Cam timing

[Jono]

More questions

I did a trial timing of my new cam last night - a Kent MD276. I am using a new Minispares lightened steel timing gear and chainset but I also have a new IWIS chain which I had intended to use until I discovered that the existing gears were not a good as I had thought (so I purchased the Minispares complete kit).

Kent say set maximum lift at 106 degree after TDC.

Having established TDC and then maximum lift (adjusting each for dwell) my cams times in at 101 degrees. A couple of questions for the wise:

1 Allowing for a bit of chain stretch is this good enough?
2 How much will the Minispares chain stretch (it's stamped AE)
3 Since I have already got it should I use the IWIS chain - if I do use it how much should I allow for stretch on this chain?
4 Should I correct the 5 degree error using an offset dowel or is it close enough?

I'm trying to be meticulous on this build as I'm treating it as an enjoyable learning experience so I don't mind spending time to get it as right as possible.

Cheers

Jon

[mk1]

Chain stretch should be minimal, I would expect less than a degree. I have always timed in to within a degree of what it should be & have never been disappointed. 5° is a heck of a way off for cam timing.

M

[dklawson]

Agreed.

You stated that you "adjusted each for dwell". How did you do that? To be confident in your measurements you need to always turn the engine in its normal direction of rotation so the timing chain slack is always taken up going one direction.

[ianh1968]

Having established TDC and then maximum lift (adjusting each for dwell)
my cams times in at 101 degrees.

You stated that you "adjusted each for dwell".
How did you do that?

Most likely by measuring the depth of a piston a nominal amount down the bore
on the up-stroke, noting the protractor reading, then wind the engine over the top
and then note the degrees at same nominal amount down the other side.

Split the difference between the two protractor readings to establish the "true" TDC.

For the cam, the method is the same - I generally go 3 thou either side of full lift and
split the difference. Best not to go too big on the readings here as the opening and
closing ramps on many cams are a different shape.

To be confident in your measurements you need to always turn the engine in its
normal direction of rotation so the timing chain slack is always taken up going
one direction.

Agreed.

5° is a heck of a way off for cam timing.

Also Agreed...
... and if you jump the chain one tooth, it will go to 4 degrees retarded...
(or another 9° advanced if you go the wrong way!) .

Mr V. reckons that 4° out is too much and anything more than 2° should be sorted.

Ian

[Jono]

Thanks chaps,

Yes, I found approximate TDC with the DTI and then zero'd the dial. Then took it an equal distance in each direction (2-3 thou on the DTI) by turning the crank and noting the degrees in each case. I then bent the pointer to the average of the 2 values and then rotated the timing disc to zero. True TDC - yes?

I then followed the same procedure by advancing the crank clockwise and watching the dial until it stopped rising somewhere between 100 and 110 degrees ATDC. Zero'd the clock again and did the same procedure as for establishing TDC. I have repeated this several times now and I keep getting a maximum lift of between 100 and 101 degrees. Kent say 106 for this cam.

It seems a lot further off than I would have expected and I thought I was doing something wrong but I have repeated it several times with similar results.

I should add that I'm doing this with the head off and the push rod on Nr 1 inlet sticking out of it's hole. It does occur to me that this might result in some error but it seems to be a method which is widely used.

Jon

[ianh1968]

<SNIP>Then took it an equal distance in each direction (2-3 thou on the DTI) by turning the crank and noting the degrees in each case.
I then bent the pointer to the average of the 2 values and then rotated the timing disc to zero.
True TDC - yes?

... possibly not, you must only turn the crank in ONE direction,
ie, the CORRECT direction, which will be clockwise when looking at the timing gear end of the crank.
Finding the "approximate" TDC then going both forwards and backwards will result in a
false reading. As you have identified, there is a certain mount of "wiggle" when the piston
goes over TDC.

This is what I do:
1) Get a dial gauge with a mag-base and stick it to the block
2) Wind the engine in the CORRECT direction until piston No1 is
about 0.100" from the top of the deck on the UP STROKE.
3) Set the dial gauge to read zero.
4) Note the protractor reading.
5) WITHOUT going backwards, keep winding the crank until the piston has
gone over the top and hits zero again, taking extreme care not to go past this point.
6) Note the protractor reading.
7) Split the difference between the before and after readings.
8) Now, keep winding the crank in the CORRECT direction until the calculated
TDC is reached. Again, don't go past it, you must stop exactly in the right place.
9) Now: SET YOUR POINTER. (Very carefully, without moving anything).

Throughout the whole process, you must ALWAYS wind the crank in the
CORRECT direction. If you accidentally go past the measurement point,
keep winding in the same direction until the piston comes up and goes
back down the next time.

Without moving the protractor, check your TDC using, say 90 thou down
and again with 110 thou on the dial gauge. If you've done the job correctly,
the calculated TDC will be where your pointer is already set to.

MY VIEW:
There is so much dwell/slop at TDC that 2-3 thou either side will not produce
an accurate reading to work out TDC - Go BIG...

For the cam, as I said before, 2-3 thou is what I use.

This is one of those jobs where if you don't do it accurately, you'd
might as well not bother doing it at all. It can take a while to do, but
if you want good results from your engine, persevere...

Ian

[Jono]

Brilliant, thanks Ian,

That all makes sense and I can see where I have probably gone wrong now.

I will do it this way tomorrow and it will be interesting to observe the difference. I will certainly persevere, I like to get things right and this engine has cost a small fortune so far I will not be spoiling the ship for a happorth o' tar as the saying goes.

Cheers

Jon

[smithyrc30]

I would also add that the DTI needs to be along the axis of the piston pin, not perpendicular to it. That way the piston rock in the bore as it goes over TDC is removed.

[dklawson]

... possibly not, you must only turn the crank in ONE direction,
ie, the CORRECT direction, which will be clockwise when looking at the timing gear end of the crank.

Exactly. That's why I asked about technique earlier. Only turn in the normal direction of rotation.

Let us know what you find re-checking the cam angle when only turning one direction.

[Jono]

Thanks guys,

Okay, could not resist checking this once more last night after Ian's post (and yes, DTI was placed in the gudgeon pin axis).

To my surprise the result is exactly as before give or take 0.25 of a degree. The cam times in at 101.5 max lift. That's after running the procedure 7 times using different methods and twice using the 'only turn it clockwise' method

I'm looking for 106 degrees so do I correct this by moving the chain one tooth (dots are currently lined up) and doing th rest with an offset key?

Cheers

Jon

[Jono]

correction - it currently times at 100.5 degrees so 5.5 degrees out

[mk1]

Get a 5° offset key this will get you within a Gnats nadgers of what you are after.

http://www.minispares.com/product/Class ... o%20search

M

[mk1]

Regarding always winding in the right direction;

If you do go a bit past a point you are after, you can always wing the "wrong" way 30 or so degrees & re-approach your target. This does save a lot of extraneous winding.

M

[rich@minispares.com]

the trick is to have a nice winding handle on the crank as well so you can get a real nice smooth wind

[R&R]

Jono, a tooth is 18 degrees, I would use a 5 degree offset key. All cams will respond to a bit of fine tuning, but it's an expensive business optimising it on a dyno, so use the Kent figure as your optimum. A couple of years ago, a guy came in with a grasstrack car that wouldn't perform, his cam timing was 24 degrees from optimum, it ran okay when it came in, but frightened him when we fixed it!

[Jono]

Thanks R&R - for some reason, and without fact checking it, I had it in my head that 1 tooth on an A series = 4 degrees

So, yes a 5 degree offset key is going to be the answer.

I still can't understand why it's so far out though - brand new Kent cam, brand new Minispares steel timing gears, chain etc. What would be the explanation or is this what might be considered normal (I suspect not).

But I guess I should just be pleased I have gone through the checking process and that I will be able to get the cam timed to the Kent recommendations. And, of course, I've learned something new!

Cheers

Jon

[mk1]

Jon,

5° is by no means extraordinary.

You are using a non standard cam & non standard timing gear, there is no way to manufacture them to any sort of tolerance when no one knows what other parts they are going to be matched with.

M

[Spider]

I've found the recent AE Chains rubbish, fit the ISWS since you have it.

I set my chain drive cams 1 - 2 deg advanced to allow for Chain stretch / settling.

Also, before checking the Cam Timing, I usually wind the assembly over at least 50 times to get an initial settling before measuring.

This is the handle I made up for winding them over

[Vegard]

That's a good idea!!

[dklawson]

Chris, I've said this to you before on another thread... you have a nice tool.