Eccentric straps

Hi all,

Looking for some advice on eccentric strap mounting. Have fitted the eccentrics to the crankshft and started to fit the eccentric straps. First one HP forward went on easily turns quite good with only coarse lapping compund so far. Next one was HP reverse - completely different.
Mounting each half dry to check fit I noticed that both straps barely fit, i.e. I can get them on but I cannot turn them, started to look for any binding spots and with repeated appying of lapping compound now am able to barely move each half by itself. As soon as I put them both on even with the bolts just stuck on I cannot move them anymore.

It looks as if the inside diameter of the strap is a bit smaller than the outside diameter of the eccentric. With only the ends on and a small gap still left on the „middle“ part I can turn them but when I press them on by hand they stick and you need considerable force to remove them, but it feels like it adheres only with the oil/grinding paste not as if the parts deform elastically in any way.

Checked with the LP straps and they seem to be the same

Right now I have the crankshaft on my workbench with two v-blocks so I can rotate the crankshaft, but I have no real leverage to keep it from rotating. I was thinking about building a lever to fit to the strap in place of the eccentic rod,

How did you do that fitting? Any trick which helped you to get along.
Spend now around 4 h on that one strap only and it seems to be as bad as in the beginning
Thanks

Cheers

Elwood

I put the eccentrics in my lathe and spun them and slowly tighten them up as the compound ground them in they are all as smooth as silk now

Hi Mark,

Thanks for this idea, was thinking about that, too, but having no lathe this might not work for me

Cheers

Elwood

do not any of your friends have one ? or a local club

Persistence. It might be just a little bit of stickieness. Do you have some engineers blue to use to find the binding areas that need addressed. You might find it is one little spot and give it a little scrape with a squared off sharp edge.

I used the 40, 60 ,80 and 120 timesaving compounds and got my eccentrics in to good order from totally bound up to silky smooth in about 12 hours of work.

Hi Lynn,

So just 8 h more
No engineers blue but used a felt tip pen and was looking for high spots.

So I might just have to get on again  

Thanks

Elwood

If it's any help to you, I used a lathe dog to turn my crank whilst lapping in the crank bearings, you could use the same method for the valve eccentrics, on the engine. They are quite cheap, and available in many sizes. I just put a piece of scrap ally between it and the crank when tightening up.

Mark.

If they are a long way off fitting it could be a manufacturing error - they do happen.
Ask STW to send you a replacement strap, bit of a bind having to wait for it, but at least the engine will finish up being built to "standard".

Hi all,

Thanks for all the advice.
I would not say they are completly off as such, only they are tolerance-wise on the low side. If pressed on by hand the strap will stick to the eccentric and with some fluid like oil or lapping paste in between will just be „glued“ on. If only the ends of the strap are on the eccentic and there still is a gap of some mm in the middle, I can (barely) turn the strap but if I push a bit more it will settle onto the eccentric and just lock up.
No need to do both halves already as I cannot turn either of them easily anyways and with both on and the bolts just in but not tightened it just locks up.

What I am trying to do (following Mark‘s suggestion) is to build a fixture to be able to turn the eccentric (removed from the crankshaft for more room and easier handling) by hand and steady the strap at the same time with the other hand. Hope this will get me on the right track.

At this moment, with the crankshaft won the workbench I just do not have a lever to turn the strap and/or the eccentric for the needed movement, like roast on a spit. Was even thinking about remounting the crankshaft to the bearings and fit the flywheel for leverage, if my „hand-spun lathe“ is a failure this might be the next try.

Cheers

Elwood

Elwood
If you remove the eccentric from the crank, clamp it with a suitably large "G" clamp on the sides and then clamp the "G" clamp spine in the workmate.
You should have enough room around the clamp to fit and turn the eccentric strap at this point.
As for a handle to turn the strap a simple piece of flat bar around 300mm long with 2 holes in one end bolted to the strap using the connecting rod bolts supplied.
Bedding the bearing should be done using a back and forward motion as round and round risks scoring circular grooves in the strap/bearing surfaces.
As Lynn said patience is the key to this part, it took me a couple of good and long days to get spot on but its worth doing properly first time and not having to re-visit later when you find it wont holed oil and hence runs hot.
Just for info my straps had bound up on the internal radius of the strap it went from bad to good all of a sudden and not gradually as I had expected it to.
Don't forget to clean all of the parts thoroughly (I used cellulose thinners) on completion of each lapping to remove all the lapping residue, then trial fit with a little oil before moving on to further lapping or reducing the grit.
Simon C

Hi again,

Update:
another 2h on HP reverse eccentric strap and, lo and behold, almost done.

Lynn you were right - Persistence - first removed the outer eccentric (HP forward) as this was working already - just to have more room. Covered the V-Blocks with cloth, my hands with gloves and started with one half only, gripping the conrod throw of the crankshaft, After about 1 h I could just about turn each of them, put them together and started to turn again.

This time, and with keeping the straps centered so they would not bind, managed to get the nuts on and to start to tighten the two strap bolts up, 1/6 of a turn each time, adding more lapping compound ever so often until just now the  two straps parts finally did touch and I can still turn the strap, with alot of effort, but I can.

Tomorrow some more and I should have finished 2 of 4 straps with coarse lapping compound, so just another 2 rounds each with medium and fine lapping compound

Thanks for bearing with me and for all your good advice.

Cheers

A reliefed Elwood.

PS My rule is to buy a tool if I missed it 3 times and was frustrated because of that. 2 times so far I missed a lathe...  

PPS@ Simon, just saw you post as I was finishing mine. Yes that would have been the next step; I just wanted not to give up and have another try this evening, and, as per above, it worked. Maybe the temperature changed so it loosened up a bit - joking - but seriously the c-clamp - vise fixture or something similar would have been the next step. All day long when off the phone and taking a break, I was sketching along on a piece of paper, let my mind wander to do a one person brainstorming session...

And yes I did as you suggested a rocking motion forward and back all the way around in kind of a pilger step truning. But, again this was only coarse, medium and fine still to come.

Thanks

Update - Feedback
Job done . Persistence was the key word. After about 10 h so far I have 2 of the four straps lapped in with fine lapping compound, 2 more and the big ends of connecting rods need to be done with medium and fine, coarse is already done. Made myself a little strap as a handle just as Simon has suggested, that did the trick.

Thanks to all.

As a in-between I tried to test fit the connecting rods to the crossheads to check sideways position of crankshaft but having trouble to fit the Grudgeon Pins into the cross heads as they are more like a press fit than a sliding fit as I would have assume. Don‘t know how tight or loose the connection should be, will see.

Thanks again

Cheers

Elwood

Hi

You need a nice bearing fit on the little end bushes. So I would check the fit there before adjusting the gudgeon pin fit in the cross head. Might just need a polish up with 800 wet and dry

Hi Lynn,
pin to cross head is tight but manageable by hand, i.e. I can push the pin into the cross head by hand.
Truoble is with the bearing at the conrod, could not ge the rod into little end.

Elwood

Hi

Using a fine half round file or a reamer of the correct diameter you can fit the little end bush to the gudgeon pin. The crosshead fit is ideal as you do not want the pin rotating in the crosshead.

Hi Lynn,

reamer it is then I assume. will have to ask my son if I can borrow a reamer from the company he works at.

Thanks

Elwood

Hi Elwood
If a reamer is unavailable then an old trick a learned years ago is to use a drill bit of a slightly smaller size than the hole in the bush and wrap the shank with fine-ish wet and dry paper then use this like you would use a draw filing an edge.
don,t forget to clean thoroughly on completion as you do not want carborundum dust in the bearing surface.
Simon

I put a small roll pin into the cross head and a small nick on the gudgeon pin so the pin cant rotate

Hi all,

Finally I can give you a thumbs-up on that.
Eccentric  and big end bearings lapped in using fine lapping compound.
Checked little end pin dia and came up with 14,00 mm on my micrometer - spot on.
Got ahold of a reamer 14H8 so 14,00 to 14,027 mm nominal.
Used reamer with plenty of cutting oil => pin can be pushed into bushing by hand, but no play whatsoever -just what the doctor ordered.
Still a interferance fit between pin and cross head, built myself a little „press“, using a long M6 bolt and two sockets , one outside dia less than 14 mm the other with inside dia larger than 14 mm so I can press the pin in and out with slide bars and valve gear mounted, at least I hope.

In the end all came together with your help, thanks for that

Cheers

Elwood