Piston to bore clearance.

[John_Progess]

I am trying to determine if I need to bore my engine. It hasn't been run in about 12 years so I don't know the condition when it last ran. What is the standard piston to bore clearance and how much ovality is permitted. I don't plan to race this thing but I do want good performance and if I have to bore the block then thats ok. It has standard pistons in it now. Thanks and have a good day!

John

 

[Randy Forbes]

The factory's recomendation is in the manual, and varies by engine type.

The BN4 is .0226" to .0008, and the MKI 3000 is top of skirt: .0032" to .0043"/ bottom of skirt: .0001" to .0016"

So as you can see, you clearly need to look this up for your specific engine.

 

[Bugeye58]

Randy, .0226" for the BN4??
Seems like an awful lot! /bcforum/images/%%GRAEMLIN_URL%%/grin.gif
Jeff

 

[Randy Forbes]

Randy, .0226" for the BN4??
Seems like an awful lot! /bcforum/images/%%GRAEMLIN_URL%%/grin.gif
Jeff

Yes it does, doesn't it. Their typo, not mine /bcforum/images/%%GRAEMLIN_URL%%/smile.gif

Probably .0026"...

 

[Bugeye58]

[/quote]
Yes it does, doesn't it. Their typo, not mine /bcforum/images/%%GRAEMLIN_URL%%/smile.gif

Probably .0026"... [/quote]

I think that would be the number I'd go with!! /bcforum/images/%%GRAEMLIN_URL%%/grin.gif
.0226" would sound like the pistons were swapping holes. Briefly!
Jeff

 

[BOBBYR]

Hi John, .022 clearance really that much.Is this engine still assembled or do you have it apart? I used to do machine work on engines for ford motor corp.. If I can help you let me know Bobby R

 

[John_Progess]

Bobby R,
The engine is still assembled. I did take off the head just to check the general condition of the pistons and valves. The engine is painted a different green than normally found on Healeys, sort of an aqua green, so I was concerned that it had been rebuilt in the past. I found out that the pistons are standard and I don't think it has been rebuilt but I don't know why the different color. Doing a very hasty check of piston bore I think it may have .001" ovality and I don't know if this is ok or if it will need reboring. Other checks may determine the answer. I would appreciate your view as a knowledgable machinist. Thanks and have a good day!

John

 

[BOBBYR]

Hi John, Piston to wall clearance is measured when the engine is disassembled.Have you ever run this motor? I'm not trying to rain on your parade but I would want evaluate it before doing a total rebuild.I think at this point I would put the head back on with a new head gasket.I hope this is a help to you. Answer back when you get the time. Bobby R

 

[John_Progess]

Bobby R,
You are not raining on my parade. Any help is appreciated. I have not run the motor but a friend of mine did see it run and he thought it had a valve leaking. I had it out of the car and then learned that the frame needs to be loaded with the drivetrain before any structural repairs are made so I put it back in the chasis. It is attached to the tranny loosly basically for weight and support. I took the head off to look at the bore and everything looked good. I don't know how many miles are on the engine. What would you suggest I check first? Thanks and have a good day!

John

 

[BOBBYR]

Hi John, Lets get some fogging oil from a marine store and coat the cylinder walls and the tops of the pistons. This oil comes in a spray can so spray enough that it will work itself into the top rings of each piston.The head should be sent out for valve job on a total rebuild, so let's do that now.If the engine does need the rebuild after you get it back together, the head is already done.What I am afraid of, is that you have too many projects going on at once.My goal ,at this point, would be to get this motor bolted to the tranny and get this motor ready to fire. Get back to me as soon as possible. Bobby R

 

[John_Progess]

Bobby,
I am retired so time is on my side, hopefully. I so have a leak in no 6 intake valve. On my other cars when I did the heads I modified them for no lead gas with stellite exhaust valves and or new seats. Is this the way to ge here? I checked no 5 bore and found it to be just slightly less that 3.283" with a ridge line at the top of less than .001". According to my manual the stock bore should be 3.281" with a piston skirt clearance at top of bore of .0032 to .0043 so it almost seems like the bore is a little tight. Am I reading this correctly? There are not alot of marine stores here in Utah as this is a desert but I will try to find this fogging oil. Is there something else in case I can't find any? Do you think it would be easier if I talked to you on the phone? Thanks again and have a good day!

John

 

[BOBBYR]

Hi John,I hope you're doing well today. How about we skip the fogging oil.Everything you have told me up to now leads me to believe that your cylinders and pistons look good.The true way to measure piston to wall clearance is as follows . Picture the block completely stripped.You would clean each cylinder with a flex hone using an up and down motion.Now clean the bore the best you can. Machine oil on a rag works well but kerosene is fine also.Remove all grit.Coat the cylinders with clean 10-40w oil.Placing a .0035 clearance tape on the full length of the cylinder you would now slide a clean piston without rings into the cylinder. The tape would remain between the wall and the piston skirt the whole way down .This is the true way to fit pistons.

I think what I am trying to do is talk you out of overhauling this engine. Reconditioning the cylinder head and getting this motor running is the direction I would go. A compression test can tell you the condition of your cylinders. Oil pressure can tell you the condition of your bearings. I know this letter is scattered and I hope you can make heads or tails of it. I am checking the specifications you have given me and I'll get back to you soon. Write back and give me your thoughts.

Bobby R

 

[John_Progess]

Bobby,
It would not be all that much trouble to get the engine started. I have the carb overhaul kit and the rest, other than the head, is just re-attaching the necessary equipment, starter, temporary fuel system, power, manifolds, securing the tranny better, etc. What is your thoughts on the head rebuild as far as no lead modifications? What about the cooling water jackets as far as gunk is concerned, or would you still strip the block and have the it boiled just not do any machining. Would you remove the main bearing caps and check clearances?
It would be nice not to have to overhaul a good engine. If it ain't broke, don't fix it. I did check no. 3 bore and had about the same readings! Thanks again!

John

 

[BobHorvath]

I don't want to shift the direction of this thread however I have a simular interest. Runs good but burning oil, some what low compression 120-130 taken @ 6300MSL. I am considering a valve job, honeing the cylinders and a new set of oversized rings, all only if the pistons are still in spec with respect to the bore. I plan to do all this with the block still in the car. Fire away please!!

 

[BobHorvath]

I don't want to shift the direction of this thread however I have a simular interest. Runs good but burning oil, some what low compression 120-130 taken @ 6300MSL. I am considering a valve job, honeing the cylinders and a new set of oversized rings, all only if the pistons are still in spec with respect to the bore. I plan to do all this with the block still in the car. Also plan to replace bearings if the crank is OK.Fire away please!!

 

[BOBBYR]

Hi John, The no lead mod.,I think is the way to go. Putting the head on and being able to check this engine out could save you time,money or both.The total rebuild of that block would be boring the cylinders .020 and then have the machine shop final fit new pistons to the exact piston to wall clearance you want.Right now I would like to get oil on the tops of the pistons and into the rings.This might not get the results you want but I would take the chance.I would not get into the lower end of this motor yet.I've got one question.With the head off,how easy does the crankshaft spin? Get back to me when possible.

Bobby R

 

[RonMacPherson]

Huh? I'm a little lost here. Oversize rings? Has Austin Healey changed the construction and design of otto cycle internal combustion engines?

If not, then you're being led down the wrong road if you think that "oversize rings" on standard size pistons will offset your oil consumption. The only possibility of ring replacement that might have any input on the oil control would be going to "total seal" type of rings. All other rings need to have a specified gap between the ends, which is by the way a very good idea to check any time rings are being replaced. Rings are built to fit the pistons, side clearance and end gap are spec'd by the manufacturer or engine rebuilder. If you do NOT spec your rings you can expect galling, or excessive blowby, either too tight, or too loose.

First thing to do is pull the pistons, take measurements with a cylinder bore gauge, measuring bore, out of round and taper. Or take it to a machine shop and have them measure it. Probably more effective and less expensive as good bore gauges get rather expensive.

 

[bob hughes]

Guys

For what it is worth

I am suffering the same oil burning problems, doing 1 litre to 120 miles, and burning it on the rear three pots. My Healey guy talked me out of re ringing as he says the it would not be worth the effort and I may be greatly disappointed with the results. He recons best put the money into a rebore and do the job properly.
I have had to recon the head as I vaporised an exhaust valve, so I whent the whole hog and had the head converted to unleaded with all new guides and valves. The engine will be one of my next projects.

Bob

 

[John_Progess]

Bobby,
I can turn the crank with a wrench on the crank nut easily. It doesn't spin but it turns over smoothly. Do you think I should still get some fogging oil in the bores? I must have drained the oil when I origionally pulled the engine as there is no oil in the sump. In the past I have just replaced the exhaust valves with stellite and all new bronze guides for the no lead modification and did not replace the valve seats if they looked ok. Is there something else I should do? Have a good day!

John

 

[anthony7777]

john p, how are you? sounds as if youve come a long way with the restoration, im not going to tell you what to do with your engine but rather explain what i would do/have done with an unknown in any situation, RIP IT DOWN! someone says it may have a valve leak? someone says it hasnt run in 12- whatever many years? youve never seen it run? humm, sounds like its time to pull it apart and find out what it aint, then again that's just me, its been mentioned here that in order to get a comprehensive evaluation of piston/piston ring sizes, ovality/clearance, evaluation of bearings etc, its got to be broken down, ah that's just me. the shortest path in achieving a goal is often the most difficult/expensive. /bcforum/images/%%GRAEMLIN_URL%%/frown.gif