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MG Midget and Sprite Technical - HGF (2 & 3) and leaking carnkcase

I pulled the head off and found that the engine is actually +60 instead of the +40 I had assumed. Not a prob, but there seems to have been leakage across the gasket plus a lot of sooting of the 2 inner piston crowns and chambers. Would this be a reasonable cause for crankcase overpressure? Lots of leakage at the rear end after a 'spirited' drive.
9.75 pistons (21253, IIRC) and I have some 21251 low comp ones in boxes. Anyone know if the rings are interchangeable? I haven't pulled the engine fully apart yet and hope that the bores are OK since they aren't scored and still have the crosshatching on them.


Also,anyone know the weight of a standard 1275 flywheel?
rob multi-sheds thomas

<<<<Would this be a reasonable cause for crankcase overpressure?>>>

I'd say yes!
Nick

15lb 6oz
David Smith

Hmmmmm. I've got 11.5 lbs on the bathroom scales. It has a deeply dished back and the outer face has a chamfer between the teeth and the cover plate. Nicely done and the bolts are all stamped/numbered.

Nick. How would it get through to the crankcase? Only seems to leak between the bores.
rob multi-sheds thomas

sounds like a home-spec job
Peter May lightened std = 10lb 14oz
MED steel = 9lb 4oz
Ally = 7lbs 14oz
David Smith

Sounds a bit light to me...most likely its been lightened...makes good seance being that they went the distance to go +.60...whats another $200


for no more then a set of deves rings cost..$50-$75, Id go that direction and tell them the pistons are the copper s 12153's so they can set you up correctly....after all they dont call them compression rings for the fun of it.

I certianly wouldnt go anything less then deves


prop
Prop

http://s82.photobucket.com/albums/j279/foggythomas/?action=view¤t=1312a.jpg
http://s82.photobucket.com/albums/j279/foggythomas/?action=view¤t=1312b.jpg
http://s82.photobucket.com/albums/j279/foggythomas/?action=view¤t=1312c.jpg

Machined flywheel at about 11 1/2 lbs, blown head gasket between 2 and 3 with some burning between the bores plus the pistons on S rods. Pleasant suprise to see that it used these rods and that the crank had been wedged, even though it appears not to be an EN40B cross-drilled one. I have a couple of those spare but am happy that this crank needs no extra work. It appears that the 21253 and the 21251 pistons have the same rings and merely a different height between the gudgeon pin and piston crown. I noticed that one company list different numbers for the piston rings for the 2 types!! I iz cofooosed.
rob multi-sheds thomas

Rob it would be difficult to find a headgasket off an A Series engine that did NOT have burning between 2 and 3 cylinders, yours is the norm and is not unusual.
I wouldn't expect that to be the cause of high crankcase pressure, although others could perhaps explain why it would?
I always believe it is as a result of higher spec engines that have dispensed with traditional evacuation techniques, mine included. :-)
Bob (robert) Midget Turbo

Rob,

Centre main not strapped by the look of it so don't rev too high.
David Billington

My guess is it wasnt broke in correctly....Im thinking they didnt use the cheap basic non-multi viscosity motor oil for the 1st 500 miles....and the clys. got glazed so the rings never real got a chance to bed in ...thus causing your blow by problem...

that or the compression rings got installed upside down


prop
Prop

that is a strange looking flywheel....I dont have one that thick or with a bevel.

prop
Prop

I bought the engine with very few miles on it for either £160 or £180 (can't quite remember) after it vomitted out so much oil that the owner of the Minor decided to get a 1300 Ford crossflow instead. It had a crank rear seal on it and an extra breather canister welded to the sump that would've fouled the chassis rail on the midget, so, had to be changed. There were bills for about £2500 and data sheets from "Simon Hawks Race Engineering" or something like that.
This is the engine that dumped out the oil in the M5 services on the way back from S50 (The OEW Lenham GT).

If I can get it re-ringed and replace the camshaft for something a little less fierce then it'll go back in with those split 45s.

Camshaft. Hmmmmmm. Who/where, what and how much?


rob multi-sheds thomas

Right. The cam is (according to the paperwork) a 286 profile, although there appears to be no markings. The last RR session gave 90.5 bhp and 112 ft/lb. Has a little chiped area on one lobe that means I am reluctant to put it back in. Drive is via a vernier system with multiple (ie 4 or more) chains linked together. Looks expensive!

Anyone selling an SW5 cam kit? Trade or P/X for a set of +60 21251 pistons without rings?

I've polished the top of the block with some fine emery on a flat board (the rudder post from my old aeroplane tailfeathers) and there is only light pitting between the 2 and 3 bores. Less that 5% of the surface is marked, so, I guess it is OK if it is flat. I hope so since the pistons are a gnats' away from the tops of the bores.

I'm going to let a stub pipe into the solid area where the fuel pump boss is located in order that I can bleed off any overpressure in the area. I've removed the 1/2 circle of upper scroll seal from the block and there appears to be no scuffing of the scroll itself. Maybe it wasn't sitting close enough and there was also a gap between the 2 halves of the seal arrangement , if you see what I mean. Lots of bitumen sealant in that area.
rob multi-sheds thomas

I'm with Bob on the head gasket - every one I've removed looks like this. Simplest way to confirm a HG leak between adjacent cyl's (prior to dismantling) is to remove one of the suspect cylinders' plugs and crank the engine. If tha gasket is leaking you'll usually hear it through the plug you have removed.

Re the rings: 21253 should have the same rings as 21251. However, given that there were two compression ring thicknesses for the 1275 pistons (1/16" and 1.5mm, which in thou are .0625" and .059" resp) you need to check that you have been supplied with the right set. (They have been mixed up in the past).
Check also that the clearance between the ring and the groove is within spec.

Paul Walbran

Thanks, Paul. I've put a mic on the rings and they are the same. The pistons have only the slightest surface scuffing on the thrust side and appear to have done only the few thousand miles that I have put on the engine.
I've been looking for a cylinder hone since the one I knew I had somewhere in the garage turns out to fit a diameter of bore that is a few mm short of the 1275 bore. I've also had a look at 'Machine Mart' for an engine stand. About £65 for the 2 items, inc delivery.
Gasket set, cam and timing wheely-thing should get me through to the finishing post.

There was no sign of HGF before the run to S50, either comp pressure being turned on the starter or by leak-down using a diffrential tester from my aeroplane toolkit.
rob multi-sheds thomas

Rob,

Very nice looking set-up.

As others have said, the head gasket looks pretty 'normal' for an A-Series, especially as there is no burning on either the block or the head.

Have you checked all the valves to make sure they are seating correctly?

The 1380 we bought secondhand used a lot of oil ~3/400 miles per litre at best! The seller said that 'it has only ever been run on full synthetic oil'. Ah! So after about 10,000 miles I had the bores re-honed by Oselli and fitted new rings to the Omega pistons.

Ran it in on a very basic 20w/50 and back to synthetic after about 1,000 miles. Oil consumption now a minimum of 1,000/lt.

With the split Webers you do need a 'good' cam to get the best from them - 276 as a minimum? I would have thought that an SW5 would be a bit too tame?

Also with no crankcase vacuum, keeping the oil in is a problem. Various thoughts on the 'proper' rear oil seal. I considered one and talked to Peter May, who said that they are ok up to around 6,000 rpm, but would not be so confident above that. Did not fit one, but do have some vacuum with the HIF6.

Richard
Richard Wale

This thread was discussed between 05/08/2008 and 06/08/2008

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