28 replies to this topic

[purple_fly06]

Hi all,

 

I'm currently rebuilding my engine after having it machined at Rob Walker Engineering. It has been offset bored 73.5mm and I requested a deck height of 0.005".

 

The engine has been line bored, the rotating assembly, conrods etc. have all been balanced and the crank has been reground and tufftrided etc.

 

They would have had to do a dry build to achieve my deck height but now I have come to put it together something seems off... almost as if its been machined on a skew.

 

I'm getting varying deck heights, one piston is 0.016" down the bore when at TDC, 0.008" above the deck when at TDC (the other two are around 0.007" down the bore).

 

- My conrods weren't labelled 1-4 when they were returned to me - could this be a cause, shall i try swapping them?

- I'm going to use a MED steel shim head gasket which is 0.030" thick. So even with the 0.015" rod extension under revving there is still clearance.

 

Is this normal on a mini build to have such varying heights? Also when turn the engine I had a lot of trouble getting it to spin freely - I've put this down to the crank being tufftrided. I understand there's a lot of jiggery pokery when building an engine but to spend the best part of £1500 on machining for nothing to go together as expected is a bit annoying.

 

Just hoping there's something obvious I've missed?

 

Thanks,

Josh

 

 

 

[GraemeC]

Yes it is normal to have variation, both accuracy of the crank stroke rod length, and piston height above the pin.  Hence the need to dry build and selectively place the rod assemblies.

However 0.024" sounds a lot of variance.

 

Are you sure they've not been marked?  Maybe punched dots rather than distinct numbers.

 

The crank should rotate freely when just the main bearing caps are installed - if not check the caps are the right way round (tangs of the bearings go together)

It should then get progressively more difficult to turn as you add each rod, especially if the rings are fitted.  Check it after each rod - if it suddenly gets much stiffer to turn then check that big end.  Again - are the caps on the right way round (tangs of the bearings go together)

[Cooperman]

A couple of thou or so is quite normal, but with that amount of variation there would seem to be a problem.

In the first instance you need to discuss it with Rob Walker and take the short engine back to him for him to measure.

It is very unlikely, well impossible really, to be the pistons. 

It could be that the rods have been re-centred inaccurately and not as a set or the crankshaft is not accurately stroked. For high performance engines it is not unusual for the crankshaft to be 'stroked & indexed' to remove any differences in journal offst from centre line and to remove any angular errors. This is an expensive operation and not all machine shops can undertake it.

I believe the tolerance on rod centres as standard is no more than +/- 0.001"

[purple_fly06]

So the crank rotated fine with the main bearing caps installed and as you say it has gotten progressively tighter to turn as each conrod has been added to the assembly but with all 4 on it is still turning.

I’ve triple checked bearings are the right way around (tang to tang) but I am 100% confident that there are no markings on them I did visually check the grinding on each of them and that seemed okay. I just made sure that when I received them back from the machine shop I kept the correct end cap with each conrod (identical to how they were returned to me).

Is it worth trying the rods in different cylinders to see if it will average the deck heights out?

[Shooter63]

Make sure you have the conrod caps on the right way round, they can bind on the side of the crank journal, as others have said the deck heights seem all over the place, what type of pistons have you got?

Shooter

[purple_fly06]

Will do another check, but 99.9% sure they’re all good.

Pistons are Omega die cast 73.5mm, 7cc dish, standard height.

[Spider]

Is this normal on a mini build to have such varying heights?

Also when turn the engine I had a lot of trouble getting it to spin freely - I've put this down to the crank being tufftrided. I understand there's a lot of jiggery pokery when building an engine but to spend the best part of £1500 on machining for nothing to go together as expected is a bit annoying.

Josh

 

Getting Varying results in Crown to Deck Heights is something I found when last using AE Pistons and you'll likely find it will be different from one side of the pin to the other. I do match my Rod lengths to each other, and also measure every Piston for their Crown to Deck Heights. As I see just now, you are using Omega Pistons, the variation here will likely be in variations in Rod lengths, it's not unusual to find as much as 0.005" between them, unless you asked for them to be matched.


Reading one of your above posts re Tight to turn over, have you trial fitted each Rod to each respective journal 'out the bottom (that is with the Rod / Piston coming out the bottom of the block and not in the Bore) ? I do this as a matter of course when building, otherwise, with the Rods, it can be hard to pick up if they are right or not. I also check with Plasi-gauge, but that doesn't always tell the whole story. As it's getting progressively tight, as you fit each successive Rod / Piston, it sounds like it could be Ring Drag to me. As a further trail assembly, drop all the Pistons out, remove the Rings, and try it again.

[sonscar]

Foolishly I though things like this is what you were paying for,if you supplied all the parts the machining should have addressed this,or am I wrong?Steve..

[Spider]

Foolishly I though things like this is what you were paying for,if you supplied all the parts the machining should have addressed this,or am I wrong?Steve..

 

Normally, it is, but all the same, it's up to the assembler to check it all and send back what's not right, but then the machinist also needs clear instructions on what the expectations are otherwise, one will only receive back something that's likely within factory specs and not 'blue printed'.

[purple_fly06]

Foolishly I though things like this is what you were paying for,if you supplied all the parts the machining should have addressed this,or am I wrong?Steve..

I agree with you Steve, I think all of this should’ve been bottomed out by the machine shop. I used them as they were highly recommended by others on tmf. I was pretty annoyed that it took over 6 months to get my parts back!

Think my plan will be to go back and accurately re-measure all of the deck heights in 4 places each and average the measurements.
Do the other checks recommended by people on here e.g remove rings, check caps etc.

Then I’ll report back... fingers crossed

[ACDodd]

Remember you only get what you pay for. Is I on the Bill?

Ac

[purple_fly06]

Remember you only get what you pay for. Is I on the Bill?

Ac

 

Thats the exact reason why i went to them after looking at a few machine shops, I didn't want an in-experience machine shop taking the job on.

 

Specification list on the bill on the image below:

https://imgur.com/a/xIAL4x1

Edited by purple_fly06, 08 February 2021 - 09:22 PM.

[imack]

No mention of a dummy build there to work out how much needs removing from the block deck to end up with the appropriate piston/deck height.

[OzOAP]

Is Rob still there?

[purple_fly06]

No mention of a dummy build there to work out how much needs removing from the block deck to end up with the appropriate piston/deck height.

 

You would have to do a dummy build to find the existing deck height, otherwise you wouldn't know how much to skin off of the block to achieve 0.005". It was also discussed via email, they haven't put it on the bill but its a given. https://imgur.com/qsvDzgu

 

I'm going to recheck like I said earlier and try to find the average values for each cylinder and go from there. Just amazes me that it can be this far out when assembling - Working in engineering myself its quite baffling to pay for something for it not to be right... I'm hoping the average values will be somewhere near the mark.