Piston to bore clearance How tight is too tight?

[trunt]

Just assembling my engine and inserting the pistons is a bear. The clearance at the skirts are so very tight that its a real fiddle getting them started in the bore. I cannot get a 1.5 thou feeler in at the skirt on any of them. I even have a 2 thou wire feeler that I cannot get in even at the oil control ring level. I checked them all and they all actually go in the bore but its tight as heck.

I have about 15 thou on the top land

The shop had the pistons and supposedly set the bores accordingly but all seem real tight compared to other engines I have built.

I see the spec is about 2-3 thou. I would measure them but don't have a decent bore gauge
So.. before |I go running to the machine shop (AGAIN!) How tight is too tight?

Terry

[Guest]

I have about 15 thou on the top land

I see the spec is about 2-3 thou.

How tight is too tight?

You will never damage an engine with clearances that are on the large side.
You will have excessive heat, and scuffing from making too tight.
You can't measure at the top land because pistons are conic.

The right clearances entirely depend on the silicon content in the aluminium.
If you check you will find Mahle typically use about 0.03mm on their pistons and that is "German spec" tightness.
(Just over 1 thou), but their pistons have plates in to inhibit expansion, and have other features.

[flying farmer]

Taken from the Haynes manual:
Grade F
Cylinder bore 3.3859 to 3.3864
Skirt diameter 3.3834 to 3.3858

Grade G
Cylinder bore 3.3859 to 3.3864
Skirt diameter 3.3834 to 3.3839

So it looks like you need 2.5 thou

Neil

[tractorjames]

Be very careful, i had 1.5 thou at skirt (machine shop i used for boring an who supplied pistons assured me to be correct)and the pile of ****e kept nipping up, knackering all 8 pistons. Increased to 3 thou on second rebuild been fine ever since.

James

[trunt]

And where I can measure - at the top land,

3.3853 min bore - 3.3650 max top land = 20.3 thou. I see about 16-18 thou

Back to the machine shop..


I have had it with machine shops. I gave them the specs and the pistons!

[trunt]

Be very careful, i had 1.5 thou at skirt (machine shop i used for boring an who supplied pistons assured me to be correct)and the pile of ****e kept nipping up, knackering all 8 pistons. Increased to 3 thou on second rebuild been fine ever since.

James

And that's what I am afraid of..

[Guest]

In view of the sheer size and weight of any multi cylinder engine like a V8.
It simply is NOT economical doing the job twice for any reason at all.

Most people get to understand this the hard way if they don't do it for a living.

Nobody profits from having to do the job twice, so best do it right ONCE, then get on with something more productive in the time economised!

[trunt]

In view of the sheer size and weight of any multi cylinder engine like a V8.
It simply is NOT economical doing the job twice for any reason at all.

Most people get to understand this the hard way if they don't do it for a living.

Nobody profits from having to do the job twice, so best do it right ONCE, then get on with something more productive in the time economised!

Well they are going to hone it out for me so yes, waste of time for both of us.

I think these guys work with modern stuff so much that they forget the extra tolerances on 50 year old engines! (But they had the sheet from the ROM!!)

[mole42]

they forget the extra tolerances on 50 year old engines! (But they had the sheet from the ROM!!)

So you gave them the data and they still got it wrong?

We really do need a naming and shaming section on this forum, we are hearing all sorts of horrors and unless we have spent the money and the time to get it right, there's no way of knowing that, for instance, seller X sells new CW&P that are not fit for purpose.

Rant over. For now.

[trunt]

I have had so much bad things happen with machine shops, aside from the delays and crazy time it took to get them to do the job.

shop A
TR250, good engine build no problems
Mini good engine block build but I gave them my transfer casing cover to get the bearing out, they melted it. Also as compensation asked them to mill my manifold, 6 months and they didn't do it. I took it elsewhere.
Shop now closed.

Shop B Mini
valve guides too tight, they were sticking, had to remove it and have them rehone.

ShopC
Good MGB build!
Missed the main caps being misaligned, gave me a rusty pitted crank back (when they had another crank that was in better shape) called me an A$$$hole for questioning his 30 years of experience.

Shop D
Bores too tight.

These guys have cost me $thousands you would have thought I WOULD HAVE LEARNED TO QUESTION ALL THE WORK!

[Mark Serj]

I think getting recommendations from others who have had personal experiences over a (preferably) long period of time is the best bet. For example, I have just had some work done by a company who a local classic car restoration firm have used regularly for quite a while with no issues. I just rang them and asked who they used for xyz (as they couldn't do it themselves) and whether they had ever had any subsequent customer issues. I then visited the company that they used and talked through the whole process, making sure I met the machine operator who was going to do the work and that he had done EXACTLY this work many times before. When you have been in engineering as unfortunately long as I have, you generally get a good or bad feeling about their skills, and just as importantly, their attitude, although I do very much recognize talk is cheap! Oh, and if the operator was still waiting to experiment with shaving, I would just walk away - no offence to the young of course.

[davidf]

I think getting recommendations from others who have had personal experiences over a (preferably) long period of time is the best bet. For example, I have just had some work done by a company who a local classic car restoration firm have used regularly for quite a while with no issues. I just rang them and asked who they used for xyz (as they couldn't do it themselves) and whether they had ever had any subsequent customer issues. I then visited the company that they used and talked through the whole process, making sure I met the machine operator who was going to do the work and that he had done EXACTLY this work many times before. When you have been in engineering as unfortunately long as I have, you generally get a good or bad feeling about their skills, and just as importantly, their attitude, although I do very much recognize talk is cheap! Oh, and if the operator was still waiting to experiment with shaving, I would just walk away - no offence to the young of course.

Unfortunately, even that can back-fire.
I had a diff rebuilt - the garage seemed very competent when I spoke to them, and showed me a TR diff they were doing. I saw mine assembled before the cover went back on, with the marks on the Crown wheel teeth looking exactly like in the manual. I am very pleased with the result - no backlash and no noise.
Recommended this garage to another user on here, and his turned out to be a nightmare.
Very disturbing for the other member, and I feel very embarrassed about it.

[trunt]

Took the block back today, they measured 1 thou of clearance and are fixing it.

One thing they did that just did not occur to me was to insert the piston upside down.. It makes it so much easier to see whats going on!

It seems they may have had a bad calibration on the bore dial for a short time.. Lucky it wasn't 2 thou ot the other way huh!

Terry

[Guest]

Took the block back today, they measured 1 thou of clearance and are fixing it.

It seems they may have had a bad calibration on the bore dial for a short time..

WTF who are these muppets?
They have no business at all doing reboring if their instruments are not 100% reliable!

1 thou?? That is GUARANTEED SEIZURE territory!

You need AT LEAST double that figure especially as it's an engine prone to running hot.
Typically 2 1/4 thou is safe.

I believe the Jag clearances were always on the wide side 3.5-4thou typically.
They rattle a good one from cold, and always have, but completely quiet once warm.

I don't care about a rattly engine cold, but sure thing I DO care about an engine that suffers a partial seizure at 130mph!

[Mark Serj]

We are talking radial clearances here guys, yes?