Vauxhall Owners Forum banner

SO glad i pulled the engine apart.

8K views 179 replies 42 participants last post by  MWF 
#1 ·
I recently bought a very nice spec bottom end off another migweb user, me being me, if im doing anything fairly hardcore i like to make sure its done right cause i cant afford to spend the money twice so i thought id strip the thing down over at a mates place where he has an excellent selection of dial guages etc i can borrow to check everything.

My god, it would have been a total disaster if anyone had tried to run it.
The actual components its made from are superb, the pistons for example i couldnt even find a fraction of a ten of a thousandth of an inch inconsitancy across all four and the biggest difference between the widest part of the largest journal and narrowest bit of the smallest journal on the crank was less than 2/10 of a thou, so i was completely chuffed to bits with the quality of the components,

Havent fully finished measuring every single thing yet as time got the better of me but as an example of the sort of extreme problems present, here are the stats for cylinder 3:
Top ring gap measured 20mm or so down the bore = 7 thou
Piston to bore clearance at top end of bore = 4 thou
Piston to bore clearance near bottom of bore = 2 thou
Those measurements taken at 7 degrees C

So that means that the ring gap at the bottom of the bore would be 7 - ( (4-2) * pi) thou, ie ZERO

ZERO ring gap and thats at seven bloody degrees, imagine what would have happened if this engine had ever been allowed to run, bearing in mind that the rings expand more than the bore does as temperature rises.

Just goes to show that no matter how much money you spend on an engine, if the person bolting it together for you is a cowboy you are only going to end up with scrap metal at the end of the day.

Thankfully the block should hopefully be salvageable as im planning on running around the 4-5 thou mark in terms of piston to bore clearance to make alloweances for the amount of heat that will be present due to the level of tune it will be ending up at, but obviously if it isnt going to come good then ill just start again with another block and can still use all the superb quality components still, so all in all im still thoroughly happy with the bits i bought, its just such a good job it was someone paranoid like myself that bought it all and not someone else blowing a couple of grand on it and then assuming it was built properly just cause it had been recently rebuilt.


Got to be a lesson in there about how important it is to check and double check everything when building a high spec engine. :beer:


Chip
 
See less See more
#3 ·
Yeah this is the bottom end i got off Rob Fairbrother, ive no idea if it was the Vaux MD lads who built it or someone else, or if they checked it or not.
TBH it doesnt really matter who it was, the point of my post isnt finger pointing or bitching or anything like that, im VERY happy with the parts i bought and the price i paid for them, i was always planning to rebuild it anyway, so it is just a case of trying to point out to other mig users that you should NEVER just assemble an engine and assume things are correct, everything should be measured and checked or it will end in an engine that either seizes or extremely quickly wears the rings out and smokes like a train, which is one of the two things that would have happened to this pretty much as soon as it was run in and opened up, so it would have been 3 grand or so (i think thats what rob spent anyway) for a bottom end that was basically worthless and would never of made any real power for any useful period of time.
In my post the other day asking for clearance details etc someone said "why are you rebuilding an engine that has just been rebuilt", hopefully now it will be very obvious why i did and encourage other people to measure things too, i get the impression that most people on here just assume when they get bores honed at an engineers that they will be correct, this is frequently not the case and any good engine builder should be checking everything as it is put together so mistakes can be rectified BEFORE they get expensive.

Chip
 
#5 ·
Its only a few hours to re-assemble, but i need to measure some clearances for bearings etc first and then order them up, so probably only a couple of weeks.
Im in no rush at all though, i dont need the engine for ages.
 
#8 ·
Yes thats exactly what im saying about the bores (especially number 3, the others are far closer), this is a very common fault on rebuilt engines, and if the ring gap on this one was a bit more generous and it had been used without inspection then it would have ran okay but just worn out more quickly where the rings were working harder cause they had to flex constantly and no one would have known why it wore out so quickly.

Its the honing process that introduces this inconsistancy not the boring.

2 thou difference, like this, is about the width of a human hair, so people think they can ignore it, but you cant if you want it reliable.
The really dangerous thing on this engine is the combination of a very tight ring gap of 7 thou (not enough on a forged piston in a bore of this size) combined with it going narrower towards the bottom of the stroke.

Chip
 
#10 ·
i remember a thread actually where tom and rob couldnt get one pistons in the bore after replacing a snapped (during fitting) ring. eventually figured out the ring ends were binding so tom slowly took material off the ring ends with an oil stone.

dont suppose thats anything to do with it ? i.e using a ring with less gap so to speak.
 
#12 ·
chip said:
TBH it doesnt really matter who it was, the point of my post isnt finger pointing or bitching or anything like that
Could u remove certain comments from the first post then, if thats the case.

The ring in question was supplied from a local engine builder who was given the piston, and asked to souce a replacement ring. The ring was fitted to the piston when it was collected.
 
#14 ·
Phil, the ring wasnt at fault, the assembly of the engine was.

When fitting rings they need to be gapped correctly at the time you fit them into the engine, the person supplying the ring did nothing wrong, whoever built the engine is 100% responsible for this, you cant expect a supplier to guess what ring gap you are going to need.

The other thing that was 100% the resonsibility of the engine builder was to run a dial gauge down the bores and check they hard be properly bored/honed, this is not a mechano set its a high performance specialist bit of engineering and needs to be assembled by someone who know what they were doing.

Also the top of the bores should have been radiused slightly by the engine builder, apart from removing the sharp edge (very bad news on a high boost motor) it would also have meant the rings didnt keep breaking when the fit was attempted.


Chip
 
#16 ·
No oddball, the block was bored perfectly, no problem there at all, the problem is that it wasnt honed correctly.
The truth is though that when you build an engine you should check that the ring gaps are correct (well in fact more than check them you have to set them, which wasnt done at all in this engine) and you have to check the bores, you cant just make the assumption that the honing is exactly how you want it.
You also have to do things like dress the top of the bores so the edges arent sharp, this isnt part of the boring/honing process, its an assembly step and it wasnt done at all, its very clear to me that whoever built this engine had very little experience of doing so and obviously simply didnt know what was actually involved and thought they could just bolt it together, the point of this post is to try and inform people on mig that this is NOT the case, and if they are rebuilding an engine then they must make sure these very simple and important bits are done or they will end up with the same problems that rob would have if he had run this engine.

Its the difference between properly building an engine, and just throwing all the parts together and hoping that everything is correct by pure chance in terms of things like ring gaps, ok fair enough the engineer who honed it (it was done by machine) is partly responsible for the mistake with the bores, but if you subcontract work out then you should firstly use someone trustworthy and secondly check the results of their work.

Phil, if you could PM me with any comment that you would like removing i will happily do so as i dont want a moderator to feel they have to remove this post as its very important that other people see this sort of thing so they dont make the same basic errors when they also build their first engine up from scratch.

Chip
 
#18 ·
oddball, yes the machinist was partly at fault here without a doubt, thats why it is SO important to always check with a dial gauge whenever you get the bores honed.

2 thou is completely unnaceptable and it should have been spotted when the engine was built and then sent back.

As i said though, thats only one in a catalogue of errors here that led to the problems with rings and would have led to a dead engine.

Chip
 
#21 ·
PS

The rest wont just "sort itself out" really as if there was for example:
4 thou at top, 2 thou at 40mm, 4 thou at 80mm, the rings would still be catching at the 40mm point as the ring gap isnt enough, also, you do need to worry about below BDC as well as potentially you can get problem with piston skirts catching the bore if it were to go too narrow at that point.

Chip
 
#23 ·
16vastra said:
i meant the rest below bdc as it hardly matters. 80mm down does tho of course
yeah 80mm is well before BDC, but my comment was that even below BDC, like at 100mm, it still matters if it comes in a lot as the piston skirts can potentially catch.

Chip
 
#24 ·
also dont forget when looking at the pressure on the rings in the middle of the bore in "sort themselves out section", thats actually the point where the piston is moving the fastest out of the whole cycle.

Chip
 
#26 ·
16vastra said:
i guess, but you would have to rebore with a chainsaw...will try it and let you know how it runs :p

you would have to ask whoever machined it just how they managed to mess it up this bad, not sure if it was a chainsaw or just a proper machine with a completely clueless idiot doing the honing, either way the effect it the same obviously.


Chip
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top