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Borescope/finding V6 TDC

Mike Armstrong

Bronze Level Sponsor
I always wondered how accurate my 33 year old damper and pointer could be. To me, finding true (I like to be exacting) TDC was a PITA. The #1 cylinder plug hole angle made using a piston stop a guessing game, same with bump turning the engine using the starter till my finger blew off the plug hole. I wished I had marked true TDC when I had the heads off.

Anyhoo, I had forgotten about that dusty borescope I had hanging on the wall, the one I used to scope a cylinder that I was 100% sure a valve cover screw had dropped into via an open plug hole (I was wrong, thank God). So I pulled #1 plug, disconnected the coil, slid in the borescope, turned off the lights and was able to rotate the engine by hand using the fan blades. This allowed me to see exactly when the #1 piston reached the very top of the cylinder (kinda cool to rotate through full cycle and see the valves and piston moving :)). This allowed me to see exactly where the pointer was lining up on the damper marks. Low and behold (or not surprisingly) it was off, by ‘perhaps’ several degrees.

https://photos.app.goo.gl/GQbypkSfYo8MgLSN6

The first photo of the damper is just for clearly showing the timing marks. The second is of the borescope. The third is to show where the pointer ended up when exactly at TDC (the yellow mark is 10 DBTDC and the red mark is 12 DBTDC). The pointer looks to be ‘after’ TDC, by how many degrees?
 
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260Alpine

Silver Level Sponsor
Mike, It looks to be a couple degrees after. Your 10 degrees is probably about 8 and 12 probably about 10.
 

260Alpine

Silver Level Sponsor
I was going by the notch in the picture on the right. If you go by the first line past the notch at "0" it would be closer to 4 off. You may want to get a rebuilt damper from Damper Dudes or Damper Doctor.
 

Mike Armstrong

Bronze Level Sponsor
Yup, I thought I t was closer to 3, maybe 4 degrees off, that’s allot. I’m surprised it doesn’t leak oil, but the seal is newer. I was just going to go to my local auto parts store, buy some timing mark tape and line it up to the new TDC mark (the white dot).
 

Bill Blue

Platinum Level Sponsor
Mike, I have to wonder why the piston stop method, which is quite accurate, was a failure. The angle of the stop should have nothing to do with the procedure. The usual method is to set the stop so rotation ends at something before TDC, say 20 degrees. Mark the stop position, reverse rotation and mark the second stop location. Halfway between the marks is TDC. Is this what you did?

As you approach TDC, crank movement becomes very large in relation piston movement, hence the need stay away from actual TDC .

Bill
 

Mike Armstrong

Bronze Level Sponsor
Hi Bill.

Bill makes a good point. At TDC, there is indeed play on the crank, probably more so as an engine wears. Using the borescope, I can see the piston rise and stop at TDC, but I can also feel the crank continue to turn with no ‘noticeable’ movement of the piston. To account for that, for several times I would visualize the piston at TDC using the borescope then rotate within the ‘crank play’ without the piston moving and try to hit the center of the ‘play’.

Question, is it normal for the piston to have ‘hang time’ during compression while at TDC before moving back down the cylinder? If so, should TDC line up on the damper when the piston first reaches TDC, and not anywhere during ‘crank play’ before the piston heads back down the cylinder?

As far as the piston stop. It’s been awhile so I don’t remember what vendor I got an apparently sub par piston stop indicator from, but when in place it would hang up and jam before the piston came up, so I took it that the angle into the piston was keeping a piston stop from working. I just ordered a different brand nicer looking one that looks like it should work better.

Like Bill said, rather than try to calculate while in that TDC ‘area’ it would more accurate to pick a point during piston travel and make your marks to find TDC.
 

260Alpine

Silver Level Sponsor
Mike, I like using a dial indicator before the heads go on. If you use a timing tape it has to be for the same diameter as your damper to be accurate. I would check your timing mark when it first comes to TDC and see how that lines up compared to slack at dwell time. It may be closer or even further off. Maybe that is how the factory does the reference mark?
 

Mike Armstrong

Bronze Level Sponsor
Mike, I like using a dial indicator before the heads go on. If you use a timing tape it has to be for the same diameter as your damper to be accurate. I would check your timing mark when it first comes to TDC and see how that lines up compared to slack at dwell time. It may be closer or even further off. Maybe that is how the factory does the reference mark?

Thanks Jim, will do.

This an interesting topic. Just talking fractional affects, especially on stock engines but, Googling I found this quote on a Ferrari Forum....

‘It's a minor difference but can be important when you consider how small the tolerances are. In every other vehicle or enviornment I have been exposed to TDC is defined and measured as the point halfway between when the piston arrives at its highest position and when the piston leaves its highest position, or the middle of the dwell period. Ferrari defines TDC as the exact point at which the piston arrives at it's highest position when the motor is turned in it's direction of rotation, and depending on a variety of variables those two places can be a couple of degrees different.’

I think this is referring to what Bill was saying and along the lines of ‘crank play’ or slack time before the piston heads back down. There is a brief period of time the damper (crank) is turning but this piston is not moving. That would seem to lead me to easily mark TDC on the damper in error as I tried to estimate that halfway point by turning the damper back and forth.
 

Mike Armstrong

Bronze Level Sponsor
So, I received a TDC Indicator in the mail. It screws in and the shaft that extends out has precise gradient marks. Nice and smooth action made it easy to rotate the engine 16 degrees before and after TDC. I ended up at precisely the same mark on the damper as when I had eyeballed the piston using the borescope. Measuring the distance between that mark and the O degree mark and comparing that to the hash marks on the damper, it measures out to be 4 degrees. That would mean I’ve been setting my timing 4 degrees farther ‘after’ TDC than I had realized, ie, instead of 12 degrees BTDC I’ve actually been setting it to 8 degrees BTDC, really? Can the damper be that far off? geez.

https://photos.app.goo.gl/aF1BdaM1sfDKj853A

(Ya, I received the Indicator with the optional long shaft, but it still fit :))

Ok, now I’m curious. Next I’m going to use a piston ‘stop’. If I end up at the same mark for a third time then there’s no doubt. I’ll still be surprised. It’s not like the damper was loose fitting and the seal was new.

EDIT 1: The other thing is it may not be that the damper is off at all, but instead, it might be the pointer. After all, that’s what I’m using to show where to mark the damper. It’s non-adjustable but can be bent or perhaps sloppy where it screws into the block.

EDIT 2: It’s not the damper that is off, it’s the pointer. I removed the pointer, which had been on there since I painted the block years ago. Since I had no recall of there being on slop on it when it was bolted on I didn’t think it was ‘adjustable’. First of all, the bolt holes on the pointer are larger than the bolts, so intentionally or not, there is enough play to slide the pointer 2-3 degrees. Secondly, though it didn’t look like it, the pointer had a slight bend in it toward ATDC. Combined I think that’s where the 4 degree mismatch to the marks on the damper was coming from.

Anyhoo, finding TDC is one thing, but after doing so you also have to line up the pointer to the O degree mark on the damper....of coarse, duh. The pointer is the variable, not the damper.
 
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260Alpine

Silver Level Sponsor
Mike, I've seen dampers off by quite a bit. Sounds like your pointer was the issue here. At 8 degrees it should still start and run OK. 10-12 you may have sharper throttle response. If you notice pinging you may have to back off a little anyway. Time it with a vacuum gauge for highest steady reading and see what it is. Did you fix the hard cold start issue?
 

Mike Armstrong

Bronze Level Sponsor
Mike, I've seen dampers off by quite a bit. Sounds like your pointer was the issue here. At 8 degrees it should still start and run OK. 10-12 you may have sharper throttle response. If you notice pinging you may have to back off a little anyway. Time it with a vacuum gauge for highest steady reading and see what it is. Did you fix the hard cold start issue?

Will do about the timing and vacuum.

Nope, never got rid of the starting problem, especially when cold. Sometimes after many attempts, a single short shot of starter fluid and it starts right up just fine. Also, the hesitation when giving gas and frequent muffled popping out the exhausts has never gone away. Tried many things over the last couple of years without change. It’s always eventually started and runs fine on the way to the occasional car show and back so the modified 2150A is still there. But I’d be willing to bet all those annoyances would be gone with a carb change.
 

Mike Armstrong

Bronze Level Sponsor

My only concern with that carb is the 350 cfm rating for use on a stock (other than headers) 2.8 engine. The stock Motorcraft 2150 1.08 venturi carb is rated at only 287 cfm. I’ve read that, for a stock 2.8, 350 cfm is too much carb and that it’s very common for guys to bolt on too much carb onto a stock engine that can’t move enough air for it.

For a direct 2100/2150 carb swap on a stock engine I’ve read that the Autoline C8072 is the carb to get.

https://www.autozone.com/fuel-delivery/carburetor/autoline-carburetor/381198_0_0_3498
 

260Alpine

Silver Level Sponsor
Mike,I've used the Holley 350 on two 3.4 GM V6's and my son's 2.8 V6 Ranger. The GM motors both had cams and headers and they worked well out of the box. The 2.8 we jetted down for the stock motor and it worked great. It has been about 10 years and I don't remember the jet #.
 

Bill Blue

Platinum Level Sponsor
I don't understand. Why, at this point, are you concerned about exact TDC? That is info needed during the engine assembly. Now is the time to give the engine what it needs to perform. If actual initial advance is 8 degrees instead of 12, do you really care? I would not, as long as it is what the engine likes. If adjusting timing +- 5 degrees around the recommendation does not improve the situation, ignition timing is not the problem.

Bill
 

Mike Armstrong

Bronze Level Sponsor
I don't understand. Why, at this point, are you concerned about exact TDC? That is info needed during the engine assembly. Now is the time to give the engine what it needs to perform. If actual initial advance is 8 degrees instead of 12, do you really care? I would not, as long as it is what the engine likes. If adjusting timing +- 5 degrees around the recommendation does not improve the situation, ignition timing is not the problem.

Bill

Hi Bill. Ya, it was just out of curiosity and looking for anything that might cure my engine annoyances. Just thought I’d check and confirm my suspicion that indeed it was off. I agree, whatever the initial advance is that seems to make the engine happy then so be it. I had just set it to what ‘I thought’ was 12 degrees assuming that would be best for the engine. Now, regardless of where it ends I’ll pay more attention to what the engine ‘wants’ as opposed to what a I think it needs.
 

Mike Armstrong

Bronze Level Sponsor
Mike,I've used the Holley 350 on two 3.4 GM V6's and my son's 2.8 V6 Ranger. The GM motors both had cams and headers and they worked well out of the box. The 2.8 we jetted down for the stock motor and it worked great. It has been about 10 years and I don't remember the jet #.

Good to know Jim thanks.
 

260Alpine

Silver Level Sponsor
Holley jet # are different than Autolite. I looked up a few instances where they put 350 Holley's on stock 2.8 V6's. Stock jet is 61 and they went down 4 to 57. That rings a bell as I think we went down 4 sizes by checking plug color.
 

Mike Armstrong

Bronze Level Sponsor
Holley jet # are different than Autolite. I looked up a few instances where they put 350 Holley's on stock 2.8 V6's. Stock jet is 61 and they went down 4 to 57. That rings a bell as I think we went down 4 sizes by checking plug color.

Got it thanks.

So the cfm difference from stock is not an issue as long as the jet size is reduced? Due you prefer Holley over the Autoline stock replacement?
 
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