Cylinder bore prep

[tricky]

Guy and forum,

A question regarding cylinder bore prep procedure, first some background.

As Guy knows but for other readers, I am building a road race engine for an integrale 16v. I entrusted the job of bottom end machining to, well a machine shop given the specified measurements and bore honing methods as stipulated by the piston supplier. When the pistons arrived in the post last week I went through the usual checks, ring end gaps and bore to skirt clearance ( feeler gauge ). Much to my suprise something didn't check out and in particular No.2 was very tight, nearly 2 thou under the correct clearance and all the others felt slightly loose. Just shows you really should never assume anything !

I returned to the shop with the block and as it turns out the engine was mistakenly bored over size by .06 mm average and not very consistantly accross the four bores. This block is basicly junk to me now and I am starting again with a spare.

I took advise from another GC forum member and bought my own bore gauge, 3-4 inch micrometer and standard slip gauge to calibrate from to minimise error. For this reason aswell finish honing will be done by me now.

Bore specs are;

Hone cylinders with 280 grit followed by, a light 380 grit brush with a 60* cross hatch.
For a faster ring seal use only the 280 grit.

Finished bore size is 84.5mm

I plan on buying a silicon carbide flex hone in the closest availible grit as stated.

My questions are;

If I get the machinist to bore and hone with 280 grit conventional method, what size should I specify him to finnish at ? In other words, should I leave an allowance undersize for my flex honing op and if so roughly how much material will the finish hone remove ? I have no previous experience with this tool.

Secondly, which way is the standard for viewing a cross hatch in terms of angle ? Is a 60 deg angle normally looked at as 120 degrees going down the bore, or 60 down the bore and 120 accross ? Obviously a good practice at this will be possible on the now dead engine.

Thanks kindly, Richard



This looks pretty, but turned out to be far from accurate.

[Guy Croft]

Just get it bored and 1st stage (parallel) honed to finished size to any finish they recommend then finish-hone yourself with a Flex-Hone.

If you know the top ring material & coatings I can tell you the grade.

With Flex-Hone you quite simply follow the instructions on the packet.

GC

[tricky]

Thanks Guy, that answers my main concern.

I don't know what the ring material is all I can really offer is a photo, the darker one has a wiper ridge and both are marked N50 on top, I'm 99% sure the dark one is the second ring ( I forgot to ask, but I will).

Regards
Rich

[Guy Croft]

Likely ductile iron or steel top rings - probably pvd chrome or similar.

The honing depends on the top ring and I suggest that 240 grade Flex-Hone will be perfectly OK.

That is what I would do anyhow. You could always ask the piston (& ring?) manufacturer for the top ring material/type..

G

[tricky]

thank you again, I will ask the question. I will be in touch soon RE; cams etc. (shouldn't be long)

Rich

[Guy Croft]

This is the manufacturer of the rings.

http://www.npr.co.jp/english/products/p ... pistonring

Unfortunately having looked I see they also do chromed rings which need 180 grade. You can see the pvd ones listed (top ring I mean) which work fine with 240 grade. Clueless as to which your top rings are Incidentally they are the ones on the left of your photo. If you hone too fine with chrome they will never bed in.

Sorry - You asked earlier about honing angle: as far as Flex-Hone and these ring types you hone at 30 deg cross-hatch for about 30 sec and then a final 10sec at 45 deg. For the latter you speed up the stroke speed. You can 'read' the angle in the oil as you're honing, see pic. The bores have been chamfered and then radiused. Using an old gasket as a protector is a good idea if the block has already been 'decked' (refaced) but I rarely do as the hone does a nice job of radiusing the top chamfer - if it can 'see' it.

Flex-Hones self-center and don't induce any change of dimension - you could hone all week at 240 grade and not measure any difference. I have in fact tried to correct a tight bore once using 80 grade and gave up after honing the blasted thing for about 2 hours.


G

[tricky]

Hello Guy just a quick follow up here,

Rings are confirmed as;

Top, Nitrided steel.
Second, Cast napier.
Oil, Stainless steel.

Thier recomended bore finnish is as above (my first post) but on further query, their engines are built using a finnished bore with diamond 320 and 500 to achevie a 10-15 RA.

What would your opinion be, based on this new information ?

Thank you in advance.

[Guy Croft]

With those top rings - 240 grade Flex-Hone.

I've discussed that issue in the past with BRM and UK agent Pacehigh so you can definitely trust my recommendation.

GC

[tricky]

I do, thanks again.

RO