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MG Midget and Sprite Technical - Help with engine modified 40 years ago..

Background: One frogeye sprite that was heavily tuned by the head mechanic at a BMC garage in the 1960's. Bought in late 70's as a wreck and then sat in a barn until 2 years ago when myself and my brother dragged it out

Spec on arrival: 948 block bored +60 with hepolite flat tops. Fully balanced crank, dampener, flywheel. 9 Spring clutch. Ribbed case gearbox with unidentifiable gears (odd ratios, short 1st and 2nd and then a long gap to 3rd and 4th) that are unmarked. Had a standard un altered 948 cylinder head and 2xHS1s loosely plonked on (can't believe they were what it was raced with). Camshaft is unmarked but measurements make me believe it is a "649" cam: 300/300 0.315/0.315 lift 1/2inch lobes. 3 to 1 medium bore exhaust system. 23d4 distributor (cooperS)

What we did: replaced the head with an unskimmed 12g295 and added twin HS2s.

Problems discovered: since. So we've had her out and about running and she was doing well (horrible 10-15mpg ;) as to be expected..) until something went bang. After some searching and discovering the first cylinder was exhausting via the inlet the tappet adjuster on the exhaust valve was found to have snapped across the oil way.

We had wondered if something like this may happen as due to the thicker head the pushrods mean the tappet adjusters were at the limits of their stretch.

Current plan: 1275 rods and replace the tappet adjusters (pressed type rocker gear)

However, I do wonder if it would be more sensible to skim the head down to improve the compression (Estimates are about 8.8 currently) due the camshaft...

This is our first foray into tuned A-series and as we're working with something that was done 40 years ago to a slightly unknown spec I would appreciate someone going "you're an idiot: Do this instead" :)

Kind regards and thanks in advance for any help

sebringsprite.blogspot.com
D Paul

Running that much lift, you need to check that the exhaut valves aren't hitting the block. It is common practice to pocket the block to provide clearance.
Dave O'Neill2

thought that was an issue with 12g240s not 12g295s....please correct me if I am wrong! hadn't considered that if that is possible.
D Paul

Lift should not be a problem with a 295 but could not harm to check.
If it is a 649 go to a high CR, 10,5-1 should do a lot better.

What type of rockers do you have?
There are several types that differ in strength.
Onno K

early pressed type.
D Paul

Weld the tops together if on a tight budget.
Or increase to forged or roler rockers which ever budget allows.
A 1.3 ratio is supposed to be the max for small bore engines

Steer clear of the sintered a+ rockers
Onno K

You might have a read from vizards book, some good insight on rockers

Im wondering if its just age and metal fatigue....the rocker has been sitting in one postion non moved under spring pressure for a solid 40 years im guessing... I cant imagine what my back would do sitting unmoved for 5 years then moved....just my 2 cents

I get a gut feeling this car was set up for hill climbs

But it sounds like you got a good find...pist some pics

Prop
Prop and the Blackhole Midget

anyone know of any suppliers for non roller 1.3 new rocker assemblies? rollers are out of budget at the moment unfortunately.

I've read through vizzard thankfully but its the overall balance of the engine that concerns me currently. I feel like we're bodging constantly and soon something bigs going to go bang. Honestly though this was the crank shaft to start!

Pictures @
sebringsprite.blogspot.com

will update a new blog post soonish when I get time
D Paul

http://www.minisport.com/mini-spare-parts/info_MSLMS0483.html

these are the only thing I've found within budget at the moment...

maybe I'll have to sell the old steel frogeye bonnet to raise some funds :( .
D Paul

Looks like a good deal to me....

Its a winner

Prop
Prop and the Blackhole Midget

Minispares has all you need.
Onno K

I had a tappet adjuster strip its thread using pressed steel rockers (even though they looked fine when fitted). Problem was the uprated valve springs on a big valve head that were too much for tappet adjuster. I replaced with forged rockers and no problems (touch wood) so far (four track days since).
Chris H (1970 Midget 1275)

we did fit uprated 200lb springs....

Think going to step back and sort her out properly. Not convinced the 649 cam is the way to go for her usage. Might drop down to a 276 kent or equivalent, skim the head alittle, get some 1.3 roller rockers, get her properly tuned and reneedled.
D Paul

This thread was discussed between 17/07/2013 and 19/07/2013

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