1800 8v DX piston clearance

Hi Folks,

I have an 83 gti tintop project at the moment, from rolling shell to daily driver by summer hopefully.

I am in the middle of rebuilding a 1800 dx engine at the moment. I have stripped the head off so far. I am going to strip the bottom end and replace the bearings etc too, but that depends on what I find.

Can anyone tell me what the tolerance is for piston clearance in the bore ? The bores looks to be in good shape and the max gap between piston and bore is 0.3mm. The Haynes spec for this is 0.07mm but that seems a bit optimistic especially for dissimilar metals.
I am not sure if I need new pistons, or just rings.

This is my first post on this forum, but I have lost count of the number of things I have fixed with the information found here.


Thanks

Alvie

Alvie said

Hi Folks,

I have an 83 gti tintop project at the moment, from rolling shell to daily driver by summer hopefully.

I am in the middle of rebuilding a 1800 dx engine at the moment. I have stripped the head off so far. I am going to strip the bottom end and replace the bearings etc too, but that depends on what I find.

Can anyone tell me what the tolerance is for piston clearance in the bore ? The bores looks to be in good shape and the max gap between piston and bore is 0.3mm. The Haynes spec for this is 0.07mm but that seems a bit optimistic especially for dissimilar metals.
I am not sure if I need new pistons, or just rings.

This is my first post on this forum, but I have lost count of the number of things I have fixed with the information found here.


Thanks

Alvie

I think youll find the clearance should be 1 thou( 0.03mm).
That 3 thou figure- 0.08mm is for absolute maximum so id not be happy to run the engine (especially with cast pistons) with clearances that wide because itll promote skirt cracking as the piston rocks at tdc.

Not only that the ring end gaps will be wide which bleeds off compression and loses power. Spec is 0.30 to 0.45 for compression rings and 0.25 to 0.40 for scraper rings.

When you strip the bottom end, take off the piston rings one at a time  (dont mix them up and keep them the right way up also- the word "top" is usually etched on them at the ring ends but it can be worn away) and fit them one at a time to the bores, then push the ring in square with the piston crown ( upside down) so that it squares the ring up in the bore.

Do it at the top of the bore just below the wear ridge ( if any) halfway down and at the bottom of travel.
Use a feeler gauge to test the ring end gap that will be present.
Record all your measurements….itll take a few hours but you need to (read MUST!) do these checks otherwise youll simply be guessing as to whether or not the measurements are correct or most likely- not.
You need to do this with each piston ring in each of their respective bores.
You can then decide on whether or not to change the rings for a new set of standard items ( depends on piston to bore clearance and bore condition) or whether to go oversized with new pistons and bore/hone etc.
If the end gaps are within spec and piston to bore clearance is correct and the bores are in good usuable condition- you then need to measure the ring side clearances.

These are the measurments of the clearance of the ring as it sits in its groove.
Again, a feeler gauge will do the job fine.
Spec for these is 6 thou or 0.15mm maximum for all the piston rings.
Ensure the grooves are clear of carbon and debris.
Dont use any abrasive materials to clean the grooves out, i use either carp cleaner and a toothbrush ( high as kite if nil ventilation used) or traffic film remover and a toothbrush before rinsing the piston in hot water.
Be aware that worn rings can have evilly sharp edges to them and if worn you can usually see the wear on the form of a dish type effect across the surface.

Theres no easy way around these checks though short of buying a "recon"….and then you dont know whether thats been done correctly either.

Let me know if you get stuck or need any other help.
 8)