Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - Printable Version

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Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - Cloud9...68 - 02-26-2012

Well, I have a question about the very first step of the cam timing procedure. The first step involves getting cylinder #1 at firing <acronym title='top dead center'>TDC</acronym>. Normally, this is a simple task, because the 968 engine has a multitude of timing marks. But how do I do this with the engine on a stand, without the flywheel (there’s no room for it between the engine stand mounting attachment and the back of the engine) or the bell housing? The procedure in the manual, and the one written by Sparky, both require the flywheel to find firing <acronym title='top dead center'>TDC</acronym>.

I suppose I could mount one of my dial indicators such that it’s arm it touching the top of piston #1, (but to do this, I’d have to figure out a way to add an extension to it, as it’s arm is only about 1-1/2” long, and didn’t come with any sort of mounting bracket). Also, I’d have to rig up some sort of degree wheel to the front of the crank pulley, note the point where the dial indicator stops moving, keep turning the crank until the indicator starts moving again, and then determine the midpoint, because the crank turns quite a few degrees while the top of the piston remains stationary at the top of the cylinder bore. Is there an easier way to do this? Thanks.

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - DaveN - 02-26-2012

This sounds overly complicated, or may I missedd the mark <img src="/forum/images/smilies/968/rolleyes.gif" class="smilie" alt="" /> When my mechanic and I did the cams there was no mark on the flywheel (aftermarket) for <acronym title='top dead center'>TDC</acronym> so he simply stuck a long screwdriver in the plug hole and rotated the engine until the screwdriver went to the top of the arc. He then fashioned the paper cam lobe cutouts as per the Porsche manual and timed the cams by placing the paper template over the lobes until they fit the cutout. There wasn't a dial indicator even mentioned; I guess that's what 30 years of doing this will do, easy peasy <img src="/forum/images/smilies/968/wink.png" class="smilie" alt="" />

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - MB968 - 02-26-2012

This method should be more accurate than something on the top of the piston since there is almost no vertical movement of the piston/per degree of crank angle at <acronym title='top dead center'>TDC</acronym>. With the pan off, take the rod bearing cap off the cylinder that it at bottom dead center when #1 is at top dead center. Mount your dial indicator on the journal, rotate the crank, and the high point will be top dead center.

Thought I should edit this post and add a few more bits of info for those who may not have used a dial indicator a lot. Do your best to make sure the dial indicator is square to the direction of the movement your are measuring. Also if you have a choice of tips for the dial indicator, use the pancake tip (your aligment to the crank isn't as critical using this tip for this type of measurement). Or, if you have the option, use a "last word" indicator rather than a dial type. A last word has a dial too, but the measuring tip is a small lever that rotates rather than the rod that slides axially with the dial type.

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - Cloud9...68 - 02-26-2012

MB - Great idea! Out of curiosity, couldn't I do the same thing with the counterweight of piston #1? And with your method, why do you have to take the bearing cap off? The biggest pain in all this is going to be building the various hold-down attachments for my dial indicators, which didn't come with anything.

DaveN - There's a lot of debate (what a shock!) about how to time the cams on a 968, from using the paper cut-outs like your mechanic did, to using dowel pins, a pair of dial indicators (which is described in the manual), to doing nothing at all. Seeing as I'm (hopefully) only going to do this one, I'm planning to follow the most involved, and hopefully most accurate method, the dual dial indicator approach. This is the method Pete from RS Barn uses, and it's the one Lear35A has been using. But whichever method is used, the first step is always locating and maintaining firing <acronym title='top dead center'>TDC</acronym> in cylinder #1. This is extremely easy to do with the flywheel and bell housing attached, but with my engine on a stand, I don't have either one at the moment.

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - Dubai944 - 02-26-2012

Setting up a degree wheel on a stand is not that hard.

In that case you don't have to try and guess where the piston and dial gauge stop, you just need to measure an equal amount of dial gauge displacement either side of <acronym title='top dead center'>TDC</acronym> and adjust the degree wheel so it reads an equal number of degrees rotation at those points. Then <acronym title='top dead center'>TDC</acronym> is exactly at 0 on the degree wheel.

A thin piece of dowel with a rounded end will work as an extension.

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - Cloud9...68 - 02-26-2012

Dubai,

Thanks, but your post raises a few questions. First, any idea where to find a dowel with a rounded end? All the ones I've seen at Home Depot and such have straight-cut ends. And how would I attach the dowel to the plunger in my gauge? Just let it rest on top, with some pre-load? Also, I'd need to find a degree wheel, which, while the procedures don't call for it, would seem to come in handy, especially since my engine is on a stand without either a flywheel or bell housing.

Sorry about the similar posts in two threads; once I get the <acronym title='top dead center'>TDC</acronym> thing figured out, I'll move the discussions to the one started by Sparky a few years ago about the cam timing procedure.

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - MB968 - 02-26-2012

[quote name='Cloud9...68' timestamp='1330278520' post='122431']

MB - Great idea! Out of curiosity, couldn't I do the same thing with the counterweight of piston #1? And with your method, why do you have to take the bearing cap off? The biggest pain in all this is going to be building the various hold-down attachments for my dial indicators, which didn't come with anything.

[/quote]

Don't use the counterweght. The machined end of the counterweight is not necessarily concentric with the centerline of the mains, nor perfectly timed with the grind of the rod journals. The counterweights are likely turned on a crankshaft lathe when the crank is turned initially and not machined after that. Later in the processing, including any hardening/heat treating, then the mains would get their finish grind to size, and then the rod journals.

I'd invest is a simple base for the indicator which usually comes with a swivel joint and the rod that the indicator mounts to. I know you can't use a mag base on our blocks, but you could bolt a plate to the pan mounting surface and attach the a mag base to that. They also make heavy steel (indicator) bases that you could just set on the pan mounting surface. If you can get a c-clamp around it all the better, but probably not necessary.

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - Cloud9...68 - 02-26-2012

Yes, I'm on my way to buy some bases, either the magnetic kind (which are currently on sale at Harbor Freight), or the clamp-on kind. I've also gotten my hands on a piece of scrap metal plate which I plan to use as the mounting platform for my gauges.

But my bigger problem is with my gauges - they don't appear to allow the use of any adapters (I need two for the job - one to touch the top of the piston, which needs a long, straight extension, and one to tough the top of the #1 intake valve lifter, which requires a bent adapter to get around the cam lobe).

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - Cloud9...68 - 02-26-2012

Quick question - I know it's not a good idea to turn the engine counter-clockwise because the variocam assembly is designed to move only in the clockwise direction, but with the cams not yet in place, is there any harm in rotating the crank back and forth a few degrees in both directions to nail down <acronym title='top dead center'>TDC</acronym>? Thanks.

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - MB968 - 02-26-2012

With no cams in place, which has to mean no timing belt, etc. all the valves should be closed, so no worry hitting them. I can't think of anything else that would be a problem with rocking the crank back and forth about <acronym title='top dead center'>TDC</acronym>.

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - KRAG50 - 02-26-2012

Assuming the cams and distributer is out, no problem with small counter clock-wise rotation. I've attached pictures of the dial indicators and extensions. You need something stout for the actual timing as you will need to rotate the engine thru a couple of full cycles to really see the accuracy of the adjustment. The single rod is bolted to the head and the dial indicator attached to it, the other picture is the extentions attached to the indicators.

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - Cloud9...68 - 02-26-2012

KRAG,

I am exploding with lust and envy. Where the heck did you find those tools? I've spent the entire day either online looking for advice and ideas, and then going to probably ten different stores (auto parts, hardware, Harbor Freight, and even a bicycle shop) looking for something that could work as the types of extensions I need for my dial indicators, which only came with the standard ~1-1/2" plungers. I didn't even find what I would consider suitable mounting brackets. My frustration level at this point is about a 15 out of 10, as I'm dead in the water, unable to make any forward progress on my engine rebuild, which I started nearly a year(!) ago. Am I going to have to order this stuff online and wait another week?!

And is that a cam block barely visible in the upper left hand corner of the top picture? <img src="/forum/images/smilies/968/wub.png" class="smilie" alt="" />

Which procedure did you follow for the cam timing? The one in the manual, Sparky's, or did you consult with Pete at RS Barn? Other than moving heaven and earth to get my hands on the right freakin' tools, any words to the (not so) wise? Thanks.

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - KRAG50 - 02-26-2012

I think my indicators are good ones, so I could purchase the extensions and they have a bracket on the back side to mount a articulating rod. I made the mounting rods that bolt onto the valve cover mounting holes. If I were you, try a hobby shop for some hollow brass tubing to put over your existing short extensions, fasten by crimping or gluing. Just remember that the one with the angle can't be allowed to rotate.

Yes, up in the corner is a cam saddle tool, I used a hunk of aluminum, but for a one time deal, you could fab one from a good hard wood like oak or maple, the rods to force them down were half inch threaded rods with a cut off spark plug welded to one end, inserted into the two and four plug holes. a couple of washers and nuts, away you go.

I bought rebuilt cams from RS Barn and Pete gave me the help to get the timing done. just remember that you are installing the cams and belts with the motor at <acronym title='top dead center'>TDC</acronym> of the compression stroke, but doing the actual timing, with the dial indicators, on the intake stroke, something that isn't explaned very well.

Words to the wise, Just take it slow and be paitent, just the timing with the dial indicators took me the better part of a day.

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - Cloud9...68 - 02-27-2012

Thanks; trying a hobby shop for some brass tubing is a good tip.

I actually had planned to make a set of cam saddles out of hardwood, supported by a stout metal strip along the top, but got stuck (a common theme with this project <img src="/forum/images/smilies/968/mad.gif" class="smilie" alt="" /> ) when I couldn't find a threaded rod to use to thread into the spark plug threads as the rods used to force down the saddles. The thought of somehow utilizing an old spark plug crossed my mind, but I don't have a welder, nor know how to weld, so I've resigned myself to installing the cams without the saddles. I know, that's definitely sub-optimal, but others have done it, and I have no choice, other than spending a ton of money at a machine shop, plus having to wait for them.

Yes, according to Sparky's (a member here) write-up, I saw that the actual cam timing is done on the intake stroke, but thanks for the reminder.

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - Dubai944 - 02-27-2012

Using a degree wheel takes the guess work out of it and once you find true <acronym title='top dead center'>TDC</acronym> you can mark a fine scribe mark on the front crank pulley and cover which is far more accurate than lining up flywheel marks etc. Use that in future for belt changes etc

If you don't have a commercial version, you can make your own for just about nothing. There are plenty of free printable degree wheels on the internet. This one I have used previously is printed, mounted to stiff card and permanently fixed to a spare front pulley which I sloted out the bolt holes to allow it to be rotated for adjustment

El cheapo dowel extension. Dril a hole in one end for the dial gauge to sit in. I imbedded one of those plastic round headed pins in the other end to give a small, friction free point to sit on the top of the piston. Real extensions are nicer, but this works.

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - MB968 - 02-27-2012

[quote name='Cloud9...68' timestamp='1330308268' post='122454']

KRAG,

I am exploding with lust and envy. Where the heck did you find those tools? I've spent the entire day either online looking for advice and ideas, and then going to probably ten different stores (auto parts, hardware, Harbor Freight, and even a bicycle shop) looking for something that could work as the types of extensions

[/quote]

These are common industrial supply items. Any shop that does machining should have these. Here's a few places to try: catalogs or on line sites for Graingers, McMaster, Fastenall, or look up industrial supply houses in your area. Or if you know a local machine shop, even one that machines engine parts they could probably tell you where to get these items locally. Good Luck

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - Cloud9...68 - 02-27-2012

Dubai,

I like the dowel idea for the straight extension - I guess I'd just have to hole that's the right size for the standard plunger ti fit in snugly. That leaves the bent one - any ideas on that? I'll try places like Grainger and Fastenall, both of which we have in Austin, per MB's suggestion.

As far as the degree wheel, yes, that seems like the best way to find true <acronym title='top dead center'>TDC</acronym>. It seems that what I'd have to do, once I affixed the degree wheel to the crank pulley, is to turn the engine with the dial gauge in contact with the top of piston #1, and watch for the dial to stop moving. Note the position on the degree wheel relative to some easily identifiable mark on the front of the block. Keep rotating the crank, and note when the dial gauge needle starts moving again. Note how many degrees the engine has rotated since the dial gauge stopped moving. Take the midpoint between the two readings, and that's firing <acronym title='top dead center'>TDC</acronym>. Make a mark on the pulley and the block for future reference.

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - Dubai944 - 02-27-2012

It's easier than that.

You set up your piston at approx <acronym title='top dead center'>TDC</acronym> (no movement). You fit the degree wheel to the front, with a suitable fixed pointer. Set it to zero to start with. Then you rotate the crank a fixed number of degrees in one direction (for example say it reads 12 degrees ATDC at your pointer), enough to get unambiguos movement on the dial gauge. Read the dial gauge. then rotate the crank back throughTDC until you get exactly the same reading on the dial gauge going in the opposite direction. Read the degree wheel. If it also says 12 degrees <acronym title='before top dead center'>BTDC</acronym>, you are already spot on. Most likely it will be off, say it says 10 degrees. If so you rotate the degree wheel (not the crank) to 11 degrees. Now when you rotate the crank back through <acronym title='top dead center'>TDC</acronym> to 11 degrees the other side you should have exactly the same dial gauge displacement.

In practise it takes a couple of rotations back and forth, but once you have the degree wheel reading exactly the same before and after <acronym title='top dead center'>TDC</acronym> with the same dial gauge reading, you know you are 100% accurate.Then you have set the degree wheel and 0 really is <acronym title='top dead center'>TDC</acronym>.

As far as extensions, unlike us peasants in the nether regions of the world, don't you live in the United States of "I can buy anything online"? <img src="/forum/images/smilies/968/smile.png" class="smilie" alt="" />

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - Cloud9...68 - 02-27-2012

I think we're saying close to the same thing. In any event, I like the degree wheel method, as it should allow absolute location of <acronym title='top dead center'>TDC</acronym>.

As far as ordering dial gauge extensions online, yes, I could do that, but I'm in a bit of a race against time (the central Texas summer heat, to be specific - I know you can relate in Dubai), in that I want to make sure the engine is 100% finished and ready to install before it starts getting hot. This is because I'm going to need the help of a neighbor (whose engine stand and hoist I'm borrowing), and I don't want to ask him to help me install the engine when it's 95 degrees with 85% humidity. At the rate I'm going, with major unforeseen obstacles at every step (each of which is completely new to me), I'm starting to have my doubts as to whether I'll make it. So, I don't want to add another week to the process by having to order parts. Also, I haven't yet found any bent extensions online as of yet. I'll call some of the local specialty tool places in town tomorrow, and if they don't have anything, I'll build my own, using the ideas of the people who have responded here.

Finding firing <acronym title='top dead center'>TDC</acronym> with engine on a stand and no flywheel - Ryan - 02-27-2012

Cloud, can't you buy a straight shaft gauge and just custom bend the end? I found a good deal on gauges w/base at Grizzly Industrial. Too bad the magnetic bases won't work on aluminium.

It looks like the special cam block tool made for Porsche just uses a section of "all-thread" in the same size as the spark plug thread.

Hope this helps