MG Midget Forum

This is #2 piston, standard?
I took the top ring off the piston and put it about an inch from the top of the cylinder and measured the gap at a loose .508mm. I think the Haynes has .408 as max.
Am I correct in estimating I need to have the cylinders bored to .20 over? If these are indeed standard. The diamond mark on the block has an 8 in it, but found no corresponding marks on the pistons...

Ok, trying to attach photos....



Edited 1 time(s). Last edit at 2017-08-16 10:04 PM by schapin9.

if there is no mark on the piston or ring to state o/s ie 020 then I'd say std
Russ

It is best to judge the condition of the cylinders by measuring the bore, and not just at one place. Several places to confirm it is round and also it is not barrel shaped, by wear. There are special cylinder micrometers made for this (and accurate enough). After confirming all 4 of them like that, then, based on what wear you've found, make the decision about what to do with them.


Note that a '74 came with low compression, 8:1 pistons, which weren't all that strong to begin with (weakness above the upper ring). To get more power out of an engine you want to run the highest compression ratio you can get away with (fuel wise), so fitting modern pistons, either standard bore if yours are in great shape, or 0.020 over if you'll be boring, with higher CR would be a wise investment. The non-USA stock were 8.8:1, and with modern fuels you can run 9.5~9.7 I think.

Use the stock pistons as paper weights.

The above is general info. Hap knows much more about this than I, but these are the basics that I've learned so far.

N

kind of side question (s) whats with the holes in 1 & 2?
and is the crank pulley nut 34mm?

The RA2272-STD are from the old Repco (then ITM) that sold replacement engine components for many vehicles. Generally considered a competitor to AE (which was part of the TNG group that owned Vandervell,Ferodo, Glyco, etc.). The big "holes" in the side by the wrist pin bores are part of the oil slot drain behind the oil control ring and a way to quiet piston skirt noise when running. Sort of a modern equivalent to the split skirt pistons of years back. These pistons are solely usable in stock engines with very conservative rpm application.

The two pistons on the right look to be original stock pistons. The '8' is actually a 'B' and is the factory "oversize" code. At that time in production, the 1275 had two oversizes, the 'A' and the 'B' in standard factory bore. These oversizes let the factory use slightly out of tolerance cylinder bores (aka big).

Double check your block in the cylinder 3/4 area where the distributor drive comes through. If you have factory sleeves protruding into this area, the block cannot be bored due to the sleeve getting too thin and potentially coming loose from the block. The block can be bored to the 1380 piston size, but may reveal a casting flaw which might have prompted the factory sleeving in the first place. Factory sleeves can be replaced if replacements can be located. I have some if you choose to go this way.

The front crankshaft dampener nut is 1-5/16". This is pretty close to 34mm being 33.3mm. Cam nut is the same.

Enjoy the day, Mike

l dont see liners below the bottom of the cylinder, seems as if theres a ridge on the bottom of the casting that the liner lands on.. 3 and 4 but is above on 1 and 2.
I think Ill wait for a good thorough measure session before commiting to a re sleeve... even if anyone here can do it.

Believe your assessment is correct. #4 looks to have an after manufacture sleeve installed to correct some situation from the past. It is not uncommon to see pistons with galling marks on the pin sides. Especially in the 3&4 cylinders. I have also seen engines where the pin loosened and wore a "slot" into the bore. Sleeving is the only fix for that kind of damage. Overall, the engine looks rather clean/low mileage after the engine repair. Some "trash" scratches on the bore in #4 where it looks like the trash got between the bore and the skirt. These should clean up with the +20 overbore.

Looks like the engine can go +0.020. The "490" photo shows a good bit of carboning on the ring lands below the compression rings. Like there was quite a bit of blow-by or compression loss. Ring gaps increase 3.14 times as much for every 0.001" increase in bore diameter. Usually the rings supplied with piston assemblies for over-bores are sized to provide the correct ring gaps with the specified cylinder clearances to the piston. So if the engine does get bored to +20, be sure to follow the recommended clearance tolerances in the piston box (should be a slip of paper or printed on the box itself). We run into this situation using pistons that are primarily used for street replacement in racing applications, like the County or Nural 9.75-1 replacements. Increase the cylinder to piston bore clearance for the additional racing heat and a bit less friction and the ring gaps go up. The fix is to use a top ring from another engine application and custom fit the ring gap and then convert the second ring into a gap-less ring by Total Seal. The oil control ring usually can tolerate the additional clearance.

Good luck with your rebuild and keep posting your progress and any questions.

I recently rebuilt a 1ZZ-FE Toyota 1.8L engine that had 265 thousand miles on the clock. It had many of the piston/ bore issues you show about your Midget engine. One problem Toyota had with these engines was oil consumption due to the carbon build up in the vent holes in the piston oil control ring groove inner bore surface. The replacement pistons have these holes that are more than double the size of stock. I had that block bored .020 and bought new pistons. The rings that came with these pistons provided almost a closed gap which gave me the opportunity to "blueprint" the ring gaps using a piston ring grinding tool that I purchased from Summit Racing. I ground the ring gaps to the tightest tolerances. The difference in power was remarkable. This engine runs nice and tight and never uses a drop of oil! My customer just took a trip from Houston to Wisconsin and back with this newly rebuilt engine! It performed flawlessly. "The devil is in the details".