Rebore issues...

[Samal Yb]

Dear Guy-croft, after verify my cylinders are perfect, the form has not being change. I must change piston rings for have broken one of the oil rings. I have the NPR piston rings in STD (75,00mm), I have to make some lines:





I have got a stone liner, is not the incrusted diamond in copper and I think it might be a very different lines.

The question: Is there a significant difference in that? How different may be the lines and the piston rings brake in? and off course

I used to assemble Lycoming aircraft engines, and they make their cylinders chrome, their piston rings have Nitriding surface.

Another question: Lycoming break in said that a new, remanufactured, or overhauled engine should receive the same start, warm-up, and preflight checks as any other engine. There are some aircraft owners and pilots who would prefer to use low power settings for cruise during the break-in period. This is not recommended. A good break-in requires that the piston rings expand sufficiently to seat with the cylinder walls during the engine break-in period. This seating of the ring with the cylinder wall will only occur when pressures inside the cylinder are great enough to cause expansion of the piston rings. Pressures in the cylinder only become great enough for a good break-in when power settings above 65% are used.

Full power for takeoff and climb during the break-in period is not harmful; it is beneficial, although engine temperatures should be monitored closely to insure that overheating does not occur. Cruise power settings above 65%, and preferably in the 70% to 75% of rated power range should be used to achieve a good engine break-in.


I also read Motoman's break-in secrets, he agree with Lycoming, and it start a controversy in my mind, the car's manufacturer advise to the customer to not run hard the engine in the first 1000 km.

How must be the engine break-in for you?


Waiting response, Iv’‚¡n.

[Guy Croft]

Hi,
sorry for missing this important post. I hope I am not too late.

Forget the Lycoming procedure. Your bores are plain spheroidal - nodular -graphite cast-iron and excessive ring-bore interaction during break-in (bedding or running-in) will damage them. There is an article in GC V/W about break-in period, I will check it's up to date and post the link below.

You need, for a start, to get a Flex Hone brush and hone with that, not with parallel stones. Read this GC 'How to' info, reproduced from something I send out with my race pistons:


FITTING RACE PISTONS

(See also cam timing & fitting/dry build data sheet)


Read this before doing anything:
Unpack the pistons, pins and rings and clean them with brake cleaner or other air-dry solvent and airline. Likewise the bearings too.
The pistons are easily scratched so be careful. Use extreme care with piston rings because if you break one ¢‚¬Å“ you¢ž¢ll probably have to buy a complete new set. If you¢ž¢re fitting race pistons to old OE rods ¢‚¬Å“ have you size-checked the rod big-end bores and piston pin bushes, and crack tested them and the bolts too? Have you checked the oil spray holes? Remember - a defective rod can break or cause seizure.
Does your crank spin freely by hand? It must ¢‚¬Å“ and if it does not you need to find out why. To fit pistons the crank must be installed ideally with the flywheel fitted so you can turn it over easily. You need a clean workbench, a vice or stand to hold the piston/rod assembly while fitting rings, a ring fitting tool and a piston ring compressor.
Have you chamfered and carefully radiused the top of your cylinder bores? You need to or you¢ž¢re going to break a ring during piston fitting.

YOU¢ž¢RE THE ENGINE BUILDER SO BE CAREFUL!

Block prep
The honing finish in cast-iron bores is very important.
For OE type rings with chrome face top ring you should bore and parallel hone to bore size then finish (plateau) hone for 20 seconds at 350-470 rpm using a 180 grit ‹Å“Flex Hone¢ž¢ (see http://www.brushresearch.com) with 10W40 multigrade engine oil to develop a 45 degree cross hatch with a further 10 secs at a slower stroke speed at 30 degree. The bore size should not exceed 0.01mm of the specified diameter. The top edge of the bores must be lightly radiused with 120 grit carborundun tape so the rings slide in without damage.
GC supplied ‹Å“aftermarket¢ž¢ race type CP and Total Seal rings are usually pvd-chromed or moly-faced ductile iron top ring (quite similar to steel) and require 240 grit Flex Hone. CP race rings require honing for 20 sec at 45 deg cross-hatch angle followed by final 10 sec overlay at 30 deg.
TS rings need less cross-hatch angle than CP types, 20-30 deg is ideal.
After honing ¢‚¬Å“ the bores need special care or you¢ž¢ll get no oil retention - hot detergent wash very thoroughly and rinse off before they start rusting - then use white cloth or good quality paper wipes and rub the bores hard with Auto Trans Fluid (ATF) until no trace of grey carborundun dust remains. The commonest cause of poor-ring bedding or wear is entrained honing dust in the bores. If you don¢ž¢t follow this procedure your rings are never going to bed-in.
Finally clean the bores with brake cleaner and dry them, and oil them lightly with engine oil ready for build.


Fitting pistons
Size check the bores with a proper bore gauge. The rings ¢‚¬Å“ or the packet will normally be marked eg: CP rings 3.327 (inches) ie: to suit bore 84.506mm. The bore ¢‚¬Å“piston clearance and desing bore size for your pistons is stated on the CP piston spec sheet (if supplied) or GC data.
Invert the pistons and offer up to the bores to check the skirt-bore clearance with a feeler gauge.
For GC CP pistons 0.0035‚ is ideal but they can be run looser, up to 0.006-0.007‚ although there is no advantage to doing this and ring gaps increase the bigger the bore.
Check ALL the ring gaps in the bores individually and ADJUST by gently filing from outside to inside with a fine (sharp) file followed by dressing of corners with a fine carborundun stone. GC recommended end gap on Fiat type TCs (80/84 nominal bore) is 12-16 thou top and second ring (though 18 thou is OK on second ring) and lower rings 12-20 thou.
Before fitting pistons to rods offer up the pistons to the combustion chambers using a gasket and locating dowels to centre them, and make sure that the piston dome does not foul the chamber. You need a minimum of 1mm between piston and head.
Fit the pistons to the rods before fitting the rings.
Make sure the piston numbers correspond to the rods, as a rule keep rod numbers on the exhaust side. On the 8V TC the inlet valve relief is of course the larger of the two. Make sure the pin clips are fully engaged and tap the pins with a soft hammer after fitting to make sure they are installed properly. Make sure the rod bolts are properly fitted.
Check one piston/rod assy in a bore to assure that the compression height of the piston is OK. You can run pistons with crown above the block deck but make sure the piston is clear of the gasket. This is especially important with gasket with split fire rings eg: Nava Linea.
If you need to check your piston-valve radial clearance (see GC dry-build data sheet) this is the time to do it.

Read any installation instructions supplied with the rings.

Clamp the rods in a vice with soft jaws or use a jig to hold the pistons steady while you fit the rings. The second and top rings require the use of a piston ring fitting tool. If you try to do it without the correct tool you¢ž¢re going to break a ring ¢‚¬Å“ you must not twist the top and second rings onto the pistons. The 3rd ring is fitted first and by hand ¢‚¬Å“ if it¢ž¢s a three piece it¢ž¢s very flexible and can be fitted spirally (wound onto the piston) but two-piece bottom rings ¢‚¬Å“ which have very low outspring - should be expanded by hand very carefully to fit them. Markings on rings generally go on top, be careful not to get rings upside down. Fit the rod bearings to the rods and oil them. Oil the crankpins too, along with a light coat on piston pins, rings, skirts. The parts do not need to be dripping with oil! Use a good quality piston ring compressor like Sykes-Pickavant, oil it thoroughly and use it to compress the rings lightly, leave about 4mm of piston skirt exposed so you can centre the piston in the bore. Line up the rod on the centreline of the bore. (It¢ž¢s a good idea to use plastic sleeve protectors on the rod bolts in case they hit the crankpins).
Make sure the end of the compressor does not catch on a ring-end. Do cyls 1 & 4 first and set the crank so the crankpins (big-end journals) are at bottom-dead-centre (BDC). You can do them at tdc but it¢ž¢s easier to see the rod numbers and fit the nuts. Sit the piston in the bore and align the ring compressor against the bore face before final tightening. When you have the rings reasonably well compressed tap the piston dome with the wooden handle of a light hammer, a sharp push should take the piston into the bore. If it sticks don¢ž¢t force it, take the piston off and investigate. Usually it¢ž¢s a ring catching the bores. If you knock the piston too slowly the rings will spring out and catch. If you force the pistons you¢ž¢re going to break a ring. Sometimes the rod gets caught on the crank! Once all the rings are in the bore, push the piston gently down by hand till the bearing engages the crank.
Do both pistons and turn the block over and put the rod caps on (make sure you don¢ž¢t mix them up) and fit the rod nuts, tightening them lightly so the caps are just secured and the bearings won¢ž¢t move when you turn the crank over. You can then turn the crank so 2&3 are at BDC and fit those pistons.

When all your pistons are installed, before you torque them up ¢‚¬Å“ double-check the setup height of the pistons relative to the block face.

Lubricate the rod bolt threads and nuts and torque up the rods. Always double-check the torque setting. If you get it wrong eg: by using 50Nm instead of 50lbf ft or use a cheap, inaccurate torque wrench or not lubricating the threads - you are going to ‹Å“throw a rod¢ž¢ later on. Mark the rod bolts with a dab of paint to remind you they have been done. Check that the engine turns over quite freely, of course there will be friction from the new rings but the crank should have a consistent feel as you turn it over and not feel tight.

A crank will generally only feel tight if:
1. A rod cap has been mixed up
2. The bearings are the wrong size
3. A bearing is getting scored by a high spot on the crank
4. The bores are not parallel
5. One of the piston rings has been fractured during installation
6. The crank is bent

Typical turning torques measured from crank nose with torque wrench, with pistons fitted and head off are:
OE rings 15lbf ft
CP race rings 10-12lbf ft
If the block is oversize (eg: bored and honed to 84.52 instead of 84.50mm) the turning torque will be less initially ¢‚¬Å“ ie: prior to break in - due to lower ring friction, but the practice of making the bores looser than GC spec is not generally recommended)

Don¢ž¢t just bolt the head on now!
The first thing to do is accurately fit a TDC pointer! Don¢ž¢t wait till the head is on.
The next phase is dry-build ¢‚¬Å“ which is an essential part of race engine prep. Read and follow the GC instructions supplied separately.
GC

[Guy Croft]

http://guy-croft.com/viewtopic.php?t=78

[sumplug]

Only thing you missed Guy, is position of rings. Ring gap position, 30 degrees to the next ring.

Andy.

[Guy Croft]

hey - maybe I missed it, maybe I didn't!

There are so many designs now that I deliberately did not quote that info.

The ring position procedure for TS rings eg. is a quite involved and I have to refer to the drawings in their *'ring instalation' data every time....!

GC


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