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MG MGA - Cylinder Honing

Guy's, i decided to repace my 1500 Block due to damage to the scroll cradle. I will utilize a 1600 block I have with it's crank that I had polished and confirmed by a machinest. My prolem is I just want to hone out the blemished cylinders were as he states I should have the block tanked, bored, and planed. I just do not want to spend that much money or time (Driving Season). Can I not just hone out the cylinder, spending extra time to smooth out any rust or blemishes? and then replace the rings on the existing pistons? I am not looking for a high performance moter. Just a decent reliable driver? Or am I wasting my money?
WMR Bill


If I were to guess I would say that he is trying to avoid any fingerpointing and a upset customer later on.

For example if the engine goes bad immediately the customer is going to blame the machinist and want the work finished under warranty, the only way he's going to feel comfortable about that is if he KNOWS its been tanked, honed and planed.

Out of curiousity are you sure the head seat is that true that its not worth having planed?
T McCarthy

I can lay a straight edge on it and it looks pretty good! besides, doesn't the copper gasket take up minor imprefections? after all it is aprox 1/8 inch think and I would think it should conform to imprefections. Please correct me if i'm simplifying this to much.
WMR Bill

one more point! I am not doing this to be cheap, The decission is to wait 5 to 6 weeks for machine work because of there busy (Racing) season, or, throw this baby back together for this season and re-build another motor i have (1800) the right way at my leasure this winter.
WMR Bill

The worst that can happen from the honning is you may burn more oil then you would with an engine rebore. Depends on what you hone with, there are the three stoned spring loaded cheapies that look like a large brake cylinder hone, or a real hone such as a Sunnen. If you feel real ambitious you can hone to the next oversize using a Sunnen or similar. Note that if you do use a Sunnen hone or one like it, the best lubricant is lard, clean up with soap and water.

Take a high quality long flat file (check that it's flat), hold it in the center with gentle pressure and run it across the surface of the head and block at an angle make sure it is always in contact with the full width or the block or head. This will take off the high points.

Some are going to read this and have a cow but it does work, just be gentle.
J Heisenfeldt

If you do decide to hone the cylinders rather than go a complete rebuild, check the cylinders for roundness and measure at least 3 different places high, mid-level and lower on each cylinder to give yourself an accurate picture of the cylinder and to order rings. Also check each cylinder for blue spots, where they may have gotten burned. This usually only happens where there was ring trouble, but you never know.
Regarding minor rust pitting, you should not really notice any excessive oil burning, unless you have vertical grooves in the cylinder walls.
Check the head surface where the head bolt threads are. Chamfering the thread holes to a slight negative angle will reduce the chances of head gasket problems, and is discussed in the archives.
Good luck.
mike parker

Ah, one of my favorite subjects, cheap repairs. If you have a cylinder hone in hand, it is easy enough to hone it to see if the bores will clean up. If there are any scratches or pit marks that you cannot hone out, then it needs to be re-bored.

Next step is to use a dial bore gauge to accurately measure the cylinders for diameter and roundness at top, middle and bottom of stroke.
If these measurements are all consistent within about 0.003" you can likely have success installing new rings on the old pistons, even if the bores are up to 0.005" oversize in places.

If you don't have a bore gauge the next best approach is to place a NEW piston ring in the bore, use a piston top to position the ring perpendicular to the bore, then use a thickness gauge to measure the ring gap. Specification for ring gap is 0.008" to 0.014" when everything is new. The gap grows as rings and walls wear. A variation of 0.003" in bore diameter will make about 0.010" variation in ring gap.

Some replacement rings may be slightly oversize (to compensate for some bore wear) so you may not get 0.008" minimum gap. In that case you need to file or grind the ends of the ring to make 0.008" minimum gap (to avoid seizing when hot). If the gap is much greater than 0.014" it makes for excess compression leakage. The engine still runs okay but may burn oil and/or blow oil out the vent ports.

These iron cylinder blocks are very robust and almost never warp (even when overheated), but the top might be burned if a head gasket was leaking, especially in the center between #2 and #3 exhaust valves. Check this with a straight edge with a bright light behind. I recommend removing all head studs, hitting the stud holes with a countersink tool to be sure there will be no raised burrs, and then hand lap the top of the block with an 8" whet stone and oil to assure there will be no raised burrs.

Same comments for the iron head. The head might be gradually bowed a few thousandths of an inch from end to end which you may see with the straight edge and light. That is okay as it will pull down flat when the bolts are tightened. Any local deviation from flatness more than a couple thou demands resurfacing, again pay special attention between #2 and #3 exhaust valves.

Head gaskets are nowhere near 1/8" thick. Thay may be 0.030" (or less) new and may compress to as little as 0.020" when installed. Use the black fiber head gasket from Felpro/Payen with NO SEALER ADDED, and you should have no sealing problems. I have never known one of these to leak as long as the mating surfaces are flat. See here:
Order the head gasket for 1975-1980 MGB, and you should get one of these. One size fits all 1500-1800 engines including standard overbores, but possibly not the over bored 1950cc modification.
Barney Gaylord

Go with the hone job...and follow Barney's info about ring fit'll be fine. However, you might consider getting the block tanked just to be sure it clean....water jacket, oil galleys, etc. I would check the valve guide clearance, replace stem seals and lap the valves while your at it. Easy enough to do.

G T Foster

Hot tanking commonly screws up the cam bearings, then they need to be replaced. Otherwise cam bearings usually don't wear much in use. If you're going to have it chemically cleaned, see if the shop can do something that won't eat the cam bearings.

For my home jobs I wash it with gasoline and rinse it with lacquer thinner before painting it. A cut down paint brush makes for stiff bristles good for scrubbing.
Barney Gaylord


FWIW, I wasn't trying to advocate one approach or another....just trying to think through why the machinist was being so insistent.

Let us know what you do!
T McCarthy


A couple more things- hot tanking will eat the engine number plate so, if you want to keep it, take it off before cleaning. Frost plugs and oil galley plugs shoul also come out. If the replacement didn't blow up, I'd skip hot tanking. Take the block to a car was and feed it quarters until the water out of the block runs clean.

Any competent engine shop can hone a cylinder. They can also tell you what you will have when they are done. The advantage of a professional hone is that it will fix slight out of round and/or taper conditions. It will also remove the top ridge. Last time I had a motor done it was under 100 dollars for honing four cylinders.

If you use a drill- mounted hone, first ridge ream the bores- this gets rid of the unworn bore section above the piston ring upper travel limit. It will make everything else easier. Most rental shops have these. While you are there, see if they have a Sunnen, Lisle or similar portable hone. These are similar to the professional units but attach to a drill.

Hope this helps,
Bill Eastman

Bill is absolutely correct. Commercial engine cylinder hones are designed as an ultra precision method of removing the minor glitches remaining from the bore and establishing the final sizes to tenths of a thousandth of an inch. Over the counter hones are not capable of doing very much to correct a tapered or out of round bore.
D Sjostrom

Be very carefull with how much you hone. If you try removing all of the taper or a deep defect you may find yourself needing the next oversize piston. It didn't sound like you wanted to go this far. As long as you can get a decient cross hatch on the bores a couple small missed areas won't kill you. It isn't going to make the motor blow up, you may just use some extra oil. Then this winter you can tear it down and do it right.
J Heisenfeldt

It's definitely worth hot tanking a block to clear out all the accumulated crud. It will then run a lot cooler.
dominic clancy

This thread was discussed between 29/07/2008 and 30/07/2008

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