17 replies to this topic

[chrisn89]

Hi Guys,

 

As the title states, in desperate need of some advice from those with the experience.

 

How tight should a freshly machined block and brand new pistons+rings be? Here's the shortened longer story:

 

• 1380 A+ block machined locally
• Pistons are the ones minispeed recommended (Powertune)
• Block was machined to a 3 thou piston gap as advised by minispeed and ties in with haynes manual figures

 

• Crank is in fine, spins freely, end float is good.
• "Dry fit" the pistons without rings, crank turns and everythings ok
• Measured ring end gap at ~15thou down the whole block for all 3 rings for all pistons (bit bigger than people say)
• Fit pistons with rings - incredibly tight 
• Managed to get all 4 pistons fitted
• Almost impossible (almost) to turn the crank over - with a 2ft bar on the flywheel retaining ring
• Everything is heavily oiled and the fact everything turns well before fitting the rings shows the bottom end isnt an issue

Surely this is too tight? I've read in places that this will free off but i've never had anything this tight before. Even if i do carry on its going to be near impossible to get the timing accurate fighting with turning the thing over.

 

Is there anything im missing here? I had a go at measuring the piston gap myself but im limited to a vernier.. I mean you expect the machine shop have done their job to 3thou and you dont expect an issue with minispeeds parts... do you?

 

I'm a bit disheartened because im about 1K into this now and dont even have a short engine to show for it as it stands.

 

Any help would be appreciated

Chris

[matty...]

When you fitted the crank, did you torque the mains up to check they are spinning freely, or were they just nipped up?

Have the rods got clearance on the little ends, between the the rod and piston? Eg rods fitted correct way round.

Correct rings for Pistons? Do they turn freely in the Piston grooves before fitting them? You can measure the groove depth with a vernier and piston ring height to check that with the ring fully compressed they are not bottoming out on the bottom of the ring grooves.

The piston/bore clearance can be measured with a feeler down the side of the skirt. It sounds like the rings aren't the correct ones for the pistons from the description. When you say the piston rings were tight, was that tight in the grooves? They should spin freely.
A well built engine should rotate with just using one hand on the crank to turn it, with no tight spots.

Edited by matty..., 06 September 2015 - 03:51 PM.

[chrisn89]

Crank is fully torqued as were the con rods for the dry run so I'm certain the issue is with the Pistons/rings.

Yes rods are free on the gudgeon pin and turns over well before rings are fitted

I can only really assume rings are correct? They came with the Pistons. Yes they turn freely in the piston before fitting.

The verniers jaws are too thick aside from the oil ring gap.

Piston bore clearance ive measure but I heard it's quite crude with a feeler gauge? With a feeler gauge it's bigger than the 3 thou so no concerns there.

only assuming but the rings supplied with the Pistons you'd have to think are the right ones? I'm giving minispeed a ring tomorrow but you'd have to think the machining and the products are two things you could count on? Frustrating..

[carbon]

+1 for Matty's comment about rings.

 

Turn engine to get piston to tdc, piston should sit central in the bore with about 20 thou radial clearance all around. Can you move  the piston slightly from side to side (at 90deg to crank axis)?

 

If it does not move then recheck the radial depth of piston ring grooves and radial depth of piston rings.

 

Depending on the design the oil scraper ring can give a lot of friction until bedded in.

[Stevie W]

Hi,

 

I tend to agree with the above comments about the rings. With pistons and rings fitted (assuming everything's is correct) the engine will be slightly tight but should "ease off" with the pistons at the top/bottom of their stroke where there is momentarily no piston movement. You certainly shouldn't need as much force as you're reporting!

 

If the rings were bottoming out on the piston grooves (from a depth point-of-view) I would've thought it would be almost impossible to compress the rings enough to knock the pistons into the bores. When fitting pistons/rings (using a ring compressor) how much force did you have to produce to get the pistons down the bores? Well oiled, you should only need the wooden handle of a hammer to gently knock the pistons down the bores.

 

Was the block offset bored seeing as it's a 1380cc?  

 

Cheers, Steve.  

[matty...]

The feeler method is a bit crude, but you really need to have dial bore gauge, air gauge.

Take one of the Pistons out check the markings on rings, on to top edge on each ring it should say the overbore size. There are different ring sets, I've know of people getting 3 of the correct type and one odd set before.

If you put the the ring on its own in the bore, like when gapping,and measure the ID of the ring

Now fit the ring in the piston, and using the end of the vernier you should be able to measure how far under the ring sits beneath the piston surface. Now Measure the ring height and piston dia either side of the ring groove, and measure the skirt.

Piston skirt - difference in piston ring dia measurement- (2x ring height) - (2 x depth ring sits beneath surface) should work out to be less than the ID of the ring in the bore, I would expect 10/20 thou difference.

Edited by matty..., 06 September 2015 - 05:53 PM.

[chrisn89]

Ok guys so I got back and measured the depth of the piston slots (I'm guessing they have a name but you know what I mean) and they're .14in in depth.

The top two rings are .127 so I'm guessing they'll close up okay?

The oil ring is around .15" including the spring. Is this to be expected or are they too big? Should I try again without the oil ring fitted?

Also I can get 10thou feeler in between piston and block when fitted without rings if this sounds ok?

Thanks for all the help,
Chris

[matty...]

Sounds like you have the wrong oil rings/ ring set? Quite often the top rings are a common size, and it's the oil ring width,depth design that's differs. I would try a piston in without the oil ring completely and see if that fits and turns nicely.

10thou would be about right if you're measuring it at the crown. The crown of the piston is smaller than the skirt.

[carbon]

Sounds like top and second rings should be OK.

 

Try fitting one piston with top and second ring, see how this feels.

 

Then I would suggest adding second piston with just the oil rings fitted, and then see how much stiffer this is to turn.

 

See Matty's post above, he beat me to it!

Edited by carbon, 06 September 2015 - 06:26 PM.

[chrisn89]

Also Matty for this:

"If you put the the ring on its own in the bore, like when gapping,and measure the ID of the ring

Now fit the ring in the piston, and using the end of the vernier you should be able to measure how far under the ring sits beneath the piston surface. Now Measure the ring height and piston dia either side of the ring groove, and measure the skirt"

im sorry but I'm not au fait with the terminology.
Height from a cross sectional POV? Also measure how far the ring sits in the piston when uncompressed?

the verniers also not that great for the piston diameter I need to grab some caliper when I get a chance to measure more accurately.

Also, forgive my stupidity, but if the ring gap is okay shouldn't that prove the rings are ok? (If it was for a smaller bore the gap would be huge... Wouldn't it?)

Edited by chrisn89, 06 September 2015 - 06:36 PM.

[chrisn89]

Okay so 100% the oil ring/piston combo. No indents on the rings just says GOE and TOP. They're goetze rings and the packet (all 3 rings come in a pack together) has part no D-51388... Any ideas?

Thanks very much yet again guys,

Chris

[matty...]

Glad you found the problem.

The height of the rings is the measurement between the inside and out side diameters. Width is the measure the thickness off the ring.

For measuring the depth of the groove, it was just another way to work out how deep the groove was, as you said the vernier wouldn't fit in the groove. If you press the ring into the groove to the bottom and measure that distance, it's more than likely that you be able to get a figure. Ideally you don't want to take lots of measurements and subtract one from another as is increases the chances of making an error, and with tight tolerances it's far from ideal, but sometimes you have to work with what you've got.

With regards to my comment about measuring the ID of the ring in the bore. The Rings can be correct for the bore size on the OD but as the ring heights vary for different Pistons and ring sets, that measurement gives you the dimension to check that the rings grooves are deep enough for the rings you have been supplied....if that makes sense?

Edited by matty..., 06 September 2015 - 08:07 PM.

[chrisn89]

Ah yeah. Thanks again for the help I'll get on to minispeed and see what they can do for me.

Thanks again Matty

Chris

[chrisn89]

Just a heads up for anyone that
Might possibly fall into the same trap..

Rang minispeed and they had a duff batch of Pistons. The oil control ring slot in the piston should be 160thou, this one was 140.

Posting back and they're sticking a good set on to the con rods for me and posting back. Unfortunate mistake but they've been spot on sorting it out.

Onwards and upwards. Thanks for the help guys

[carbon]

What make were the duff pistons?

 

Likely to be quite a few now out there...