Assessing an unknown engine

[hashman]

Great work rob and great reading.
Its another good reference for the self builder.
Thanks for recording it.

Karl

[RobFrost]

Seeing that reminds me I have a Flexhone suitable for 84mm cylinders if you want to borrow it?

Very kind of you Gary. I'll bear that in mind nearer the time.

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[RobFrost]

Where the magnesium oxide was dissolved it all looked so good yesterday. After a run through the dishwasher I appear to have outdone myself with this very special brown finish.



"A little knowledge is a dangerous thing" I hear you cry and I have to agree that might just apply to my chemistry skills.

Tentatively I'd say the magnesium chloride dissolved easily but left very fine iron oxide behind. Running that through the dishwasher turned it to brown iron hydroxide.

I didn't get a photo but the iron cleaned up relatively nicely with a fine brass brush. I'm in two minds now whether to wax the magnesium parts or get them coated.

I previously got this lovely old dial / bore gauge.



German too by the looks of it. I wonder if it's measured an old 911 before.



I find pleasure in using these old things - maybe it's a bit like driving an old Porsche. I have no clue if a modern one is made so well, but look at this tiny ball bearing on the end of the probe.



Anyway I digress... the purpose of the dial gauge tonight was to measure the crank runout. I dropped bearings 1 and 8 into the case and placed the crank on. My starting objective was simply to work out how I was going to mount the gauge rigidly ro the case, to get a good reading. But I soon established (to my surprise) that I could simply pin it down with a finger and get accurate and repeatable measurements.

Runout at the centre journal was 0.035mm - inside the spec of up to 0.04mm.



That was something of a relief, because these 66mm cranks are getting pretty rare now, especially with full size journals. And I want a short stroke engine rather than going to 70.4mm.

It says in the book you can measure it at the flange so as to exclude journal out of round from your measurement, but I don't know what flange that means and how it is possible.

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[Bruce M]

I use modern (gentle) wheel cleaner on magnesium crankcases. The stuff that creates purple streaks when used on wheels as it dissolves the iron brake dust residue. Not the old “wonder-wheels” stuff though which is nasty strong hydrochloric acid.

[911hillclimber]

Wonder Wheels...used to use that a lot some years ago, stings on the skin!

Autoglym wheel cleaner seems to be just right.

Not sure you can get chromate coating on magnesium today, chemistry may be too evil for today's world?

[RobFrost]

After checking the runout yesterday, I downloaded a spec sheet for the crankshaft. According to that, the 0.04mm spec in Dempsey only applies to the later crank, with the 66mm crank having a .03mm runout limit.

After a little research on t'internet I found this guy smashing his crankshaft with a steel cold chisel.

https://youtu.be/DjqBbYyaNXk?start=100

I think we all know where this is going...

I contemplated getting a brass chisel, but after a little reassurance from Pelican forum that this method is tried and tested on 911 cranks, I rounded off the end of this cold chisel.



I got it just right to match the radius either side of the crank journals.



The method is; you find a high spot on the crank, you rotate the crank to put that to the bottom then you strike into the corners of the journals either side. By the powers of voodoo, this lifts the journal in-between where you hit.

Other videos I had seen, took a gentler approach than the one above and I thought it better to follow their "little and often" methodology. I'm probably hitting harder than it looks here.

https://photos.app.goo.gl/ERra5KMLbcTapPLt5

I spent about an hour measuring, striking and remeasuring, and working up and down the crank, dialling in the runout at each journal.

In the end every journal was less than 0.01mm runout.

https://photos.app.goo.gl/3Bj7gtVFkhn3TtMu6



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[RobFrost]

Work paused on the cylinder heads while I awaited a little 9mm/12mm steel sleeve to make this drift for the valve guides.



I think I've made it too wide at 8.97mm, as it's quite tight in the exhaust guides. Tight is good, as it reinforces the guide and prevents it deforming in the head, but needs to be loose enough to get in and out.


With an engineer's hammer I tapped out a couple of intake stems.



No problem. It was reassuring to have a feel for how much force was being used and to feel the guide give.



I think the exhaust guides will need a pneumatic hammer, but the idea of that scares me a bit. It will be less controlled in terms of speed and power, but I do think it will be better in terms of hitting straight. With the hindsight of having slid the pneumatic hammer over the drift, I would have made the drift a little longer. But I left that for another day as everyone was in bed by now.

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[Bruce M]

If it’s possible, I was told to drill out the guide and leave a step that is on the far side of the head. As you hammer on the guide the contact point causes the guide to be in tension (so stretches) instead of compression which can mushroom, broaching out the head bore as it is pushed through. Never done the job myself though so not speaking from experience.

[RobFrost]

If it’s possible, I was told to drill out the guide and leave a step that is on the far side of the head. As you hammer on the guide the contact point causes the guide to be in tension (so stretches) instead of compression which can mushroom, broaching out the head bore as it is pushed through. Never done the job myself though so not speaking from experience.

Thanks for the advice. Sounds like a good approach. I'm committed now to this method, but if I experience squeaky bum syndrome with the force required, I could yet have a rethink. It's entirely possible these exhaust guides have been driven in with excessive interference.

Graham is right. Aluminium does expand marginally more than both phosphor bronze and manganese bronze with heat, so that may help.

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[RobFrost]

I'm not 100% sure why the exhaust valves are so stuck. But I did get one out and it's clear they're in worse condition on the outside. I guess the hot gases get forced up the sides under pressure.



The guides are all around 13.05mm which I believe is standard spec, not oversize, because they're supposed to have 0.05mm interference in their hole.



It was a relief to get them all out and the home made drift took a lot of punishment.



Before the last one I realised I'd exceeded the fatigue threshold of the 12mm collar and it was coming away.



Consequently I think I scratched one of the holes.




I peened it all round with the ball peen, spot welded it all round and smoothed it off again to drift out the last exhaust valve.



After they were all out I cleaned up with a bottle brush and alcohol, before cleaning out with a tissue. This is also how I clean my ears.



The widest hole at its widest point is about 13.06mm so I guess that will guide the choice of reamer and guides to go back in. They definitely need reaming as some, especially the exhaust, are out of round at the cylinder end.




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[RobFrost]

Dempsey says first oversize is 13.2mm and design911 have 13.26mm guides so it looks like those are the sizes. Interference is meant to be 0.05mm so that will give me one hundredth to play with when honing them clean.

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[impmad2000]

Got these if they are any use to you. 1st oversize

[RobFrost]

Got these if they are any use to you. 1st oversize

Tim those look perfect if they're first oversize. Do you have a micrometer? The only thing slightly confusing is I'm expecting first oversize to be 13.26mm and they look like they might say 13.08mm.



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[impmad2000]

Bought from Pelican parts 10+ years ago, but happy to put a micrometer on them to check

[Ollie]

Re: Cleaning Magnesium, Soda Blasting works very well. Gets all the much off without damaging the mag.