DTI Gauge for timing JX engine

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DTI Gauge for timing JX engine

Postby si macc » 10 Oct 2019, 19:53

Hi,

Struggling to find a basic DTI Gauge for checking the timing on my JX engine. Following the vince waldon post about timing IDI diesel engines and he states he used a 'cheapie'. I'm looked about on ebay/amazon but can't find one with 25mm travel (plenty of 10mm ones...). Anyone got any recommendations? Don't want to spend more than £30 unless really necessary?

Thanks

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Re: DTI Gauge for timing JX engine

Postby davidoft1 » 10 Oct 2019, 20:37

buy a used good one of ebay, the cheap ones really are junk
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Re: DTI Gauge for timing JX engine

Postby ZsZ » 11 Oct 2019, 08:09

you can approximate it with a digital caliper
I bouht this set later: https://www.ebay.de/itm/VW-T3-T4-Messuh ... SwA3dYDdcU
but this one looks better:
https://www.ebay.de/itm/VW-T3-T4-LT-Mes ... Sw~gRVkaM8
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Re: DTI Gauge for timing JX engine

Postby Mr Bean » 11 Oct 2019, 08:56

This has only just occurred to me reading this post but:
As far as I can work out measurement of TDC by piston displacement even using a digital depth gauge reduces in accuracy as TDC is approached:
Bearing in mind that the relationship between crankshaft angle and piston displacement is not linear, It just occurred to me that if you would remove the core from a spark plug and (very firmly as you wouldn't want to lose it down the bore ), fix a short bar into it so as to arrest the piston say an inch BTDC then gently rotated the crank back and forth between these two positions then a very accurate TDC position wound be found halfway between these positions. Or if no head present one could us a simple non-calibrated but fixed depth gauge.

Not sure if this is applicable on a diesel engine though what with having no spark plugs and it would require that the crankshaft sits centrally in line with the bore :!: See how complicated a simple concept can get... :roll:
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Re: DTI Gauge for timing JX engine

Postby tobydog » 11 Oct 2019, 09:13

Do you need a DTI with 25mm of travel?
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Re: DTI Gauge for timing JX engine

Postby Mr Bean » 11 Oct 2019, 20:03

tobydog wrote:Do you need a DTI with 25mm of travel?


Sorry Reading my post I may not have explained myself very well - this system doesn't need a DTI just a method of stopping the piston as it approaches TDC and a protractor. Here goes again:

A blank timing disk is attached to the crankshaft - the larger the diameter the more accurate will be the measuring.

The idea is to arrest the piston part way up the bore where the relationship between crankshaft rotation and piston displacement is closer to unity. This would be when the crank is 90 degrees to the bore axis but a reasonable distance would be say 25 mm BTDC. This could be done if the cylinder head is off using a tool similar to a carpenters marking and mortice gauge set and locked at an arbitrary position of say 25mm. If more likely the cylinder head is present a fixed length modified spark plug could be used. Note- fixed length.

So rotating the crankshaft clockwise the piston will be stopped at a position set by the fixed depth gauge. The disk should be marked against a fixed reference point on the crankcase.

Then the crankshaft would be rotated anticlockwise through BDC until it is arrested by the depth gauge again. The disk should be marked again against the fixed reference point on the crankcase.

The second mark will be a considerable number of degrees from the first depending on the length of the depth gauge which is as previously mentioned, arbitrary providing it is the same for both clock and anti clock marking operations.
A third mark can now be added exactly halfway between the two marks this being Top Dead Center. The disk can then be marked in degrees either side of the middle mark so as to indicate degrees before and after TDC. Again not sure if this would work on a diesel with no spark plug hole.
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Re: DTI Gauge for timing JX engine

Postby tobydog » 11 Oct 2019, 20:56

Mr Bean wrote:Bearing in mind that the relationship between crankshaft angle and piston displacement is not linear

Yep, sinusoidal 8)
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Re: DTI Gauge for timing JX engine

Postby si macc » 12 Oct 2019, 06:12

ZsZ wrote:you can approximate it with a digital caliper
I bouht this set later: https://www.ebay.de/itm/VW-T3-T4-Messuh ... SwA3dYDdcU
but this one looks better:
https://www.ebay.de/itm/VW-T3-T4-LT-Mes ... Sw~gRVkaM8
Thanks all for the advice. This one looks good as I need all those parts anyway. Cheers!

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Re: DTI Gauge for timing JX engine

Postby si macc » 12 Oct 2019, 06:13

tobydog wrote:Do you need a DTI with 25mm of travel?
Was wondering as it seemed so common to find 10mm ones, just that post states it so was following that. The kit mentioned about on has 0-10mm so must be OK.

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Re: DTI Gauge for timing JX engine

Postby davidoft1 » 12 Oct 2019, 09:46

I’m assuming you need the dti to set the injection pump timing ?
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Re: DTI Gauge for timing JX engine

Postby si macc » 12 Oct 2019, 10:27

davidoft1 wrote:I’m assuming you need the dti to set the injection pump timing ?
Yeah, that's right.

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Re: DTI Gauge for timing JX engine

Postby colinthefox » 12 Oct 2019, 16:38

You don't need 25mm of displacement, 10mm is plenty, but you do need a longish leg to reach inside the pump to the piston. If the leg's not long enough, just tape a bit of wire to it to make it longer. I use the depth gauge on my vernier caliper. With care it's accurate enough.
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Re: DTI Gauge for timing JX engine

Postby si macc » 12 Oct 2019, 21:12

colinthefox wrote:You don't need 25mm of displacement, 10mm is plenty, but you do need a longish leg to reach inside the pump to the piston. If the leg's not long enough, just tape a bit of wire to it to make it longer. I use the depth gauge on my vernier caliper. With care it's accurate enough.
Thanks, good to know!

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Re: DTI Gauge for timing JX engine

Postby tobydog » 12 Oct 2019, 21:45

Mr Bean wrote:
tobydog wrote:Do you need a DTI with 25mm of travel?


https://www.google.com/search?client=fi ... +a+machine
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