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Grinding oversize

hohocc

Well Known Member
The cylinders from my 320 have all suffered from corrosion in the bores, 2 have honed within limits and the corrosion is removed, however the other 2 need to be ground .010 oversize, and given I can't see any point in having half the cylinders at a larger bore than the others I'll either replace all 4 or bore all 4.
Given I live at the bottom of the earth there is only 1 (certified) shop in the country that can do the work, naturally they know how to charge. I'm told they remove the head from the cylinder to do the job.
I have also been introduced to another local shop that does certified work on other aircraft engines (so they must have some process and QC in what they do), who tell me they are able to bore my cylinders without removing the heads (non certified of course as they work on vintage engines). Their charges are more reasonable.
So the question really is: Is removing of the head a universal part of boring Lycoming cylinders oversize, or is it something that some shops do and some not? I've no doubt the latter shop can do a good job, but what is normal practice stateside for this job?
Thanks!
 
State side

In the lower 48 it would be more common to just exchange all 4 for reworked or new jugs. A good shop that does know how can bore them with the heads on. I would think that shipping would be a big item to give you paused for exchanging them. Sorry, I do like your home digs and would like to fly there in my bird some day as well. Yours as always, R.E.A. III #80888
 
ECI used to seperate the head from the cylinder for a couple of reasons. Sometimes I'd get one with a head that was crooked. The shops in the States that oversize cylinders don't remove the heads for grinding and I personally wouldn't use one that had been rescrewed. How much money are they charging per cylinder?
 
Yes unfortunately the $ to exchange them with a US business would probably be better spent just buying new, however these cylinders don't really owe me much (the rest of the engine was in good order), and they are only 300 TTSN so hence my interest in grinding them.
It's a few months since I got prices but from memory the certified shop was about $330US per cylinder (plus some freight), and the local place was about 60% of that. So if leaving the heads on is normal and acceptable (which it appears to be) then I think it's reasonable to send them to the local guy. The shop concerned has a very long and good reputation so I've no concern about the likely quality of work, and of course the certification is no issue in a homebuilt aeroplane.
Robert, yes New Zealand is a really nice place, lots of good scenery to look at, especially in the South Island, although the North is great too, but very different from the South. If you get down this way please get in touch! Like the Australians we do at times despair at freight costs and taxes but it's a good place to live.
 
Aerhed, yes I did wonder if they didn't occasionally end up crooked! I can see how in manufacture it would be relatively straightforward to get head and cylinder together but the forces involved in getting them apart must be scary. I understand they use a bar several metres long to do it, I'm not in a position to criticise but if they don't need to come apart that seems a lot less drama.
Thanks!
 
I don't think having them all the same size is a big deal. Less than 0.1 square inches out of 20.7. If it were me I would just bore the 2 that need it then put them across from each other (1&2 or 3&4) and not worry about it. should balance out fine. Just my 2 cents. Let the flaming begin. :D
 
Don't forget that you'll need new pistons with the bored out cylinders. You need to look at the piston weights. If the .010 over pistons are the same weight as Standard, I think I would only bore the two in need, but would need to think about that a bit. If the weights are more than a few grams different, you'll need a matched set; at least on the opposing cylinders sharing a crank journal. I believe that Lycoming allows about 12 or 15 grams of imbalance or weight difference in reciprocating mass. Don't forget to account for tolerance stacking. I wouldn't worry about imbalanced CI volume's. You'll have more thrust variability from imbalanced mixtures than the CI difference, especially on a carb.
 
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If oversized you need to do both sides. You can do opposite pairs according to lyc, but I'd do all four.
 
Good chat

Sounds like you are on the right road here. A point of interest for you as it is your call on how to proceed with this. It is very commend in the racing world to match the weights of all the moving parts and a good shop can do that while you are apart on this job. You would not pull the crank shaft and have it balanced on this one, but the rods, rest pins, and pistons can be matched on the bench with out much effort on the part of the shop. Even if you use two oversized pistons at 10 over and set them on the opposed cheeks, those two can be made to match the two that are standard. You just need a good scale and the know how of where to take the needed weight away from the heavier mass. I would not be afraid to fly behind one that that had been balanced out in this way. This configuration would be more in tune than that of a factory new standard production unit. Just two pence of thought for you. Hope this helps, Yours, R.E.A. III #80888
 
The engine has had a sudden stoppage and has been dismantled and checked by a certified shop, the components are all inhibited/in storage until closer to when they need to be assembled, so the whole thing is a known quantity, the most significant problem is the bores so I'm pretty happy with my engine overall.
I had been considering the possibility of a compression (and therefore piston) change anyway, given the bore state I'd be buying at least a couple of pistons, so grinding 4 and getting 4 pistons seems sensible enough to me.
Since it's in a million bits I had considered things like crank balancing but am undecided at this stage, the shop/engineer that will be assembling the engine considers it a good idea, and the opportunity to match piston/pin/rod weights is a very good point too.
Thanks for all the input, the original question is covered and you've given me other points to consider too!
 
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