especially for hydraulic cams.

To measure the cam, it is best to have access to the lifter with a dial indicator, but you can get close with a hydralic one. You can also do the lift measurement at the valve, but then you are reling on the rocker arm ratio being correct, and you are also assuming that there is no part deflection anywhere.

Anyway, the recommended way to do this is with a cam degree wheel. Basically it is the protractor you talked about using, but is calibrated a full 360 degrees. This bolts to the harmonic balancer. If youhaven't ever seen a degree wheel, this is what it is:

The pro engine builders use a very large diameter wheel to get their camshafts degreed in as close as possible.

Once you have something like this, you can rig up some kind of a pointer for the wheel. I used to use a piece of a metal coat hanger and bend it to shape. For your purposes this will be plenty accurate. Now you need a dial indicator to measure the lift at the valve or at the lifter. If you do it at the valve, you need to know the rocker arm ratio for your engine. I will assume for my explanation that it is 1.5. If you are meauring the valve lift you will need to turn the engine in its normal rotation and take a reading on the degree wheel when the valve reaches .075 lift. You could crudely do this with a dial caliper as well by meauring from the top of the valve spring retainer to the valve spring seat on the head.

Record the reading on the wheel. I will use 5 degrees BTDC as an example. Now turn the engine until the valve is at maximum lift and record that. (.450 for my example). Rotate the engine until the valve comes down to .075 lift again and record that number (25 ABDC for my example). You can now determine the intake lobe and where it is reaching peak lift. Take the BTDC number, add 180, and add the ABDC number to get 220. This is your duration at .050 tappet lift. To determine the lobe center angle, divide the duration in half (110) and "add" that to your intake opening point. (5 deg BTDC +110 rotation = 105 deg ATDC) this would mean your cam lobe reaches peak lift at

105 degrees ATDC. Cam lobe lift is simply valve lift divided by rocker ratio, so .450/1.5 = .300 lobe lift.

Repeat the procedure for the exhaust valve. I will use 40 degrees BBDC for valve opening. Now turn the engine until the valve is at maximum lift and record that. (.450 for my example). Rotate the engine until the valve comes down to .075 lift again and record that number (10 ATDC for my example). You can now determine the intake lobe and where it is reaching peak lift. Take the BBDC number, add 180, and add the ABDC number to get 230. This is your duration at .050 tappet lift. To determine the lobe center angle, divide the duration in half (115) and "add" that to your exhaust opening point. (40 deg BBDC +115 rotation = 115 deg BTDC) this would mean your cam lobe reaches peak lift at 115 degrees BTDC.

The last thing you need to determine is the lobe separation angle. This is simply the average of the 2 lobe center angles, so (105 + 115)/2 = 110. This is the split betwen the lobes. You can also figure out that the cam is installed 5 deg advanced by subtracting the lobe separation angle from the intake lobe centerline, or 110-105.

Now that I typed all of this, I see it may be easier to pop the rear cam plug out of the block and see if there is a part number there ;-). Many of the manufacturers put the number on the back, but some put the number on the face where the timing gear sits, so this is a bit of a crap shoot.

Chris

I do understand the geometry you are talking, but I was looking for something more basic. You know, valve starts moving here, stops there. Regular cam has so much, modified cam has something else.

No special tools besides a plastic protractor needed.

If they all start and stop the same, then the upper movement is the way to figure it like you say.

Popping the back cap off makes perfect sense!

There are no numbers on the front.

Mike

c wrote:

Cool, that's the back side?

Mike

"L.W.(ßill) Hughes III" wrote:

Yup it is. Notice there are no bolt holes or locating pins/keys for the timing gear.

Chris

Ok, it will be worth popping the cap out to see what this one says.

I might have a sweet motor hiding in my garage....

Mike

c wrote:

I can decode many of the numbers Bill. If he posts them here, I will see what I can do.

Chris

Thanks. I am off bush running for a few days. I will see if I can pop it apart when I get back.

Mike

c wrote:

I don't have any of the stock AMC stuff, but over the years of building engines I've aquired a lot of the numbers and decoding steps from the aftermarket cam makers. The one thing you have to be careful of on the cam numbers is that there is a number for the cam blank, and then a grind number. The cam blank number is useless to you and I because one blank can be used for several different grinds, and to make matters worse, some of the smaller cam grinders buy their blanks from the bigger cam companies.

Chris