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Repairs to 4S cylinder head

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david james 111/08/2010 16:21:17
70 forum posts
I have ruined the thread holding the exhaust to my saito FA-180 cylinder head
 
The thread is unusual (14mm by 1mm pitch) and i don't think helicoils are available.
 
I know these can be repaired in the states. Does anyone know who can repair them here.
 
Thanks
Ed Darter11/08/2010 17:12:09
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A friend had a similar issue with his Saito 82a, Helicoils for that are available, but not common. However - when we tried it back in a model the helicoil lasted about 5 minutes before it worked its way out. He ended up just buying a new cylinder head..... with the 82 at least there is so little meat around the manifold that you don't have a lot to work with.
Bruce Richards11/08/2010 17:18:54
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1849 forum posts
A friend of mine just fixed this problem on a OS by tapping a slightly coarser thread in the head (it was not too badly damaged) and making an adapter to go between the head and the exhaust pipe. The original pipe was silver soldered into the adapter. I can find out exactly what thread he used if you are interested?
Steve Hargreaves - Moderator11/08/2010 17:24:13
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Hate to be the bearer of bad news but Ed is probably right....its new head time. As well as the helicoil you would need the correct drill & special tap PLUS these would need to be "squared off" at the end to ensure the hole/new thread went right to the bottom.
 
It might be worth trying Just Engines & seeing if they can suggest a solution or even repair it for you. Alternatively why not send it to the States for repair if you know who can do it.....Postage won't be too much I shouldn't think & if you're lucky & insure it the head might get lost & you could buy a new one courtesy of the Royal Mail
Ed Darter11/08/2010 22:41:17
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Steve is right - the Tap had to be modified to enable him to cut the new thread deep enough to stand half a chance. Bruce, not sure how similar the OS would be to the Saito, worth a look but on the Saito 82a at least there is literally only a few mm of casting before you have opened it up to fresh air....
Martin Harris11/08/2010 23:44:47
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I tried an experiment recently on an old OS 60 which seemed to work (pre-Surpass with separate rocker boxes - replacement heads for these are virtually unobtainable) although the old boy I did it for has abandoned our club in a bit of a huff so I can't give any update on its longetivity. The thread was stripped and the surrounding material had cracked.
 
I made a threaded plug out of hexagonal steel bar (extended with a plain extension into the port which was axially concentric) and drilled out the remnants of the thread enlarging the hole to as large a diameter as seemed reasonable. After "V"ing the cracks, I bolted the head to a piece of aluminium (to protect against warping) inserted the plug and filled the gap using Lumiweld. Once it had hardened I unscrewed the plug while still as hot as possible. I then re-tapped the hole (after grinding the excess off the plug tap) filed the face flat and threaded in the exhaust stub with some trepidation which, to my relief, held up to the locknut being tightened fully.
 
Stable door time - to avoid having the problem in the first place:
 
(1) Always oil the thread before inserting the header.
 
(2) Tighten the header as far as it will go and back it off to the desired angle i.e. less than 1 turn to ensure the maximum thread engagement. (I'm sure some people just thread it in to the position where the locknut happens to have been put on by the manufacturer for transit!)
 
(3) Make sure the locknut is tightened well and check it regularly.

Edited By Martin Harris on 12/08/2010 00:03:04

Bruce Richards12/08/2010 09:14:25
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1849 forum posts
Yes the tap had to be ground off to enable the thread to be cut to the bottom of the whole.
Steve Hargreaves - Moderator13/08/2010 08:55:39
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Good advise from Martin there.....cleanliness is next to Godliness where these fine threads are concerned.....always make sure they are clean & there are no nasty bits of hard carbon around to chew up yer thread
 
Personally I always wind a few turns of (plumbers) PTFE tape around the threads on the gland nut before threading it (carefully, to avoid any cross threading) into the head. This does three things....1) it helps prevents galling of the threads. 2) It seals the thread & stops any oil leaking through the joint 3) it acts as "threadlock" to help stop the nut coming undone
 
Another thought......always make sure you can take the cowl off without undoing the manifold....the less you have to fiddle with the exhaust the longer the thread will last.....after all you wouldn't have to undo the exhaust manifold on yer Lycoming to get the cowl off yer Cessna.....
david james 113/08/2010 10:12:22
70 forum posts
I wish i had received the good advice about arranging the cowl so you can take it off without disturbing the exhaust. If I had followed that, i don't think i would ever have got into this mess.
 
Anyhow, after consulting some American web-sites, my son-in-law, who is visiting the US, is going to post it to Clarence Lee for repairs at a cost of about £30 incl postage back. I could probably have got one of my mates in the club to rethread it to a larger size and make up an adapter but, at this price, a professional repair seemed good value. Just Engines were no help.
David Turner 513/08/2010 10:32:08
106 forum posts
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I've fixed a few of these ... in the way already described; usually.
 
I'm pretty sure that I could fix yours, but the cost of the taps is prohibitive for one-off jobs.
 
The irony is that I've never had to fix one of my own engines!
Bruce Richards13/08/2010 13:32:10
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Steve Hargreaves - Moderator13/08/2010 14:29:23
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£30 sounds like a bargain to me......much cheaper than a new head methinks.....
John Laird14/08/2010 09:21:50
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Clarence Lee is one of the best engine men in the states and has been doing this type of repair for yonks.  I dont recall seeing any bad press on his work.

Edited By John Laird on 14/08/2010 09:22:15

David perry 114/08/2010 11:28:11
932 forum posts
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I did the same to an SC52 FS a few years ago.  I tried varioius fixes but all to no real long term avail.  I suspect the cost of a new head casting for a SC is going to be so much its just worth buying a new engine.
 
Martin Harris14/08/2010 12:43:45
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SC heads are a fairly reasonable price (and many SC/ASP parts are dimensionally identical to OS parts and around half the price ) but I think you're probably going to be correct as I believe David's engine is a Saito  on which the head and cylinder casting are a one piece item. - $108 on a quick search...

Edited By Martin Harris on 14/08/2010 12:44:10

Steve Hargreaves - Moderator16/08/2010 09:01:03
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David, Perkins list the SC52 head at £51 inc the valves...they don't seem to offer just the head. An ASP52 head (no valves) is £30 & as Martin says the ASP & SC motors are alledgedly the same animal....I say alledgedly because I have no personal experience regarding the interchangeability of spares between these two engines......
 
Interestingly I have just bought a new SC32 prop driver to fit an ancient OS32.....its a perfect fit & only £1.65 (the ASP version was £4.95 & the OS versions were "sorry mate...can't get them anymore!!!")
 
I have to say that $108 for a new head & cylinder on a big Saito sounds a lot cheaper than I would have thought given that a new motor is well over £400...

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