Stripped thread.

[J D 8]

   One of my Irvine 46's has a striped thread in one of the crankcase head bolt holes. 

   Can a thread this small [4mm I think] be fixed with a coil insert or any other method.

It is a well used Mk 3 and was still going well until it started to blow near the front most head bolt which was then found to be loose with thread gone. Stain on liner where blowing

[Andy Stephenson]

JD,

If the engine is what Just Engines refer to as a Mk IV then they have spare crankcases in stock.

I mention this because there always seems to be some confusion about Irvine's version numbers. It may even be compatible if it isn't the same version.

Edited January 13, 2023 by Andy Stephenson

[J D 8]

      Thanks Andy, The Mk4 is the Japanese made one and any way I am not looking to spend the best part of £80 on a 25year old engine that still ran well but has just this one issue.  Cheers John.

[Denis Watkins]

Double check those head bolts JD8, as if they are M3,

It can easily and cheaply be tapped out to M3.5

Edited January 13, 2023 by Denis Watkins

[Martin Harris - Moderator]

Chronos tools do thread insert kits from 3mm upwards (https://www.chronos.ltd.uk/omega-search/?q=Thread repair) and even small UNF sizes - useful for US market equipment.  Maybe a little expensive for a one-off at around £15 but very handy to have to hand and a lot cheaper than a replacement crankcase. 

[Paul De Tourtoulon]

Just looking at my Irvine, there is 1cm of thread can you use a longer screw ?.

 

[Engine Doctor]

Coil inserts are readily available from M2 upwards. Kits are available as said already from around £15 to £20 ish.The deciding factor on whether its suitable to use a helicoil is the ammount of metal between the bolt and the cylinder. To fit a 3mm helicoil repair the hole will have to be drilled and tapped considerably larger to accept the coil. It might just be possible but will be close .

Tapping the hole to 3.5 mm thread will be easier but looking at the pic an m3.5 will cut into the cylinder flange and will need grinding to clear .The cyl head will probably need drilling for clearance and head of the bolt may need grinding down a little to fit into the head recess. Simple job so check first and take your time. Good luck.

Edited January 14, 2023 by Engine Doctor

[kc]

There is an old but very simple trick that works well for stripped threads - put in a long bolt ( or stud) and use a nut on the top to tighten instead of tightening the bolt into the alloy.   Won't look neat but usually works.    Especially useful for exhausts too.

It cost me half a crown ( 12.5 pence) to learn this trick when my cars mechanical fuel pump broke down in 1962 - the country garage mechanic fixed the loose top in a minute this way.   Saved the cost many times over since then!

[J D 8]

            Thank's to everyone for the ideas, may well try the long bolt/stud @ nut first as there does seem to be a section of undamaged thread further down the hole.  Let you know how it goes.

[J D 8]

        Update, the long stud not a goer as thread is gone all the way down. A close look and it seems like some sort of alloy corrosion has eaten up from the open lower end of the hole. As ED says there is just not enough room for a helicoil and I recon little "meat" or space for a 3.5 tap. 

[RottenRow]

If the hole is open-ended, could a nut perhaps be fitted with a long bolt or stud all the way through?

Or some other method of securing the bottom of a long stud such as a pin through it?

Then a nut on the protruding stud to hold the head down.

 

Brian.

[Martin Harris - Moderator]

I’d take another look at helicoiling John.  I don’t have a 46 to hand but I’ve looked at a spare 53 crankcase and although the hole is very close to the flange of the liner there is plenty of “meat” below it and a finished insert won’t impinge on the top of the liner. 
 

Hopefully the 46 would be very similar. 

[Andy Stephenson]

Martin, if as JD8 says this is a Japanese made version it is likely to be different to the 53 as AFAIK the OS never made the 53 size. Looking back I have to take issue that this is in fact an OS made engine because it would have had an OS carb, the one in the picture is an Irvine Jetstream carb only used on UK made engines.

[Richard Wills 2]

that looks like an English made mk3 46 to me.  OS did make a 53, the mk2.

[J D 8]

             The engine is an English made Mk3 with Jetstream carb, bought new by myself in late 90's. Probably my most used engine by a long way.

 

[J D 8]

          It is up and running. In the end went with something like RR above suggested. I was not sure a coil would have worked the problem being either corrosion or even perhaps a poor casting leading to corrosion hidden by the red coating, as noted above the hole for this head bolt was open ended, the others are not.

  Something of a bodge but hopefully a good fix is fine by me, time will tell.

   In pic you can see a steel nut between the fins to locate head bolt. Bit of Dremel to make it fit.

   I fitted new bearings as I had some and then another snag, with back plate on engine was very stiff. Checked seat of bearing ect but in the end I could only think was new bearing a tad deeper. Made a thin card gasket for backplate and engine once again free

 Just have to find the muffler and it will go in my winter fix up of battered Wot 4.

 

[Glenn Philbrick]

I would so as kc has suggested, run a longer bolt or threaded rod down as far as you can, thread lock it in place then use nuts to hold the head tight. I think the Irvine engine is prone to this problem we have an old club trainer with an Irvine and most of the head bolts have stripped and been replaced by threaded rods and nuts.