Stuart D10 reversing gear timing – eccentrics

[Matt Stevens 1Participant @mattstevens1]

[Matt Stevens 1Participant @mattstevens1]

Hi All,

So I am in the process of finishing a Stephenson's reversing gear set (from Stuart) for my D10.

Generally speaking I thinking my machining is pretty accurate and I have the eccentrics pinned at 120 degrees apart per the instruction. I did this by marking the first eccentric out using trigonometry, drilling, then placing this against the second eccentric (flipped over)to drill that one.

Anyway – when it comes to setting up the timing, I have adjusted the eccentric strap to expansion link rods such that I get equal travel in both the forward and reverse settings…..next I have set the eccentrics such that the valve timing is correct when say in the forward direction – that is – just before TDC, the top port starts to open to cushion the piston travel and likewise at BDC it is also correct (ish/valve may need filing slightly to get it totally accurate)…..anyway, now when I switch to reverse and turn the crank over in the opposite direction, the timing looks to be way off….like about 60-70 degrees off! Why?

I can only conclude that the eccentrics are not set correctly. I can understand a couple of degrees in accuracy, but so much?!?! I have seen the principle of sliding eccentrics such that you can fine tune, but is this practical on a D10 given the size? Am I going to have to make new eccentrics as my existing ones are nicely pinned together with red Loctite. Maybe heat can separate them?

Any other advice?

Thanks

Matt

[JasonBModerator @jasonb]

120 apart gives a bit more lead (30deg) than I usually use but should still be OK

If you look at the engine from the eccentric side when the crank pin is at the top nearest the cylinder you should have the max throw of one eccentric at about 4 o'clock and the other at about 8 o'clock

Sounds like you may have put the eccentrics on 180 out of position.

[JasonBModerator @jasonb]

Or with the craknpin at the bottom the max throws should be at 10 & 2 O'clock positions with the 120deg between them.

This is for the No 1 but D10 is the same setup

[Matt Stevens 1Participant @mattstevens1]

So just as feedback….i set the other side at 180 degrees out after playing around with the timing, and i believe that 180 degrees is correct which in my head makes sense anyway…..i'd like to understand where this 120 degree theory come from!

I just hope i can see the first side again using heat other wise i will have to make new Eccentrics

[HopperParticipant @hopper]

A propane torch should weaken red Loctite, unless you used the high temp stuff.

[JasonBModerator @jasonb]

As a general rule of thumb the valve should lead the crank by 90degrees plus the lead which is between 10 and 30degrees.

Looking at that pic I posted if you imagine that the crank pin is a BDC eg 6 oclock and that the crank is rotating clockwise then the back eccentric is a 10 oclock. with 30 degrees of a circle per hour that is 4 x 30 =120deg which is the same as 90 + 30 so your lead is 30degrees.

Do the same for the other eccentric with the crank turning anti clockwise and you get another 120degrees.

Therfore 360 – (2 x 120) =120degrees between the two eccentrics.

If lead was 20 degrees you would have 90+ 20=110 deg each side, 360- (2×110) = 140deg between eccentrics

[Author]

Posts