Harvs.com.au – Tom Harvey

The Diagnosis Doctor – 1275cc MG Midget Engine Rebuild

Posted in Cars, MG Midget by Tom Harvey on January 15, 2009

The reason that there was more smoke blowing from the exhaust than observed at a Bob Marley concert has become clear:

  1. The bore was extremely glazed, almost to a mirror finish
  2. The piston ring gaps were more than double the recommended size (~26 thousands as apposed to 12-14 thousands)
  3. The piston rings had been annealed. This is the tempering process metals under go when exposed to repeated heating and cooling. This was made obvious by the degree to which the piston rings could be stretch up and down when they were removed. The piston rings are made from cast iron so they should be brittle, snapping after very little deforming – these did not.

All these problems would have contributed to the excessive consumption of oil, which was so bad we were thinking of buying shares in The Penrite Oil Company.

So what’s the solution?

  1. We had the bore honed. The reason to have the bore honed and not re-bored was simple – cost. There was nothing wrong with the pistons (AE 21253 + .40″) so we wanted to avoid having to replace them. To re-bored and buy a new set of pistons was going to be ~ $500.
  2. Buy a new set of AE 21253 Piston rings. Not the cheapest set of rings by any means at ~$185, but still cheaper than the re-bore and new pistons.

While we’re at it and the engines out of the car we also decided to do the following:

Fit a new RE-13 camshaft. This is a fast road cam produced by an Australian firm called Russell Engineering. It is a good cam for around town as it’s power is very useable even at low revs. We got the cam on exchange for a very reasonable ~$150, it was noticeably lumpier than the standard, visibly having a lot more duration. To get the most out of the cam we also chose to replace the cam followers with light weight cam followers ( ~$105).

We had the crankshaft, flywheel and clutch pressure plate dynamically balanced. This seemed to be an absolute bargain at ~$100, especially when we got them back and realised how much material had be shaved off – BMC/Leyland were definitely knocking out their components at a price!

All this was made very easy by Colin Dodds from Sprite Parts who was extremely helpful, a wealth of knowledge and more than happy to work to a fairly restrictive budget. A big thanks to Colin!

With a relatively small amount of money and a little time we should end up with an engine than is more reliable, has a little more power and hopefully consumes a little less oil (sorry Penrite).

Let the rebuild begin…

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3 Responses

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  1. Flemming said, on April 27, 2009 at 4:17 am

    Dear Tom, (I suspect this isn’t really why you have a blog but as a blog-respondee-virgin thought I’d give it a go): We have a 1970 MG Midget that we have owned and loved since the mid 1980’s. It’s probably done around 80k miles (we were away for 16 years!). It has been well maintained. Over the years it has blown the odd head gasket or three (excessive speed, expressway’s middle of summer). This is the only major work we have had done. We have no reason to believe there was any major work done before we bought it. A couple of months ago it started to do something a little strange. It started each morning as usual but after 10 minutes running starred to blow smoke (lots) for 3-5 minutes then stopped smoking and ran fine for the rest of the day. A local mechanic looked at it, pressure tested and said it was the rings i.e. burning off oil that leaked down when the engine was cold overnight. The cost of servicing the rings was lots ($5,000). He then said we could probably think about it for a while as it could be many months (up to 12) before it really got bad. Anyway, a week ago it lost lots of compression to become un-drivable. We tried another mechanic (to get a cost comparison). He said it needs an engine rebuild that will start at $4,000 but that there was a big risk in even starting this because, for the 1250cc models, it was not possible anymore to get crankshafts, so he could get well into the work and find he cannot finish it. I checked the rego which says it has a 1274cc engine.

    Just a couple of questions: You seem to have been able to get a crank shaft, have you heard of difficulties in supply? Do the kind of repair bill numbers noted above; look in right ballpark given your experience? We are in Sydney – Do you know anyone who may be well experienced … and reasonable on price (I know there is a conflict in this question)? If the crankshaft cannot be sourced, do you know if replacement engines are available anywhere. I used to work on cars a long time ago and am pretty good at following the manual (other guidance)…, this looks like a huge job, is it impossible for an amateur mechanic. I note that even if the answer is yes, I then have a time and space issue!. Any comments on any of the above will be greatly appreciated. Cheers, Flemming

    • Tom Harvey said, on May 5, 2009 at 1:38 am

      Hey Flemming,

      I’ll just let you know that I am by no means an expert, but here are my 2 cents.

      The cost of new rings is about $200. To get the full benefit, you’ll need to have the bore honed. If you’re unlucky it may not be possible to just hone the bored, and it may need to be rebored. If it is rebored you’ll then need new pistons and rings.

      I’m not too sure how difficult it would be to get a hold of a crack shaft, but then again you probably won’t need one. There was no need to replace ours and unless you’re racing it I’m under the impression that they hold up pretty well.

      I’d suggest you give Colin Dodds at http://www.spriteparts.com.au/ a call.
      He’s a top bloke and wealth of knowledge for all things Midgets and Sprites, and his prices are very reasonable – I’m sure he’d be interested in helping you out with a rebuild.

      Cheers,

      Tom

  2. Guy Chester said, on July 15, 2010 at 4:34 pm

    Tom,

    I have just finished rebuilding a Midget 1275 (1970) using the same logic as you – new rings, honed etc. Reassembled it would not turn over (this is my 15th or so engine rebuild) Thinking I had screwed something up in a “duh” moment I disassembled again but all was correct (crank truns fine with no binding with pistons out). Relubed everything and reassembled with correct torque settings and ring gaps etc. It still will not turn over – only if I elevate the engine and use a bloody great break bar on the crankshaft pulley bolt can I get it to spin.
    Any ideas at all would be very helpful.

    Thank you

    Guy (Victoria, BC Canada)


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