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MG Midget and Sprite Technical - 1500 dilema

hi everyone,
can anyone assist with a couple of problems? firstly i have 2 pistons 3 and 4 rising higher in the bore by around 2mm (see picture showing 2 and 3 at tdc) you can see clearly that 3 is flush with the recess and 2 is down the bore some way, 1 and 4 when at tdc show the same picture. will this be a problem when i refit the head or will it be able to run ok for general motoring?.
the reason i removed the head was because the coolant system was presurring and blowing water out of all the perished hose, i could see nothing obvious on the head or gasket but have had the head skimmed anyway but i have also noticed a small scar/deppresion between cyl 3 and a water course on top of the block which can also be seen in the picture. this depresion is very slight maybe only a couple of thou, so could anyone tell me if this was the root of the origional problem?
and finally i have had a look at the new head gasket and seen that there is a kind of gusset between cyls 1/2 and 3/4 which would sit on top of the points where the bores have been recessed which would stop the gasket from sitting flush on top of the block.(see pic 2) when i look closly i can see that a previous gasket has left a stain where the gusset has sat before but the old gasket is still at the machine shop with the head so i cant see what effect the gusset actually has.
so those are the problems i have noticed and my questions are:- should i get the rest of the engine out and have the block machined?, scrap it and look to get a short engine? or put it back together the way it is and join a recovery service?

dave

da cook

pic 2 gasket compared to top of block with closer view of depression between cyl 3 and water way

da cook

Hi Dave

It does look as though your problem started at that water way, it looks like a re-grind is the order of the day as long as everything else on the bottom end is fine, ie, main bearingsd, big ends etc..
Did you do a compression test before you stipped it (this would have pointed to the problem if compression was down on that cylinder)

Hope you resolve the problem..


Regards, John..
j b biggs

hi john, thanks for that, i did run a compression test 1st and thats what added to my confusion a little. the results were cyl 1 160 psi, cyl 2 115, cyl 3 182 and cyl 4 175. so i immediatly suspected cyl 2, but when i did the strip i found push rods 3 and 5 considerably bent which correspond to inlet valves on the 2nd and 3rd cyls so i didnt think the compression test would hold much value, the higher reading on the last 2 cyls i put down to the extra 2mm of lift that im getting on the pistons. i have got the head back now so im just going to put it back together now and see what happens and then consider an engine out total rebuild once i have completed the welding and got it through an mot.
the joys of owning a classic eh :)

dave
da cook

What's the history of the car?

Have you owned it for long...maybe a during a previous rebuild, 2 pistons were replaced, not a whole set & made for the difference in height?

I think I'd put it back together & possibly spray on a light coat of copper coat on the head gasket (it's good for helping fill in any imperfections in the machining of the head and/or block.

Personlly I'd put it back togther & drive it, see what happens & go from there.
Dave Rhine ('78 1500)

hi dave,

i really dont have any history for the car, i picked it up a couple of years ago knowing that there would be some welding required, the engine ran fine after being jump started but i could tell that it needed some attention. i did get the car for what i thought was a very reasonable price but the guy who sold it claimed he had no history for the car as he also bought it as a project but had no time to take it on and he in no way appeared sad to see it go. when i took a new battery to it a couple of weeks ago it started blowing water from all the perished hoses and steam from the exhaust, so i sent for a head gasket and removed the head when the rain halted the welding operation. after removing the head i could see very heavy pitting where some foriegn objects in the last 2 cylinders had eroded the head, but the valves were heavily encrusted in all 4 cylinders so i considered the damage to be historic, and there were signs that the head and block had been milled at some point and thats when i started this thread.
i had the head back yesterday after having a further 12 thou machined of it by grinding to ensure that it is true and flat, and after looking at many pictures of similar heads over the past week or so i would guess that in total 1/8" (3.5mm) has been removed in total, but i put it all back together yesterday afternoon anyway and ran the engine for over 3 hours with no signs of any problems, no water leaks, no pressurisation of any hoses, no steam from the exhaust, and the temp gauge sitting comfortably under the mid point. this was all done statically as the car is still up on blocks with no wheels on waiting for the welding to be completed. when i went back to it this morning the water level was fine and it started on the 1st turn of the key. my only concern now is the high compression ratios that im getting due to all the machining, the book say's the compression ratio is 9:1 which i guess should give me 9 bar on the meter but the actual readings are 12 bar (175psi) on cyls 1,2 and 14 bar (200psi) on 3,4 which is understandable due to their extra travel. this is satisfying from the head seal point of view but i dont know what other effects this will have, along with the fact that i will be running on unleaded. i suppose only time and a few miles under the wheels will tell.
da cook

Hi Dave,

I did try to respond to your other thread a while back, but for some reason the board ate my response so I guess it's orphaned out there in cyberspace!

"Firstly i have 2 pistons, 3 and 4 rising higher in the bore by around 2mm...... Will this be a problem when I refit the head, or will it be able to run ok for general motoring?."

YES it's a problem. Read on!

"I have had a look at the new head gasket and seen that there is a kind of gusset between cyls 1/2 and 3/4 ......I cant see what effect the gusset actually has."

The 'gusset' is quite normal and sits against the head face. If you look carefully it sits below the height of the tops of the fire rings and so doesn't cause any problems. It is there to prevent fire distortion owing to the close bore centres on the outer pairs of cylinders.




Turning to your motor,

From what I can see from your photo's, whoever rebuilt that engine before never equalised the piston heights which is bloody CRIMINAL!

This is rather critical on the Triumph 1500 (as well as the late 1300s) owing to the recessed bores. The pistons need to be set at around 15-25 thou down the bore at TDC. If this isn't done you will get problems at higher rpm with piston growth/rod stretch which (unless the head is for a racer and set up for 'pop-up' pistons) will ultimately lead to gasket failure.

The recessed bores additionally can become a point of detonation initiation in tuned motors - this may become an issue with your increased static CR (QV later). This may be what has lead to the erosion between No3 Cylinder and the waterway but without closer inspection I can not say.

For fast road / performance motors I deck the block flat, equalise the pistons and use the earlier non-recessed bore gasket. That way you run the engine in a sense as it was originally designed and you don't get problems.


WRT to your cylinder head, firstly you are better placed with skimming a 1500 head because it is taller with larger combustion chambers than its earlier (but visually similar) Triumph 1300 brethren. However 3.5mm DOES sound rather a lot.

I mill around 70 thou on fully ported heads (1.8mm) to gain a static CR of 10.5:1, although admittedly this is on motors that have been overbored so you could take a little more off on a standard bore. Looking at your motor I doubt it has been bored because the maximum you can get away with safely on the recessed bore gasket is 20 thou.

With 10.5:1 static CR these motors will run nicely on 95 RON on a 37/63 73/27 Fast Road Cam. However the standard cam in the 1500 is rather 'mild' at 18/58 58/18 and may cause problems with this sort of static CR.

The upshot of all this is that you may well get detonation problems with your motor as is on 95 RON, although you might get away with it on 97 RON. If you DO get detonation problems, a pound to a pinch of salt says that it will start at the bore recess. If you are lucky and catch it early you might only blow the gasket, if not you will burn out a piston crown, probably number 3 because this is the cylinder that runs hottest on these motors.

So, all I can suggest without actually getting my hands on your motor, measuring everything, and buretting the head, is to 'suck it and see'.

If you do get problems let me know because it wouldn't be TOO difficult to exchange the head for a standard one with somebody in the racing world (because 1500 heads are the bees knees on race 1300s and run stupidly high CRs so they get extensively milled).


I am intrigued when you say you had 2 bent push rods. Until last year (when I stripped down a customer's motor), I had NEVER seen this in 25+ years of playing with these motors (even on high RPM race screamers)!

It could be you have rocker shaft wear (high mileage Triumph 4 pots are notorious for this) or it could be valve guide wear - it was the latter in the motor I rebuilt last year. Both need checking - I put in new rockers and shafts almost as a matter of course these days. If it is the latter then you need the guides replaced, because if you don't the problem will merely re-manifest itself. If you are going to put in new guides, then you might as well go the whole hog for bigger valves and unleaded seats at the same time.
Deborah Evans

hi deborah,

thank you for your most comprehensive and informative reply ;) i got it back together now and will be running it in the suck it and see mode certainly for the purpose of getting it to its mot and for a few country miles while we still have some decent weather. the reason for me doing this is to restore the car fully so that i can sell it to get some funds to do a couple of BGT's that have been sitting around at the back of my mums for a few years, and for this reason i want it right and i want it reliable. so bearing in mind what you have said i will do a bench rebuild before i put a sale sign in the window.
i havent encountered the term equalise pistons before as when i have previously rebuilt engines many years ago i would replace all 4 or 6 pistons with standard parts to keep things equal anyway.
when i also picked up the old head gasket yesterday i had a close look at it and it to looked like it origionally had a gusset feature in it of soft metal, and this appeared to have been compressed flush into the softer gasket material so i felt confident enough to go ahead with refitting it anyway.
the push rods appear to have been in a bent condition for quite a while given the amount of abrasion on the outside of the curve and quite possibly been refitted in that condition by the guy that had previously done the rebuild and not necessarily put back in the correct order and may have come from the 2 damaged cylinders (they were in 3rd and 6th slot not 3rd and 5th as i stated earlier). i did satisfy myself about the condition of the rockers and guides with both being suprisingly in quite good condition.


*stop press*

i got this far with a reply when i took someone around to show them my progress, the car started 1st time again but it sounded like the oil pump rattled a couple of times trying to pick up, the motor then ticked over normally for about 10 secs and stopped, the starter would not then engage just giving a click from the soleniod, so i removed the plugs and tried spinnig it by hand but it was solid, i then took the rocker box cover off to check the valve gear and this was still all intact but there was signs of water contamination in the oil even though there was no apparent loss of water, on removing the filler plug the water was still up to the top. the oil was drained out while i was working on the head with fresh oil and a new filter fitted before running the engine but i dont know if there was any of the old oil/water mix trapped in the sump pan below the oil drain plug.
so now i guess the sucking and seeing didnt last long and a bench rebuild is the only way forward,,,,at least i will be able to do a full and proper job on the engine bay now though.
thanks to all who have replied to this thread especially you deborah i will take care to follow your comments during the rebuild.

dave
da cook

This thread was discussed between 14/06/2010 and 16/06/2010

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