piston stop tool OK?

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miker
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piston stop tool OK?

Post by miker » Thu Oct 10, 2019 7:51 pm

Would it be OK to use a piston stop tool on our DOHC engines to find TDC? I ask because it seems that to use this technique, one has to rotate the engine in both directions.
MikeR (mirafiori.com since 1995)


1977 Fiat 124 Spider
Previously owned:
2012 Fiat 500 Prima Edizione #236 (now owned by my son David)
'86 Bertone X1/9
'81 Fiat Spider 2000 #236
'78 Fiat 131 four door
'76 Fiat 128 4 door
'74 Fiat 128 4 door
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engineerted
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Re: piston stop tool OK?

Post by engineerted » Thu Oct 10, 2019 8:06 pm

Yes,and a degree wheel. It will let you find BDC the rotate 180 degree to TDC after the stop tool removed.
1978 124 steet car
1974 124 EProd race car
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fiatfactory
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Re: piston stop tool OK?

Post by fiatfactory » Sat Oct 12, 2019 6:02 am

Positive stop method of determining TDC is usually done when the cylinder head is off, and usually with something quite a large diameter (like the flywheel) mounted to give a degree of accuracy.

The Fiat DOHC engine HAS a TRUE TDC marker already... remove the front crankshaft pulley and on the crank mounted toothed timing gear there is a machined in scribe mark, this lines up with a cast in nub on the front seal carrier plate... more than accurate enough for fitting a timing belt, but not the level of accuracy needed if you were going to try and determine cam timing to within a degree of crank rotation.

Cam timing (generally) wouldn't be somethng you'd go about determining with the engine fully assembled and installed, too difficult to get a degree wheel of large enough diameter to get any accuacy to the results....if you didn't do it when you were putting the engine together, then you've sort of missed the boat on this one.

I'll usually determine full lift point of each cam, and scribe a mark(s) against the cam and the rear cam bearing housing, so when the engine is in, checking/ altering with accuracy is quite easy... again this full lift is much easier to determine with the head removed and a dial gauge on the valve head... working from the shim side makes it difficult to get the dial gauge dead perpendicular, and any variance (angle to the dial gauge) here results in an error of sine to the measurements.

The issue is always the dwell (both cam FL point and also crank / piston TDC) around the point your trying to determine.

SteveC
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miker
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Re: piston stop tool OK?

Post by miker » Sat Oct 12, 2019 12:05 pm

My application is pretty simple, so the cast in TDC should be fine if I read you correctly.

All I want to do is adjust my stock cams to the stock timing with adjustable cam wheels because I have a much thicker than normal head gasket due to having blown a stock gasket a few years ago. I have higher compression pistons that were installed when I was going to have cams with more overlap. I couldn't use these cams because the engine failed Oregon DEQ testing. So back to stock cams, but too much dynamic compression and a blown head gasket 20K miles later. Anyway, the thicker head gasket eliminated my detonation problem but left the stock cam timing a ½ belt tooth off.
MikeR (mirafiori.com since 1995)


1977 Fiat 124 Spider
Previously owned:
2012 Fiat 500 Prima Edizione #236 (now owned by my son David)
'86 Bertone X1/9
'81 Fiat Spider 2000 #236
'78 Fiat 131 four door
'76 Fiat 128 4 door
'74 Fiat 128 4 door
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friedman
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Re: piston stop tool OK?

Post by friedman » Sat Oct 12, 2019 1:53 pm

You could probably use the stock timing marks for the crank pulley for what you are trying to do. That is, using the timing mark plate attached to the front of the motor. But this has been an interesting thread regardless.
Carl in Virginia
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83 Bertone
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