11 replies to this topic

[foggy]

I am stripping and rebuilding my 1275gt engine as i need to replace the crank.

 

I have just had a look at the bores clsoely and they are all ok apart from number 4,which i am a bit concerned about and wanted some peoples opinion who have done all this before.

 

At the top of the bore there seems to be 2 slight almost "indent" like marks, Looks like where the rings would have been sat when the piston is at the top of the bore.

 

The marks aren't "stepped" but you can feel 2 suttle indents which are smooth. (and you can see them in the correct light)

 

I will take a picture when i get home tonight to see if the camera will pick them out.

 

But in the meantime if anyone understands how i'm describing it, some help in the best next step would be appreciated. ie would i get away with honing or am i looking at a rebore?!

 

Cheers

[Cooperman]

The 'indents' represent the bore wear.

When the block is stripped you need to measure the bores to see how much wear and ovality there is.

It may be possible to hone the bores and fit new rings, but if in any doubt have the block re-bored to the next size up and fit new pistons.

It would be a pity if you went to the trouble and work of fitting a new crank and then had an engine which smoked.

[minimadles]

Like cooperman said of your going to the trouble of replacing the crank it makes sense to sort the bores out if you can feel bore wear with your finger nail micrometer that's over a thou of wear so just to hone it out your removing 2 thou so best to rebore and have a nice fresh bottom end than regret it later

[foggy]

Yea I'm sure your both right. I measure them and the top of the bores are 70.55 -.59mm
Will add photo too. If i fo get it bored out what could i go to.i guess i may aswell make it a fair hit bigger to increase the cc.

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[foggy]

Cylinders 2 and 3 r showing similar wear to.number 4.hence the picture

[Cooperman]

Best advice is to go to the next size up. If you go to the maximum and it subsequently needs a further re-bore the block will probably be scrap, and they are never going to cast any more original blocks. That is why blocks are becoming more expensive these days. Unless going racing, the theoretical increase in power/torque by going to the biggest bore will not really be noticed. Accurate and thorough build quality and top quality parts is more important

It is not just the step that indicates wear. Most of the cylinder wear is at mid-stroke as this is where piston speed is at its maximum, so the effect is a 'barrel-shaped' bore. Bores also become slightly oval with use due to the piston pressing more on one side of the bore than the other on the firing stroke.

A newly honed engine with new rings can still smoke, or soon start smoking after such a rebuild.

[foggy]

Thanks for the advice. Luckily i work about half a mile down the road from slark race engineering.so will probably take it there. Any idea roughly on the cost i should expect to pay for a rebore?

[minimadles]

It's about £15 per cylinder your main expense will be the new parts required like cooperman go to the next minimum rebore you can get away with so you get more bites of the cherry

[foggy]

I would just be able to buy new pistons and re-use the current con rods couldn't I? or would it be best to change them too?

[minimadles]

Yes reuse old con rods but have them checked over just to be sure

[foggy]

I have measure the middle and top of the bores and they are as follows.(please give me your thoughts) in mm.
Top:Mid
70.65: 70.64
70.60: 70.64
70.47: 70.60
70.64: 70.58

Would i get away with a hone?or does it need a rebore?

[Cooperman]

For what it costs a re-bore is best. With only a hone the engine can easily start burning oil quite quickly.